Chapter 9: Control and Coordination
In the early 1900s, two physiologists in London — William Bayliss and Ernest Starling — were trying to settle a question that seemed already settled. Everyone “knew” that when acidic food left the stomach and entered the small intestine, the pancreas began pouring out its digestive juice because a nerve carried the message. Nerves, after all, were the body’s messengers.
So the two men did something decisive. In an anaesthetised dog, they cut every nerve running to the pancreas. By the accepted theory, the pancreas should now have fallen silent. Then they introduced acid into the small intestine — and the pancreas responded exactly as before, releasing its juice.
If no nerve carried the message, something else did. They reasoned that the intestinal lining released a substance into the blood, and that this substance travelled through the circulation to reach the pancreas. They named it secretin. A few years later Starling gave a name to this entire class of blood-borne chemical messengers: hormones.
That single experiment revealed the second great communication network of the body. The nervous system speaks in fast electrical signals along fixed wires. The endocrine system speaks in slow chemical messages carried by blood to every corner of the body. Together, these two systems keep an organism in control of itself — and in step with the world outside it.
Diagram
Fig. 9.0 – Opening illustration: split panel showing (left) a neuron carrying an electrical impulse along its axon to a target muscle, and (right) a gland releasing hormone molecules into a blood vessel that reach a distant target organ; label “electrical signalling” and “chemical signalling”.
Each of these questions will be answered as you work through this chapter — watch for them as they appear.
9.1 Nervous System in Hydra
Why a Multicellular Body Needs Coordination
A unicellular organism keeps life simple. One single cell senses its surroundings, feeds, respires, excretes and responds — all by itself. There is no one else to inform, and nothing to organise.
A multicellular body cannot work that way. Its organs and organ systems must each be controlled, so that no single part runs faster or slower than the body needs, and they must also be coordinated, so that the systems act together rather than independently. Only then can the organism maintain homeostasis — a steady, balanced internal environment (steady temperature, steady blood glucose, steady water and salt levels) in spite of changes outside.
Both plants and animals show control and coordination, but they do it differently. Plants have no nerves. They coordinate by sending chemical signals, and these signals bring about various types of movements — phototropic movement (growth towards light), chemotactic movement (movement in response to a chemical) and thigmotactic movement (response to touch, as when a tendril curls around a support).
Animals, especially the higher vertebrates, show a gradual increase in the complexity of control and coordination, and they use both electrical signals and chemical signals. In this chapter you will study how the nervous system develops across different animal groups, and then study the system in humans in detail.
Sponges: Division of Labour Without Nerves
The porifera (sponges) are the most primitive of the animal phyla. They contain several different types of cells, each carrying out a different job, yet a proper nervous system is completely lacking in them.
So how does a sponge work efficiently at all? These multicellular animals show division of labour among their cells. Cells become specialised for particular activities such as digestion, respiration, excretion and others, and the body functions because each cell type does its own task.
Later, in the higher animal phyla, these different cells form tissues, then organs, then organ systems. Once that happens, coordination becomes unavoidable: the systems must work in step with one another for smooth internal functioning of the body. At the same time, the organism must be able to respond and coordinate with respect to various stimuli or changes in the external environment.
The Two Coordinating Systems
In lower animals such as Hydra and Planaria, the nervous system alone achieves this function of coordination. In higher and more complex animals, the work is shared by two coordinating systems — the nervous (electrical) system and the endocrine (chemical) control system. We will study these two separately in this chapter, although in the living body they always work in coordination with each other.
Nervous Coordination Begins: Hydra
Hydra is a cnidarian, and it shows the diffused nervous system — the most primitive nervous system found in animals. The cnidarians are therefore the first animal group to show a true, simple nervous system. Before them (in sponges) there was none at all.
The system is built from just two kinds of cells: sensory cells, which detect the stimulus, and nerve cells or neurons, along with their fibres, which carry the message. There is no brain, no cord, and no control centre of any kind.
The nerve cells are not gathered into a bundle or a centre. They are scattered or distributed throughout the body and are interconnected to each other by synapses — junctions between the fibres of neighbouring neurons — so that the whole set of cells forms a continuous mesh called the nerve net, as shown in Fig. 9.1.
Diagram
Fig. 9.1 – Hydra with its nerve net; label tentacles, body wall and the nerve net spreading through the body.
There are two nerve nets in Hydra, and both lie in the mesoglea (the jelly-like layer between the two cell layers of the body). One net is connected towards the epidermis (the outer layer), and the second towards the gastrodermis (the inner layer lining the gut cavity). Sensory cells are scattered in the body wall and in the tentacles.
What the Nerve Net Can and Cannot Do
Sense organs are lacking in Hydra. It has sensory cells, but these are not gathered into an eye, an ear or any other organ. Similarly, its neurons have fibres, but there are no separate sensory and motor nerves — no dedicated “incoming” and “outgoing” wires.
Because of this arrangement, the nerve impulse in Hydra shows no polarity and no direction. Activation of a sensory cell can happen at any point on the body, and from that point the impulse can be carried throughout the body in any direction. The result is a movement of the body or of the tentacles — for example, catching of prey during feeding.
Flowchart
FC 9.1 – Response in Hydra: stimulus (touch of prey) → sensory cell excited → nerve net (impulse spreads in all directions, no polarity) → contraction of body/tentacle cells → prey captured.
Why the Diffused System Matters
The diffused type of nervous system is the first important landmark in the evolution of the nervous system. It is the moment when animals first acquired dedicated signalling cells.
It has not disappeared, either. It is seen in the ctenophora, and — remarkably — in the enteric system or gut wall of higher animals, including humans.
9.2 Nervous System in Planaria (Flatworm)
Planaria is a flatworm belonging to the phylum Platyhelminthes. It marks the next great step after the nerve net: it is the most primitive animal to possess a central nervous system (CNS) — a definite control centre to which messages are brought and from which instructions are sent out. In Planaria this central nervous system is located on the ventral (lower) side of the body.
The First Brain
At the front of the animal lies a mass of nervous tissue called the cerebral or cephalic ganglion. A ganglion is simply a collection of nerve cell bodies grouped together. This ganglionic mass appears like an inverted U-shaped brain, and it lies in the anterior or head region.
From each ganglion, nine branches arise towards the outer side, carrying signals to and from the front end of the body. Notice in Fig. 9.2 how everything in the animal is arranged with reference to this head-end mass — this concentration of nervous tissue and sense organs at the anterior end is the beginning of a trend that eventually produces the human brain.
Diagram
Fig. 9.2 – Ladder-type nervous system in a flatworm such as Planaria; label eyespot, brain (cerebral ganglion), the pair of ventral nerve cords, and the transverse nerves (commissures) joining them.
The Ladder Pattern
Ventrally, from below the ganglia, arises a pair of ventral nerve cords (VNC) — long nerve cords that run backwards along the length of the body. These two cords are interconnected to each other by transfer nerves or commissures, arranged in a ladder-like manner. The two long cords are the side rails of the ladder and the transverse commissures are its rungs, which is why this design is called the ladder-type nervous system.
The peripheral nerve plexus arises laterally from the ventral nerve cords, spreading out into the body. This peripheral nervous system also includes sensory cells arranged in lateral cords along the body, and single sensory cells scattered in the body — a leftover of the older, diffuse plan.
A pair of photosensory structures, the eyes (eyespots), are located on the dorsal side of the brain. These are true sense organs, gathered at the head end — something Hydra completely lacked.
The Evolutionary Trend So Far
Look back now at the examples of Hydra and Planaria, together with the cockroach and humans that you have studied earlier. Placed side by side, they show the gradual evolution or change of the neural system, and the trend is summarised in FC 9.2.
Flowchart
FC 9.2 – Evolution of the nervous system: no nervous system (sponges) → diffuse nerve net, no centre (Hydra) → ladder-type CNS with cerebral ganglia (Planaria) → segmental ganglia with a nerve cord (cockroach) → highly centralized brain and spinal cord with a complex peripheral network (humans).
Two things change along this sequence. First, there is a high level of specialization in the formation of neurons as electrically signalling cells — the cells themselves become better and better at carrying signals. Second, the system as a whole changes, gradually shifting from a diffuse neural system to a centralized nervous system.
That centralization means the expansion of the system into a properly organized arrangement involving a brain, which then gradually expands in size and in the number of functions it performs. Along with this comes the centralization of the various sense organs, which assist in coordinating the internal environment of the body with the external environment.
Finally, there is the evolution of a complex networking system that efficiently transmits signals between one part or organ of the body and another — the peripheral wiring without which a large brain would be useless.
9.3 Neural Tissue
Neural tissue is the material out of which the whole nervous system is built. It consists primarily of two types of cells: the neurons (nerve cells), which actually carry the signals, and the neuroglia or glial cells, which support, protect and service the neurons.
Before going further, one piece of vocabulary must be settled, because examiners are fond of it. The word nerve is used for a bundle of axons lying outside the CNS. For exactly the same structure lying inside the CNS, the word tract is used instead.
Nerves themselves are of three kinds. A sensory nerve carries only sensory fibres, a motor nerve carries only motor fibres, and a mixed nerve is one having both types — sensory and motor fibres — running together in the same bundle.
How the Tissue Becomes a System
All of these — neurons, neuroglia and nerves — along with the nervous organs, make up the nervous system of the higher animal, and together they bring about coordination and control of the various activities of the body.
The work is done in a fixed order. Receptors bring in sensory inputs towards the central nervous system. Processing is then carried out in the CNS. Finally, through motor commands, the response is sent out. The nerves arising from the cytons of the CNS travel throughout the body, transmitting nerve impulses to or from the CNS, and this general plan is summarised in FC 9.3.
Flowchart
FC 9.3 – Working plan of the nervous system: stimulus → receptor → sensory (afferent) input → CNS (integration and processing) → motor (efferent) command → effector (muscle or gland) → response.
Neurons — the Nerve Cells
Neurons are the structural and functional units of the nervous system. (You may recall the structure of a neuron studied in earlier classes.) Each multipolar neuron has three parts — the cyton or cell body, the dendron, and the axon. As you follow Fig. 9.4, notice how the shape of the cell already tells you the direction in which the message travels: many short processes gathering signals in, one long process carrying the signal away.
Diagram
Fig. 9.4 – A multipolar neuron; label cyton (cell body), nucleus with nucleolus, Nissl’s granules, neurofibrils, dendrons with fine dendrites, axon hillock, axon, collateral, telodendrons and synaptic knobs.
The cyton
The cyton has a distinct central nucleus with a nucleolus, surrounded by neuroplasm (the cytoplasm of the neuron). A clear film of cytoplasm surrounds the nucleus, and around it lie the neurofibrils, the Nissl’s granules and other cell organelles.
Nissl’s granules are riboprotein components — that is, they are rich in ribonucleoprotein, and so they are the protein-manufacturing machinery of the cell. They play an important role in the synthesis of the enzyme required for the formation of the neurotransmitter, the chemical by which one neuron passes its message to the next. Neurofibrils, in contrast, play an important role in the transmission of the nerve impulse.
The dendrons
Dendrons are the many small conical processes arising from the cyton. Each is highly branched into fine dendrites. Nissl’s granules and neurofibrils can both be seen at the base of the dendrons. Dendrons transmit the message towards the cyton.
The axon
The axon is a single, long, usually unbranched process arising from the cyton at a slightly raised region called the axon hillock. It consists of a bundle of neurofibrils, and Nissl’s granules are absent in it.
Terminally, the axon gives out branches called telodendrons, and along its length it may give out lateral branches called collaterals. Axons carry the messages away from the cyton. The terminal branches finally attach to a muscle, a gland, the skin, or the telodendrites of another neuron.
The interconnection between two neurons, or between a neuron and a motor organ, is called a synapse. Most synapses are axo-dendronic (axon of one neuron to dendron of the next), but a synapse may also be axo-axonic, axo-somatic (axon to cell body) or dendro-dendronic. The working of the synapse is taken up in detail in the next section.
Where the Parts of a Neuron Lie
The cytons are generally found inside the brain and spinal cord (that is, in the CNS) and in the ganglia. Small groups of cell bodies lying inside the white matter of the brain are called basal nuclei.
A bundle of axons — a nerve — may be covered in one of two ways. In non-medullated (unmyelinated) nerves it is covered only by the neurilemma. In medullated (myelinated) nerves it is covered both by the medullary sheath (the fatty myelin sheath) and, on the outside, by the neurilemma. This difference matters enormously: conduction of impulse by the medullated nerves is 50 times faster than in the non-medullated nerves.
Outside the axons come three layers of connective tissue, one inside the next. The connective tissue covering around a nerve fascicule (a small bundle of fibres) is called the endoneurium. A few nerve fasciculi with their endoneurium are surrounded by connective tissue called the perineurium, and a still larger bundle of nerves is covered on the outer side by the epineurium. Blood is supplied to all the nerves to provide oxygen and nutrients — nervous tissue is highly active and cannot survive without a constant supply.
Neuroglial Cells
The neuroglial cells are far greater in number than the neurons. Most of the supporting cells of the nervous system are derived from the same embryonic tissue layer (the ectoderm) that produces the neurons themselves. The term neuroglia refers to the supporting cells of both the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
They are not simply packing material. As Table 9.4 shows, they build the myelin sheath, defend the brain, maintain its chemical environment and produce its fluid — jobs without which neurons could not function at all.
Diagram
Fig. 9.3 – Types of neuroglial cells: ependymal cell, oligodendrocyte, satellite cell, astrocyte, microglia and Schwann cell, each drawn and labelled.
Table 9.4 : Types of Neuroglial cells and their functions
| Location | Cell type | Functions |
|---|---|---|
| CNS (Central Nervous System) | Oligodendrocytes | These cells have few branches and mainly form the myelin sheath around the central axons, which form the white matter of CNS. Myelin, an insulating sheath, is made up of protein and fatty substances. It allows quick transmission of electrical impulses. |
| CNS | Microglia or brain macrophages | Small sized cells with few branches. These are derived from monocytes and act as macrophages. They go to the site of injury, dead neurons and cell debris in the CNS. They mediate immune response in the CNS. |
| CNS | Astrocytes | Star shaped cells and the most abundant glial cells of CNS. They have varied roles in the brain — secretion and absorption of neurotransmitter and maintenance of the blood–brain barrier (BBB). They regulate the transmission of electrical impulses within the brain. |
| CNS | Ependymal cells | Form a single layer of squamous or columnar, often ciliated epithelial cells lining the ventricles or brain cavities and the central canal of the spinal cord. Mainly responsible for the production, and probably also for the circulation, of CSF in the brain ventricles and central canal. |
| PNS (Peripheral Nervous System) | Schwann cells | These are the most abundant glial cells of PNS. They produce the myelin sheath around medullated nerves of the PNS. |
| PNS | Satellite cells | They support the functions of neurons. |
9.4 Synapse
A neuron does not touch the next neuron. Between the two lies a junction with a minute gap — the synaptic cleft — and the message is carried across that gap by a chemical bridge of neurotransmitter. This junction is the synapse.
Everything a nerve fibre can and cannot do — how strong a stimulus must be, how quickly it can respond a second time, how fast the signal moves — follows from the properties listed below. Learn them as a set; NEET questions frequently pick one property and test whether you can apply it to a situation.
1. Properties of Nerve Fibres
a. Excitability / Irritability – The nerve fibres, on account of the presence of a polarised membrane (a membrane with opposite charges on its two faces), have the ability to perceive a stimulus and enter into a state of activity. Without that charged membrane there would be nothing for a stimulus to disturb.
b. Conductivity – It is the ability to transmit the excitation, that is, to pass the disturbance along the fibre instead of letting it die at the spot where it began.
c. Stimulus – A stimulus is any detectable physical, chemical or electrical change in the external or internal environment which brings about excitation in a nerve, muscle, organ or organism.
In order to be effective, the stimulus must have a minimum intensity, called the threshold stimulus. A subliminal (weak) stimulus, which is below this minimum, will have no effect at all. A supraliminal (strong) stimulus, which is above it, will produce the same degree of impulse as the threshold stimulus — being stronger does not make the impulse bigger.
d. Summation effect – A single subliminal stimulus will have no effect. But if many such weak stimuli are given in quick succession, they may together produce an impulse, due to the addition or summation of stimuli. Individually each is too small; arriving together, they cross the threshold.
e. All or none law – The nerve will either conduct the impulse along its entire length, or will not conduct the impulse at all, as in the case of a subliminal or weak stimulus. There is no partial impulse and no half-strength impulse.
f. Refractory period – It is the time interval (about a millisecond) during which a nerve fails to respond to a second stimulus, however strong it may be. For that brief moment the fibre is simply unavailable.
g. Synaptic delay – The impulse requires about 0.3 to 0.5 milliseconds to cross a synapse. This time is required for the release of the neurotransmitter from the axon terminal and for the excitation produced in the dendron of the next neuron. The delay is the price paid for using a chemical bridge instead of a direct electrical connection.
h. Synaptic fatigue – The transmission of the nerve impulse across the synapse halts temporarily due to the exhaustion of its neurotransmitter. The store of transmitter in the vesicles is finite, and repeated firing can empty it faster than it is replaced.
i. Velocity – The rate of transmission of the impulse is higher in long and thick nerves. It is higher in homeotherms (warm-blooded animals, whose body temperature stays high and constant) than in poikilotherms (cold-blooded animals). The velocity of transmission is higher in voluntary fibres (100–120 m/second in man) as opposed to autonomic or involuntary nerves (10–20 m/second). It is similarly faster in a medullated nerve, as the impulse has to jump from one node of Ranvier — a gap in the myelin sheath — to the next, instead of travelling through every point of the membrane.
The Two Sides of a Synapse
At the synapse, neurons communicate with one another. The neuron carrying an impulse to the synapse is the pre-synaptic neuron. The neuron receiving the input at the synapse is the post-synaptic neuron, or the generator region, which may instead be a gland or a muscle. A synaptic cleft, a small intercellular space of width about 20–30 nm, lies in between the two cells.
The process by which the impulse from the pre-synaptic neuron is conducted to the post-synaptic neuron or cell is called synaptic transmission. It is a one-way process, carried out by neurotransmission — the transmitter is stored on one side only, and the receptors lie on the other, so the message cannot run backwards.
2. Types of Synapses
a. Electrical synapse
In this type of synapse, the gap between the neighbouring neurons is very narrow, and the synapse between such closely placed neurons is mechanical. An electrical conductive link is formed directly between the pre- and post-synaptic neurons. At the gap junction, the two cells are within almost 3.8 nm of each other.
Transmission across this gap is faster, since no chemical has to be released, diffuse and bind. However, it depends entirely on the connections located at the gap junctions between the two neurons. Electrical synapses are found in those places of the body requiring the fastest response, as in the defence reflexes. They are bidirectional, allowing transmission in either direction, or may be unidirectional.
b. Chemical synapse
These are specialized junctions through which cells of the neural system send a chemical signal to other neurons and to non-neuronal cells, such as gland and muscle cells. The synaptic gap here is larger than that in the electrical synapse: it is 20–40 nm. A chemical synapse between a motor neuron and a muscle cell has a special name — the neuromuscular junction.
There are three components of a typical chemical synapse: 1. The presynaptic terminal (mostly an axonic terminal). 2. The synaptic membrane of the post-synaptic cell (usually on the dendrite of the next neuron, or on a gland cell or muscle cell). 3. The post-synaptic neuron.
Transmission Across a Chemical Synapse
The impulse travels along the axon of the pre-synaptic neuron to the axon terminal. Most presynaptic neurons or axons have several synaptic knobs at their ends or terminals. These knobs have arrays of membranous sacs, called synaptic vesicles, that contain the neurotransmitter molecules. The whole sequence is shown in Fig. 9.5.
Diagram
Fig. 9.5 – Synaptic transmission; label presynaptic neuron, nerve impulse, synaptic end bulb, voltage-gated Ca²⁺ channel, Ca²⁺ ions, synaptic vesicles, neurotransmitter, synaptic cleft, neurotransmitter receptor, ligand-gated channel (closed and open), Na⁺ and K⁺ movement, postsynaptic potential and the new nerve impulse in the postsynaptic neuron.
When an impulse reaches a synaptic knob, voltage-sensitive \(Ca^{2+}\) channels open and calcium (\(Ca^{2+}\)) diffuses inward from the extracellular fluid, because calcium is far more concentrated outside the cell than inside it.
The increased calcium concentration inside the cell initiates a series of events that fuse the synaptic vesicles with the cell membrane of the presynaptic neuron, where they release their neurotransmitters by exocytosis into the cleft.
Once the neurotransmitters bind to the receptors of the post-synaptic cell, the action is either excitatory (turning a process on) or inhibitory (turning a process off). This depends entirely on the nature of the neurotransmitter involved — the same signal can therefore mean “fire” at one synapse and “do not fire” at another.
Once the impulse has been transferred across the synapse, an enzyme such as cholinesterase destroys the neurotransmitter, and the synapse is then ready to receive a new impulse. Without this clearing step, the transmitter would sit on the receptor and keep the post-synaptic cell switched on permanently.
Flowchart
FC 9.4 – Chemical synaptic transmission: impulse reaches synaptic knob → voltage-gated Ca²⁺ channels open → Ca²⁺ influx → synaptic vesicles fuse with presynaptic membrane → neurotransmitter released by exocytosis into cleft → binds receptors on postsynaptic membrane → ligand-gated channels open → if excitatory → postsynaptic potential → new nerve impulse; if inhibitory → process turned off → cholinesterase destroys neurotransmitter → synapse ready for next impulse.
9.5 Transmission of Nerve Impulse
The neurons are cells with some special features, and the most important of them is this: the cells are excitable. A nerve impulse is a wave of bioelectrical or electrochemical disturbance passing along a neuron.
The transmission of the nerve impulse along the long nerve fibre or axon tube is the result of electrical charges across the neuronal membrane during the conduction of an excitation. So to understand the impulse, we must first understand the charge that sits on the membrane even when nothing at all is happening.
The Resting Membrane: A Charged Battery
Each neuron has a charged cellular membrane, with a voltage that is different on the outer and the inner side of the membrane. The plasma membrane separates the outer and inner solutions, which contain different chemical compounds but have approximately the same total number of ions.
The external tissue fluid has both \(Na^+\) and \(K^+\), but there is a predominance of \(Na^+\) and \(Cl^-\) outside. Inside the fibre — that is, in the intracellular fluid — \(K^+\) is the predominant ion. This condition of a resting nerve is called the polarised state.
The polarised state is established by maintaining an excess of \(Na^+\) on the outer side. On the inside there is an excess of \(K^+\), along with large negatively charged protein molecules and nucleic acids, which are far too big to leave the cell.
The Sodium–Potassium Pump
No membrane is perfectly sealed. Some amount of \(Na^+\) and \(K^+\) always leaks across it, which would slowly destroy the ionic difference. The \(Na^+/K^+\) pump in the membrane therefore actively restores the ions to their appropriate side.
Working against the concentration gradient and the electrochemical gradient, \(Na^+\) is forced out of the cell and \(K^+\) is forced inside. This process is called the sodium pump or the Na–K exchange pump. Because it pushes ions “uphill”, this active process requires ATP energy.
The difference in the distribution of \(Na^+\) and \(K^+\) on the two sides of the membrane produces a potential difference of \(-50\) to \(-100\) millivolts, the average being \(-70\) millivolts.
Resting Potential
This potential difference seen in a resting nerve is called the resting potential (\(-70\) millivolts). It is mainly due to the differential permeability of the resting membrane, which is much more permeable to \(K^+\) than to \(Na^+\).
Because of this, slightly more \(K^+\) diffuses out than \(Na^+\) moves in, causing a slight difference in polarity. In addition, the negatively charged proteins and nucleic acids trapped inside the cell make the overall charge negative on the inside and positive on the outside.
The Gates in the Membrane
The nerve membrane not only has leakage channels, through which ions trickle continuously, but also has many gated channels for \(Na^+\) and \(K^+\). These are also called voltage-gated channels, because they open and close in response to changes in membrane voltage.
These channels are what enable the neuron to change its membrane potential to the active potential in response to a stimulus. The \(Na^+\) and \(K^+\) gated channels are separate, so the transport of these two ions is done separately. However, during the resting potential, both these gates are closed and the membrane resting potential is maintained.
Generation of Nerve Impulse
1. Depolarization
The origin and maintenance of the resting potential depends on the original, perfectly balanced state. Any change or disturbance to the membrane will cause \(Na^+\) to enter into the membrane and lower the potential difference (that is, make it less negative than \(-70\) millivolts).
This lowering of the potential difference itself makes the membrane more permeable to \(Na^+\), so there will be a rapid influx of \(Na^+\). This self-reinforcing property is peculiar to a nerve membrane: entry of a little sodium invites the entry of a great deal more.
The voltage-gated \(Na^+/K^+\) channels are special in 2 ways: they can change the potential difference of the membrane as per the stimulus received, and the gates operate separately and are self-closing.
During the resting potential, both gates are closed and the resting potential is maintained. However, during depolarization the \(Na^+\) gates open but not the \(K^+\) gates. This causes \(Na^+\) to rush into the axon and bring about depolarisation — the opposite of the resting polarity.
The extracellular fluid (ECF) now becomes electronegative with respect to the inner membrane, which becomes electropositive. The value of the action potential is \(+30\) millivolts to \(+60\) millivolts. This reversal at one spot triggers depolarisation in the next part of the fibre, while the spot itself already starts going to repolarisation, as Fig. 9.6 shows.
Diagram
Fig. 9.6 – Polarisation and depolarisation along a nerve; show a resting (polarised) axon with \(Na^+\) outside and \(K^+\) inside and the Na–K pump, then an excited axon with a depolarised patch where the stimulus was applied, and then an axon showing polarised, depolarised and re-polarised regions in sequence.
2. Re-polarization
The change in polarity from the depolarized state back to the original state is done by the process of repolarization. It occurs after a short interval called the refractory period.
The large number of \(Na^+\) ions now present on the inside causes a drop in the permeability of the membrane to \(Na^+\), and at the same time makes it more permeable to \(K^+\) ions, by opening the \(K^+\) voltage gates and slowly closing the \(Na^+\) gates. This action is a localized activity, confined to the patch of membrane concerned. \(K^+\) ions pass out very rapidly, as compared to the slow entry of \(Na^+\).
In this period, the \(Na^+\) gates are closed, the \(K^+\) gates are open, and the \(Na^+\)–\(K^+\) pumps become operational, restoring each ion to its proper side.
This process of producing a wave of stimulation \(\rightarrow\) causing depolarization \(\rightarrow\) repolarization is repeated continuously up to the end of the axon terminal. It is a self-propagating process: each depolarised patch acts as the stimulus for the patch just ahead of it, so the impulse regenerates itself all the way along and never fades out.
Flowchart
FC 9.5 – Chart 9.7: Steps in generation and conduction of a nerve impulse: application of stimulus on a resting nerve → permeability of membrane changes and it becomes more permeable to Na⁺ → Na⁺ ions diffuse into the neuron from the ECF → number of positive ions inside the axon increases → membrane potential changes from −70 mV to about +30 mV (action potential) → polarity reversed, negative outside and positive inside (depolarization) → repolarisation: at the peak of the action potential (+30 mV) the Na⁺ channels close, K⁺ gates open, membrane becomes more permeable to K⁺ → K⁺ ions diffuse out of the axon → the inside of the membrane becomes less and less positive and finally negative once again → axoplasm inside becomes negatively charged and ECF positively charged → resting state restored, wave passes on to the next region.
Diagram
Fig. 9.7 – Graph of membrane potential (mV) on the y-axis against time (milliseconds) on the x-axis; mark the resting potential at \(-70\) mV, the threshold, the rising phase of depolarisation, the peak at about \(+30\) mV, the falling phase of repolarisation, and the return to resting level.
Saltatory Conduction in Medullated Nerves
In medullated nerves, the insulating fatty myelin sheath prevents the flow of ions between the axoplasm and the ECF. Ions simply cannot cross where the sheath covers the fibre.
As a result, the transport pump and the gated channels can operate only in the region of the nodes of Ranvier, where the myelin sheath is absent.
The action potential therefore cannot travel as a smooth, continuous wave of membrane depolarization. It has to jump from node to node. This process is called saltatory conduction, and it takes place at the rate of 120 m/second. It is faster than the continuous conduction in a non-medullated fibre in the ratio \(50:1\).
9.6 Human Nervous System
The nervous system in humans is well developed and complex. It is broadly classified into three parts, viz. the CNS, the PNS and the ANS. Everything you have learnt so far — the neuron, the synapse, the impulse — now goes to work inside this organised structure.
Flowchart
FC 9.6 – Organisation of the human nervous system: Nervous system → Central nervous system (CNS) → brain, spinal cord; Nervous system → Peripheral nervous system (PNS) → cranial nerves, spinal nerves; Nervous system → Autonomic nervous system (ANS) → sympathetic nervous system, parasympathetic nervous system.
1. Central Nervous System (CNS)
The brain and the spinal cord are the parts of the CNS, and both lie along the mid-dorsal axis of the body. The brain is enclosed within the brain box or cranium of the skull, whereas the spinal cord occupies the vertebral canal of the vertebral column.
Bone alone is not enough. Inner to these bony coverings lie three protective membranes called the meninges, which protect the brain and spinal cord. From outside inwards they are:
a. Dura mater — It is the outermost, tough, non-vascular, thick and fibrous meninx, and it is attached to the inner side of the cranium. It is separated from the underlying arachnoid mater by the subdural space, filled with a serous fluid.
b. Arachnoid mater — It is the middle, thin and non-vascular layer of connective tissue, having a web-like appearance. It is separated from the pia mater below by a narrow subarachnoid space, filled with cerebrospinal fluid — CSF.
c. Pia mater — It is the innermost, delicate, highly vascular membrane, and it lies in close contact with the CNS itself.
Cerebrospinal Fluid (CSF)
The CSF is a lymph-like extracellular fluid, secreted by the choroid plexuses of the pia mater, present inner to the subarachnoid space, in the ventricles of the brain and within the central canal of the spinal cord. It is secreted by the pia mater, the choroid plexuses, and the ependymal cells lining the ventricles of the brain and the central canal of the spinal cord.
The three openings in the roof of the medulla oblongata help in draining out the CSF from the brain to the outside, so that fluid is removed as fast as it is made.
The CSF does a great deal of work. Together with the meninges, it acts as a shock absorber, protecting the brain and spinal cord from mechanical injuries — the brain effectively floats in it. It maintains a constant pressure inside the cranium. It helps in the exchange of nutrients and wastes between the blood and the brain tissue. It helps in the supply of oxygen to the brain, and it protects the brain from desiccation (drying).
CSF is a slightly alkaline fluid with a specific gravity of 1.005. A total of 100–120 cc of CSF is present in and around the CNS.
A. The Human Brain
The study of all aspects of the brain is called encephalology. The brain can be divided into three main parts — the forebrain, the midbrain and the hindbrain. Their full division is set out in FC 9.7.
Flowchart
FC 9.7 – Chart 9.8, Parts of the brain: Brain → Forebrain (Prosencephalon) → Rhinencephalon (reduced olfactory lobes, 2), Telencephalon (cerebrum, divided incompletely into two cerebral hemispheres), Diencephalon (epithalamus, thalamus, hypothalamus); Brain → Midbrain (Mesencephalon) → cerebral peduncles or crura cerebri, corpora quadrigemina; Brain → Hindbrain (Rhombencephalon) → Metencephalon (cerebellum, pons varolii), Myelencephalon (medulla oblongata).
a. Forebrain
The forebrain consists of the olfactory lobes, the cerebrum and the diencephalon.
i. Olfactory lobes — These are highly reduced in the human brain, and are covered by the cerebrum from all sides except the ventral side. Each lobe consists of an olfactory peduncle and an olfactory bulb. Their small size in humans reflects how little we depend on smell compared with a dog or a rat.
ii. Cerebrum — It is the largest part of the brain, making up about 85% of the total brain. It is divided into a right and a left cerebral hemisphere by a deep median, long fissure. The two hemispheres are internally connected to each other by a thick band of nerve fibres called the corpus callosum, so that the two halves can share information.
The outer surface of the cerebrum is called the cerebral cortex, while the deep inner part is the cerebral medulla. The cerebral cortex has an outer thin region composed of grey matter, and the inner medulla is composed of white matter.
The surface of each cerebral hemisphere is greatly folded, by many convolutions called gyri and grooves called sulci. These greatly increase the total surface area, for accommodation of the vast number of nerve cells — the sheet of cortex is far larger than the skull that holds it, so it must be crumpled to fit in.
Each cerebral hemisphere is further divided into four main lobes by three deep sulci. These are: - the central sulcus, which demarcates the frontal lobe from the parietal lobe; - the parieto-occipital sulcus, which separates the parietal from the occipital lobe; - the lateral or sylvian sulcus, which demarcates the temporal lobe from the frontal and parietal lobes.
Since these three sulci are not complete, the lobes are not clearly demarcated from each other. A fifth median lobe, called the insula or insular cortex, is folded deep within the lateral sulcus.
The grey matter of the cerebral cortex mainly consists of the cell bodies of billions of neurons, along with non-medullated fibres and dendrons. The white matter mainly has axons of myelinated nerves — its white colour comes from the fatty myelin itself.
Functional areas of the cerebrum
1. Frontal lobes — They have the motor area, which controls voluntary motor activities or movements of muscles. The premotor area is the higher centre for involuntary movements and for the autonomic nervous system. The association area is for coordination between sensation and movements. Broca’s area, the motor speech area, translates thoughts into speech. Expression of emotions, intelligence, will-power, memory and personality areas are also located in the frontal lobe.
2. Parietal lobes — They are mainly for somaesthetic sensation: pain, pressure, temperature and taste (the gustatoreceptor sense).
3. Temporal lobes — They contain centres for smell (olfactory), hearing (auditory), speech and emotions.
4. Occipital lobes — They have the visual area, mainly for the sense of vision.
The area of contact between the temporal, parietal and occipital lobes is the centre for Wernicke’s area, or the intelligence centre. It helps in the understanding of spoken and written words.
The cerebrum thus shows all three types of areas — sensory, motor and association.
Basal nuclei or basal ganglia are grey masses present within the white matter, or lying on the lateral sides of the thalamus. The basal ganglia or nuclei of the cerebrum receive neurotransmitters from various parts. They help the cortex in the execution of activities at the subconscious level — for example, writing slowly, or typing rapidly without watching the keys. The corpus striatum, at the floor of the cerebrum, is the largest basal nucleus.
Diagram
Fig. 9.9 – Sagittal section of the human brain; label cerebrum, central sulcus, parieto-occipital sulcus, lateral ventricle, corpus callosum, thalamus, hypothalamus, pituitary gland, pineal gland, choroid plexus, midbrain, pons varolii, medulla oblongata (together the brain stem), cerebellum with arbor vitae, and the spinal cord.
iii. Diencephalon — It is the part of the forebrain that contains the epithalamus, thalamus and hypothalamus. It lies below the corpus callosum and above the midbrain. It encloses a single cavity termed the third ventricle or diocoel, which communicates with the two lateral ventricles of the cerebrum through a narrow opening called the foramen of Monro.
The epithalamus is the thin, non-nervous roof of the diencephalon. Anteriorly it is fused with the pia mater to form the anterior choroid plexus, and from its dorsal wall it is connected to the pineal gland through a pineal stalk. (The pineal gland is discussed in detail under chemical coordination.) Earlier this gland was thought to be vestigial, but later it was found to produce the hormone melatonin, the sleep-inducing hormone, which is also related to reproductive behaviour.
The lateral thick walls of the diencephalon form the thalami. They mainly contain grey matter. The habenular commissure connects the two thalami. Different parts of the brain are interconnected by the RAS (Reticular Activating System) through the thalami.
The thalamus is called the relay centre, as it transmits all sensory impulses except those of olfaction (smell) to the cerebrum — it is the gatekeeper of the cerebrum, connecting the anterior lateral ventricle to the iter posteriorly. The narrow cavity of the diencephalon is the IIIrd ventricle or diocoel. It connects anteriorly to the two lateral ventricles by a single opening called the foramen of Monro, and posteriorly to the IVth ventricle or metacoel through a narrow duct of Sylvius or iter.
Diagram
Fig. 9.10 – Ventricles of the brain, shown in coronal and sagittal planes; label the two lateral ventricles, interventricular foramen (foramen of Monro), third ventricle, cerebral aqueduct (iter), fourth ventricle and central canal.
Hypothalamus — It forms the floor of the diencephalon. It is richly supplied with blood vessels (the hypothalamo-hypophyseal portal vein helps in the feedback mechanism for hormonal control). It maintains homeostasis, the internal equilibrium of the body, and it controls involuntary behaviour.
Like the cerebrum, the hypothalamus also contains hypothalamic nuclei in its white matter (refer to the figure of the pituitary gland in Section 9.10), with neurosecretory cells involved in the production of the hormones oxytocin and vasopressin.
The hypothalamus is the link between the nervous and the endocrine system, and it has the higher centres for the endocrine system. It regulates heart rate, respiration, blood pressure (B.P.), body temperature, and water and electrolyte balance. It has centres for hunger, thirst, sleep, fatigue and satiety, and for the secretion of the glands of the stomach and intestine. It also produces neurohormones that stimulate the pituitary gland.
A complex neuronal circuit called the limbic system is formed by the hypothalamus, the amygdala, parts of the epithalamus and thalamus, the hippocampus and other areas. It appears to be responsible for emotional reactions, motivational drives and memory.
The floor of the hypothalamus continues as a downward projection called the hypophyseal stalk or infundibulum, which connects it to the hypophysis (pituitary gland) both physically and functionally, by the secretion of neurotransmitters (details in chemical coordination). The inferior surface of the hypothalamus also bears the optic chiasma (the crossing of the two optic nerves) and a pair of mammillary bodies, which are unique to the mammalian brain and are responsible for recollective memory.
b. Midbrain
It is located between the diencephalon and the pons varolii. It contains the cerebral aqueduct or iter, which connects the third and fourth ventricles.
The corpora quadrigemina are four rounded elevations on the dorsal surface of the midbrain. The two superior colliculi are involved in visual reflexes, and the two inferior colliculi are relay centres for auditory reflexes — those that operate when it is necessary to move the head in order to hear sounds more distinctly.
The midbrain also contains, on its inferior surface, two thick fibrous tracts called the cerebral peduncles or crura cerebri. These tracts of ascending and descending nerve fibres come from the RAS and connect the cerebrum with the midbrain.
Near the centre of the midbrain is a mass of grey matter scattered within the white matter. It is called the red nucleus. It plays an important role in controlling posture and muscle tone, in modifying some motor activities, and in motor coordination.
c. Hindbrain
The posterior region of the brain is called the hindbrain. It consists of the pons varolii, the cerebellum and the medulla oblongata.
The pons varolii appears as a rounded bulge on the underside of the brain stem. (The brain stem consists of the midbrain, pons and medulla, and continues up to the spinal cord.) The pons contains a cross band of nerve fibres connecting the cerebrum, the cerebellar lobes, the medulla oblongata and the spinal cord. It also contains several nuclei. In effect, it is a bridge — every message travelling between the higher brain and the cord passes through it.
The cerebellum is the second largest part of the brain, and consists of two lateral hemispheres and a central vermis. It is composed of white matter with a thin layer of grey matter, the cortex. Here the white matter intermixes with the grey matter and shows a tree-like pattern called the arbor vitae.
The surface of the cerebellum shows convolutions (gyri and sulci). A number of nuclei lie deep within each lateral or cerebellar hemisphere. Over 30 million neurons lie in the cortex. Three pairs of myelinated nerve bundles, called the cerebellar peduncles, connect the cerebellum to the other parts of the CNS.
The cerebellum is an important centre which maintains the equilibrium of the body, posture, balancing orientation, moderation of voluntary movements, and maintenance of muscle tone. It is a regulatory centre for neuromuscular activities, and it controls rapid activities like walking, running and speaking. All activities of the cerebellum are involuntary — though they may involve learning in the early stages.
The medulla oblongata is the posterior conical part of the brain, and it continues as the spinal cord. It has inner grey matter and outer white matter. It controls involuntary vital functions like heart beat, respiration, vasomotor activities and peristalsis. It also controls non-vital reflex activities like coughing, sneezing, swallowing, vomiting and yawning.
The cavity of the medulla is called the IVth ventricle or metacoel. Its roof has the posterior choroid plexus for the secretion of CSF. The posterior choroid plexus also shows 3 openings — a pair of lateral foramina of Luschka and a median foramen of Magendie.
B. Spinal Cord
The spinal cord is the part of the central nervous system that forms the lower extension of the medulla oblongata of the brain. Like the brain, it is covered and protected by a bony covering and by membranes. It lies within the neural canal of the vertebral column and is surrounded by the three meninges — dura mater, arachnoid mater and pia mater. The cerebrospinal fluid secreted by the pia mater forms a fluid cushion around the spinal cord, and also within it, inside the central canal.
Externally, the spinal cord appears as a long cylindrical rod, 42 to 45 cm long and 2.0 to 2.5 cm broad. It is broadest at its anterior end, gradually tapers into the conus medullaris (at the level of L1 to L2), and continues as a thread-like filum terminale, ending posteriorly. The spinal cord shows two swellings along its length, called the cervical and lumbar swellings.
31 pairs of spinal nerves arise from the lateral sides of the spinal cord. These nerves are concentrated in the region of the cervical and lumbar swellings and around the conus medullaris — the swellings exist precisely because the limbs need extra nerve supply. The bunch of nerves in the hind part of the spinal cord, along with the filum terminale, appears like a horse’s tail, and is therefore called the cauda equina.
T. S. of the spinal cord
The spinal cord is dorsoventrally flattened, due to the presence of a deep, narrow posterior fissure and a shallow, broad anterior fissure. A central canal can be seen in the centre. The fissures divide the spinal cord incompletely into a right and a left side.
The grey matter is somewhat H-shaped or butterfly-shaped and is on the inner side, while the white matter is on the outer side — the reverse of the arrangement in the cerebrum. Look carefully at Fig. 9.11 and confirm this for yourself.
Diagram
Fig. 9.11 – T. S. of the spinal cord; label posterior median fissure, posterior median septum, anterior median fissure, central canal, ependyma, butterfly-shaped grey matter with dorsal, lateral and ventral horns, surrounding white matter, dorsal root with dorsal root ganglion, and ventral root.
The fissures divide the grey matter into six horns — namely the dorsal, lateral and ventral horns (one pair each) — while the white matter is divisible into 6 columns or funiculi, namely the dorsal, lateral and ventral funiculi.
The dorsal and ventral horns extend out of the spinal cord as the dorsal root and the ventral root of the spinal cord respectively. Of these, the dorsal root is connected to the dorsal root ganglion, which lies just outside and lateral to the spinal cord. It has an aggregation or collection of unipolar sensory neurons.
The association or inter-neurons lie inside the grey matter. They receive the signal from the sensory nerve, integrate it, and direct the response towards the motor neurons lying towards the ventral horn. The lateral horns have neurons of the autonomic nervous system (ANS). The nerves arising from these neurons emerge out from the ventral root of the spinal nerve.
The white matter consists mainly of bundles of myelinated nerve fibre, called the ascending and descending tracts. The ascending tracts conduct sensory impulses from the spinal cord to the brain, and these lie in the dorsal column or funiculi. The descending tracts conduct motor impulses from the brain to the lateral and ventral funiculi of the spinal cord.
Functions — The spinal cord is the main centre for most reflex actions. It provides a pathway for the conduction of sensory and motor impulses to and from the brain. It provides nervous connection to many parts of the body.
2. Peripheral Nervous System (PNS)
The peripheral nervous system connects the central nervous system to the different parts of the body having receptors and effectors. Depending on their connection to the CNS, the peripheral nerves are classified into two main types: - Cranial nerves — connected to the brain. - Spinal nerves — connected to the spinal cord.
Cranial Nerves
These nerves develop from the brain in all amniotes (reptiles, birds and mammals). There are 12 pairs of cranial nerves. Roman numbering, I to XII, is used to denote them. These nerves originate from, or terminate into, the brain.
According to their function, these are classified as sensory (I, II, VIII), motor (III, IV, VI, XI, XII) and mixed (V, VII, IX, X) nerves. The details of the cranial nerves present in the human body are presented in Table 9.12.
Table 9.12 : Cranial nerves — nature and functions
| No. | Name | Type | Origin | Organs innervated | Functions |
|---|---|---|---|---|---|
| I | Olfactory | Sensory | Olfactory bulb | Epithelium of nose | Smell |
| II | Optic | Sensory | Side of diencephalon | Retina of eye | Vision |
| III | Occulomotor | Motor | Floor of midbrain | Eye muscles (4 of the 6 eye muscles) | Movement of eyeball |
| IV | Pathetic | Motor | Floor of midbrain | Eye muscles (1 of the 6 eye muscles; forehead scalp) | Rotation and movement of eyeball |
| V | Trigeminal (Dentist’s nerve) — a. Ophthalmic (sensory), b. Maxillary (sensory), c. Mandibular (mixed, largest) | Mixed | Ventral side of pons | Nasal cavity, upper eyelids, forehead, scalp, conjunctiva, lacrimal gland; mucosa of nose, palate, upper teeth, upper lip, lower eyelid, parts of pharynx; lower teeth, skin over mandible and cheek, side of head in front of ear, muscles of mastication | Sensation of skin touch, taste, jaw movement |
| VI | Abducens | Motor | Pons | Muscles of eyeball, lateral rectus muscle | Movement of eye |
| VII | Facial (bearing geniculate ganglion) | Mixed | Pons | Facial, scalp and neck muscles; lacrimal, sublingual, submandibular, nasal and palatine glands | Facial expression, movement of neck, secretion of tears, taste, salivary secretion |
| VIII | Auditory (vestibulocochlear) — i. Vestibular, ii. Cochlear | Sensory | Pons | Internal ear | Hearing and equilibrium |
| IX | Glossopharyngeal | Mixed | Side of medulla oblongata | Pharynx, tongue, salivary glands | Taste, salivation and swallowing |
| X | Vagus (Pneumogastric) | Mixed | Side of medulla oblongata | Larynx, trachea, pharynx, alimentary canal, heart, lungs, pancreas, blood vessels | Visceral sensations and visceral movements like breathing, cardiac slowing, gastric and pancreatic secretion, gastrointestinal movements |
| XI | Spinal accessory | Motor | Side of medulla oblongata | Neck and shoulder muscles; reflexes of thoracic and abdominal viscera, larynx, pharynx | Movements of larynx, pharynx, neck and shoulder |
| XII | Hypoglossal | Motor | Side of medulla oblongata | Tongue muscles | Movement of tongue |
Spinal Nerves
Thirty-one pairs of spinal nerves originate from the spinal cord. They are mixed nerves, and they provide two-way communication between the spinal cord and parts of the upper and lower limbs, the neck and the trunk.
Table 9.13 : Number and types of spinal nerves
| Group | No. of pairs | Region of origin from vertebral column |
|---|---|---|
| Cervical | 8 (C1–C8) | Neck |
| Thoracic | 12 (T1–T12) | Thorax |
| Lumbar | 5 (L1–L5) | Abdomen |
| Sacral | 5 (S1–S5) | Pelvis |
| Coccygeal | 1 (Co1) | Coccyx |
Formation of a typical spinal nerve
All spinal nerves are of the mixed type, i.e. they have some nerve fibres that are sensory and some that are motor. Each spinal nerve is formed inside the neural canal of the vertebral column by two roots — the posterior or dorsal sensory root, and the anterior or ventral root.
The sensory nerve comes in from the dorsal root ganglion, where the cell bodies of the sensory neurons are located, while the anterior or ventral root gives out the motor nerve. The dorsal sensory and the ventral motor nerves then join together to form the mixed spinal nerve. It emerges out from both sides of the spinal cord through the intervertebral foramen, as shown in Fig. 9.14.
Diagram
Fig. 9.14 – Formation of a spinal nerve; label grey matter, white matter, dorsal root, dorsal root ganglion, ventral root, the mixed spinal nerve, and its three branches — ramus dorsalis, ramus ventralis and ramus communicans — with the sympathetic ganglion.
As soon as it emerges out of the vertebral column, the spinal nerve shows three branches: - Ramus dorsalis — goes from the skin, and to the muscles, of the dorsal side. - Ramus ventralis — the largest of the three; it supplies the organs and muscles on the lateral and anterior side. - Ramus communicans — the smallest of the three, given out from the 1st thoracic up to the 3rd lumbar (L3) spinal nerve. It joins the sympathetic ganglia.
Flowchart
FC 9.8 – Formation of a spinal nerve: dorsal root (sensory fibres, cell bodies in dorsal root ganglion) + ventral root (motor fibres from ventral horn) → mixed spinal nerve → emerges through intervertebral foramen → branches into ramus dorsalis, ramus ventralis and ramus communicans (ramus communicans → sympathetic ganglion).
Reflex Action
A reflex action is a sudden, spontaneous, automatic, involuntary response to a stimulus. The response to the stimulus is said to be involuntary, as it is carried out without any conscious effort by the brain. The path along which the action is carried out is called the reflex arc. (In your earlier classes you have studied reflex actions.)
Flowchart
FC 9.9 – Reflex arc: stimulus → receptor → sensory (afferent) neuron → dorsal root → spinal cord (inter-neuron in grey matter) → motor (efferent) neuron → ventral root → effector (muscle/gland) → response.
Types of reflex actions
1. On the basis of control over the actions - Cranial reflexes — carried out by the brain; slow action response; e.g. watering of the mouth on the sight or smell of good food. - Spinal reflexes — carried out through the spinal cord; urgency for a response is required, so these are quick-acting; e.g. withdrawal of the leg while stepping on something hot or pointed.
2. Based on previous experience - Unconditional reflexes — these do not require any previous experience; e.g. sneezing, coughing, yawning, hiccuping. - Conditional reflexes — these actions are based on previous experience; e.g. swimming, dancing, cycling. Initially these actions are voluntary, while learning is being done; later, after perfection, they become involuntary. They were first studied by E. Pavlov, on salivation in a dog at the sight and sound of a bell.
3. According to the number of synapses involved - Simple monosynaptic reflex — it involves only sensory and motor neurons; e.g. the knee-jerk reflex. - Complex polysynaptic reflex — it involves sensory, internuncial (inter-) and motor neurons; e.g. cycling, swimming.
Flowchart
FC 9.10 – Chart 9.15, Types of reflex actions: Reflex actions → (1) by control: cranial reflexes (brain, slow, e.g. watering of mouth) / spinal reflexes (spinal cord, quick, e.g. leg withdrawal); (2) by previous experience: unconditional (no experience needed, e.g. sneezing, coughing, yawning, hiccuping) / conditional (based on experience, e.g. swimming, dancing, cycling; studied by E. Pavlov); (3) by number of synapses: simple monosynaptic (sensory + motor only, e.g. knee jerk) / complex polysynaptic (sensory + internuncial + motor, e.g. cycling, swimming).
Afferent and Efferent Nerves
The human nervous system is divided into CNS, PNS and ANS. The PNS consists of the network of nerves arising from, or going to, the CNS. Accordingly, the peripheral nerves are classified as: - a. Afferent nerves — the afferent nerve fibres transmit sensory impulses from a tissue or organ to the CNS. - b. Efferent nerves — the efferent nerve fibres transmit regulatory or motor impulses from the CNS to the various peripheral tissues and organs.
However, according to recent studies, the extent of the PNS has been broadened to incorporate the ANS. According to this view, the PNS is divided into: i. the somatic nervous system, and ii. the autonomic nervous system.
The somatic nervous system relays impulses from the CNS to the skeletal or voluntary muscles of the body.
3. Autonomic Nervous System (ANS)
The autonomic nervous system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
The ANS consists of a special set of peripheral nerves that regulate the activities of involuntary organs like cardiac muscles, smooth muscles, glands, etc. In this system, impulses are conducted from the central nervous system by an axon that synapses with an autonomous ganglion. This first neuron is the preganglionic neuron. The second neuron in this ganglionic pathway has an axon that extends from the autonomic ganglion to an effector organ, and is known as the postganglionic neuron.
The autonomic nervous system consists of the sympathetic and the parasympathetic nervous systems.
Diagram
Fig. 9.16 – Autonomic nervous system; show the parasympathetic outflow (cranial nerves III, VII, IX, X from the medulla oblongata, and sacral outflow) on one side and the sympathetic outflow with its chain of sympathetic ganglia and solar plexus on the other, with arrows to the eye, salivary glands, heart, bronchi, stomach, liver, pancreas, adrenal gland, intestine and bladder, and the opposite effect of each system labelled at every organ.
a. Sympathetic Nervous System (SNS)
It is also called the thoraco-lumbar outflow. It originates in the thoracic and lumbar region of the spinal cord (T1 to L3) and consists of 22 pairs of sympathetic ganglia, which lie on a pair of sympathetic cords on the lateral sides of the spinal cord.
The pre-ganglionic nerve fibres are short and the post-ganglionic nerve fibres are long. Adrenaline and noradrenaline are produced at the terminal ends of the postganglionic nerve fibres at the effector organ, and hence these are also called adrenergic fibres.
The sympathetic nervous system controls body activities during emergencies — the fight or flight response. It has an excitatory and stimulating effect on most organs of the body, except on the digestive and the excretory organs, which it slows down instead.
b. Parasympathetic Nervous System
It is also called the cranio-sacral outflow. It consists of branches from the cranial (III, VII, IX, X) nerves, the sacral (II, III) and the spinal (IV) nerves. It consists of ganglia which are very close to, or within the wall of, the effector organs.
Here the pre-ganglionic nerves are long and the post-ganglionic nerves are short — exactly the reverse of the sympathetic arrangement, because the ganglia sit right at the target organ. Acetylcholine is produced at the terminal end of the post-ganglionic nerve at the effector organ, and hence these are also called cholinergic fibres.
The parasympathetic nervous system is antagonistic to the sympathetic nervous system. It brings back to normal all the activities which are stimulated by the sympathetic system. Hence it is also called the housekeeping system. It has an inhibitory effect on most organs. However, the activities associated with digestion, peristalsis and micturition, which are inhibited by the sympathetic system, are accelerated by the parasympathetic system.
Table 9.17 : Comparison between the sympathetic and parasympathetic nervous system
9.7 Sensory Receptors
How are we able to recognise the smell of the first showers of rain, or a sudden change in the weather? How do we hear the chirping of a bird and know which bird it is? How do we see and enjoy the colours of nature after sunrise? The answer to all these questions is the presence of a sensory system in our body. It consists of simple to complex structures called sensory receptors.
Sensory receptors are specialized structures in the body that receive the various stimuli coming from the external or internal environment. The nature of a receptor is defined by the type of stimulus it can respond to — for example, a photoreceptor responds to light, and to light only.
But what does a receptor actually do? When a specific type of stimulus is received by the sensory neuron (the receptor), it generates an action potential in it, and this action potential is then conducted in the form of an impulse. These impulses are conducted to the different functional areas of the brain for processing and interpretation.
This is a point worth pausing on. The nerve fibre from the eye and the nerve fibre from the ear both carry exactly the same kind of electrical impulse. What makes one “light” and the other “sound” is not the impulse — it is which receptor generated it and which area of the brain receives it.
Classification of Receptors
Receptors are classified on the basis of their location, their function, and their sensitivity to specific stimuli. Their classification is given in Chart 9.18.
Chart 9.18 : Types of receptors
| No. | Name/Type of receptor | Location | Function |
|---|---|---|---|
| I | Exteroceptors — receive external stimuli | ||
| a | Phonoreceptors | Internal ear — organ of Corti | Sound reception |
| b | Statoreceptors | Internal ear — semicircular canals | Receptors for maintaining balance and equilibrium |
| c | Photoreceptors | Retina of eye | Receive sensory stimuli for vision |
| d | Thermoreceptors | Skin | Receive sensory stimuli for heat (caloriceptors) and cold (frigidoceptors) |
| e | Mechanoreceptors | Skin | Sensitive to mechanical stimuli like touch, pain, pressure, deep pressure, etc. |
| f | Chemoreceptors — gustatoreceptors | Taste buds of tongue | Sensitive to the tastes of sweet, salt, sour, bitter and umami |
| f | Chemoreceptors — olfactory receptors | Olfactory epithelium of nose | Sensitive to about 10,000 different smells |
| II | Interoceptors — receive stimuli coming from within the body | ||
| a | Enteroceptors | Internal body organs | Sensitive to stimuli coming from internal organs, such as hunger, thirst, pain and osmotic change |
| b | Proprioceptors | Joints, muscles and tendons | Detect changes in the movements of joints, tendons and muscles; pain, tension; sensitive to vibrations |
| c | Baroreceptors (also considered mechanoreceptors, receiving signals from internal organs) | Walls of atria, venae cavae, aortic arch, carotid sinus | Sense changes in B.P. so as to restore homeostasis through vasodilation or vasoconstriction |
Flowchart
FC 9.11 – Classification of receptors: Receptors → Exteroceptors (external stimuli) → phonoreceptors, statoreceptors, photoreceptors, thermoreceptors (caloriceptors, frigidoceptors), mechanoreceptors, chemoreceptors (gustatoreceptors, olfactory receptors); Receptors → Interoceptors (internal stimuli) → enteroceptors, proprioceptors, baroreceptors.
Eye
The eyes are a pair of sensory organs of vision. They are located in the orbit of the skull, with a cushion of fat around them. Each eye is spherical or rounded, and is called the eyeball.
The eyes are protected within bony sockets, and also by the eyebrows, the upper and lower eyelids, the eyelashes and the lacrimal (tear) glands. Movement of the eyeball within the orbit is controlled by 6 sets of muscles.
The wall of the eyeball is made up of 3 layers: (1) sclera, (2) choroid and (3) retina — from outside inwards, as seen in Fig. 9.19.
Diagram
Fig. 9.19 – Sectional view of the human eye; label sclera, cornea, choroid, ciliary body, iris, pupil, lens, suspensory ligament, aqueous chamber with aqueous humor, vitreous chamber with vitreous humor, retina, fovea, blind spot and optic nerve.
1. Sclera / sclerotic
It is the outermost layer, made of dense fibroelastic connective tissue with collagen fibres. It provides attachment to the eyeball muscles — the tough coat is what the muscles can pull against.
The anterior thick, transparent part of the sclera is the cornea. It is slightly bulged out, for focussing light on the retina. The sclera is provided with blood vessels; however, the cornea is devoid of them. The cornea is nourished instead by the aqueous humour and also by the lacrimal secretion.
The exposed part of the sclera and the entire cornea are covered by a transparent membranous covering called the conjunctiva. It provides protection and lubrication to the cornea.
2. Choroid / Uvea
It is the middle, vascular and pigmented layer. It is not a complete layer, and it can be divided into 3 regions — a. the choroid proper, b. the ciliary body, and c. the iris.
a. The choroid proper — It lines the sclera. Due to its pigmented nature it prevents internal reflection, so that light does not bounce about inside the eyeball and blur the image. The blood vessels of the choroid provide nutrition and oxygen to the retina.
b. Ciliary body — It is a thick, muscular, ring-like structure at the junction of the choroid and the iris. Its epithelium secretes aqueous humor. Attached to the ciliary body are the suspensory ligaments, which hold the lens. The ligaments and the muscles of the ciliary body help in the adjustment of the size of the lens.
c. Iris — At the junction of the sclera and the cornea, the vascular part of the choroid sharply bends into the cavity of the eyeball, forming a thin and coloured partition called the iris. It is perforated in the middle by an aperture called the pupil. Smooth muscles of the iris help in regulating the size of the pupil, depending on the intensity of light entering the eyeball. The pigment in the iris determines the colour of the eye.
Lens
It is a transparent, elastic and biconvex structure. It is suspended in the eyeball by the suspensory ligaments.
The lens and the suspensory ligaments divide the cavity of the eyeball into a small anterior aqueous chamber, filled with a clear watery fluid, the aqueous humor, and a posterior large vitreous chamber, filled with a jelly-like vitreous humor. The vitreous humor maintains the shape of the eyeball and maintains the pressure that keeps the lens in position.
3. Retina
It is the innermost, delicate, non-vascular, light-sensitive layer. It has 2 regions: (a) a single layer of pigmented, non-sensory part lining the iris and the ciliary body, and (b) a sensory part lining the choroid.
It has an outer pigmented part and an inner nervous part. The inner nervous part is transparent and is made of 3 layers: 1. the outer photosensitive layer, made of rod and cone cells; 2. the middle layer of bipolar nerve cells; 3. the inner layer of ganglion cells.
The nerve fibres from the basal end of the ganglion cells collectively form the optic nerve.
Diagram
Fig. 9.20 – Structure of the retina; show light arriving from above and label nerve fibers, ganglion cell, amacrine cell, bipolar cell, horizontal cell, cone, rod, pigment epithelium, choroid and sclera, with an arrow showing the path of light down to the rods and cones and the path of the impulse back up to the optic nerve.
Now look carefully at Fig. 9.20 and notice something strange. The rod and cone cells lie deep in the retina, so that light has to pass through the ganglion cells and the bipolar cells before reaching them. The photoreceptors face away from the light — an inverted arrangement that the vertebrate eye has retained.
The blind spot is an area diagonally opposite to the lens. It is the area of the retina from where the optic nerve and blood vessels leave the eyeball. There are no rod and cone cells in this region — and therefore no vision at that point.
An area lateral to and above the blind spot is called the yellow area or macula lutea. At its centre is a depression called the fovea centralis. It has the maximum density of cone cells, and it is the place of formation of the sharpest vision.
Photoreceptor cells
These are of two types: (a) rod cells and (b) cone cells. They contain light-sensitive proteins termed photopigments.
The cones are responsible for daylight (photopic) vision and for colour vision, while the rods function in dim light (scotopic) vision. The purple-red protein called rhodopsin is present in the rods, and it is a vitamin A derivative.
The cones are of three types, each having its own characteristic photopigment, which respond to red, green and blue lights. Various combinations of these cones and their photopigments produce the sensation of different colours. The sensation of white light is produced due to the simultaneous, equal stimulation of these three types of cones.
The optic nerve consists of the fibres arising from the base of the ganglion cells. It leaves the eyeball from the posterior side and carries visual impulses from the retina to the brain.
Generation of image
The light rays from the object pass through the conjunctiva and the cornea, then through the pupil, on to the lens, and are focused on the retina to form an image. In the visual area of the cerebrum, the nerve impulses are analysed and the image formed is recognized.
Flowchart
FC 9.12 – Changes in the retina when light rays fall on it: light falls on rod and cone cells in the retina → breaking up of light-sensitive pigments by the specific wavelength of light → stimulation of rod and cone cells and generation of nerve impulse → nerve impulse transmitted to bipolar nerve cells → nerve impulse transmitted to ganglion cells → nerve impulses in the axons of ganglion cells converge and leave via the optic nerve → nerve impulse transmitted by the optic nerve to the brain → perception of the image by brain cells in the visual area of the cerebrum.
Ear
The human ear is called the stato-acoustic organ, and it has two functions — hearing and body equilibrium. Anatomically the ear is made up of three parts: the external ear, the middle ear and the inner ear.
Diagram
Fig. 9.21 – Structure of the human ear; label auricle (pinna), earlobe, auditory canal, eardrum (tympanic membrane), tympanic cavity, ossicles (malleus, incus, stapes), temporal bone, semicircular ducts, vestibular nerve, cochlear nerve, cochlea and auditory (Eustachian) tube.
External ear
The external ear consists of the ear pinna, the auditory canal and the tympanic membrane.
In humans, the ear pinna is an immovable part, supported by an elastic cartilage structure. It leads into an auditory canal. The pinna collects the sound waves and sends them into the auditory canal.
The auditory canal ends at the ear drum. It transfers the sound waves to the ear drum. There are very fine hairs and wax-secreting sebaceous glands in the skin of the pinna and the auditory canal — these trap dust and insects before they can reach the delicate membrane.
The tympanic membrane is a delicate, membranous structure which transmits the sound waves to the middle ear. It is formed of connective tissue, covered with skin on the outside and a mucous membrane on the inside.
Middle ear
The middle ear consists of a chain of three ear ossicles, called the malleus (hammer), the incus (anvil) and the stapes (stirrup — the smallest bone in the body).
On receiving the vibrations from the tympanic membrane, the ear ossicles amplify the vibrations and transfer these to the cochlea. This amplification matters: sound has to pass from air into the fluid of the inner ear, and fluid is far harder to move than air. Without the lever action of the ossicles, most of the sound energy would simply bounce back.
A short Eustachian tube connects the middle ear to the pharynx. It equalises the air pressure on both sides of the ear drum.
Internal ear
The internal ear consists of the labyrinth. The labyrinth consists of a bony labyrinth and a membranous labyrinth. These are filled with perilymph and endolymph respectively. The membranous labyrinth consists of three parts: the vestibule, the semicircular ducts and the cochlea.
The cochlea is a coiled duct and contains three fluid-filled chambers, separated by Reissner’s membrane and the basilar membrane. The upper chamber, towards the vestibule, is called the scala vestibuli, and the bottom chamber is the scala tympani; both are filled with perilymph. The middle chamber is the scala media, and it is filled with endolymph.
The organ of Corti is a pea-sized structure located on the basilar membrane, which forms the floor of the scala media. This is the actual hearing receptor — the phonoreceptor of Chart 9.18.
The organ of Corti has a sensory epithelium over the basilar membrane. This sensory epithelium is in contact with a gelatinous tectorial membrane. The sensory cells have sensory hairs on their free ends, and so are also called hair cells. In between the rows of hair cells are present supporting cells.
Hair cells have long, stiff microvilli called stereocilia on their apical surfaces. Above these stereocilia is the jelly-like tectorial membrane. This organ acts as a transducer, converting sound vibrations into nerve impulses.
Diagram
Fig. 9.22 – Human internal ear; label bony labyrinth, membranous labyrinth, anterior, posterior and lateral semicircular ducts, ampulla of semicircular canal, endolymphatic sac, utricle, saccule, vestibule, cochlea, cochlear duct, oval window and round window; alongside, a section through the cochlea labelling vestibular canal (scala vestibuli), cochlear duct (scala media), tympanic canal (scala tympani), basilar membrane, organ of Corti and auditory nerve.
Internal Ear and the Mechanism of Balance
Besides the cochlea, the inner ear also has the vestibular apparatus, which is composed of three semi-circular ducts or canals and the utriculo-saccular region with the otolith organ.
All three semi-circular canals lie in different planes, at right angles to each other — which is precisely why a movement of the head in any plane can be detected by at least one of them. These canals are filled with endolymph.
The base of each canal has an ampulla, in which there is a sensory spot called the crista. The cristae help in maintaining equilibrium.
The vestibule has two sensory spots, viz. the macula of the saccule and of the utricle. The macula consists of hair cells and supporting cells. The tips of the hairs and the cilium project into a thick gelatinous sheath called the otolithic membrane. Within this membrane are secreted minute particles, the otoliths or otoconia. These are made of \(CaCO_3\) and protein.
The macula and the crista are the receptors sensitive to the position of the head with respect to gravity. The three semicircular canals are arranged such that movement in any plane can be detected by these cells, and thus the balance and posture of the body is maintained.
Receptors for dynamic balance lie in the cristae of the ampullae, while those for static/linear balance are in the maculae of the utriculus and sacculus.
Mechanism of Hearing
The pinna of the ear receives the sound waves and directs them to the eardrum. The eardrum vibrates, and these vibrations are amplified and transmitted through the ear ossicles to the endolymph inside the cochlea.
This generates a wave in the endolymph. These waves induce ripples in the basilar membrane. These movements in the basilar membrane cause the hair cells to press against the tectorial membrane.
This generates a nerve impulse in the afferent neurons. The impulse is sent to the brain via the auditory nerve. The auditory cortex of the brain finally decodes the sound.
Flowchart
FC 9.13 – Mechanism of hearing: sound waves collected by pinna → auditory canal → tympanic membrane vibrates → ear ossicles (malleus → incus → stapes) amplify the vibrations → vibrations passed to endolymph of cochlea → waves in endolymph → ripples in basilar membrane → hair cells of organ of Corti press against tectorial membrane → nerve impulse generated in afferent neurons → auditory nerve → auditory cortex of brain → sound perceived.
9.8 Disorders of Nervous System
The nervous system is an organ system like any other, and it too can fall ill. When neurons die, when a particular neurotransmitter runs short, or when abnormal proteins pile up in the brain, the effects show up not as pain in one spot but as changes in movement, memory, mood or behaviour.
Psychological Disorders
Psychological disorders, commonly called mental disorders, are a wide range of conditions that affect the mood, thinking or behaviour of a person. They affect multiple areas of life and create distress for the person suffering from them.
Some of the major categories of psychological disorders are: - ID — Intellectual disability (formerly known as mental retardation) - ASD — Autism spectrum disorder - BD — Bipolar disorder - Depression - Anxiety disorder - ADHD — Attention Deficit Hyperactivity Disorder - Stress-related disorders
These are medical conditions of the nervous system, and like other medical conditions they can be diagnosed and treated by trained professionals. That is an important point of understanding: a disorder of the brain deserves exactly the same attention, and the same absence of blame, as a disorder of the heart or the kidney.
Parkinson’s Disease
Degeneration of the dopamine-producing neurons in the CNS causes Parkinson’s disease. Dopamine is a neurotransmitter used by the pathways that connect the basal ganglia with the cortex, and you have already seen (Section 9.6) that the basal ganglia help the cortex carry out movements smoothly at the subconscious level. Take away the dopamine, and those pathways can no longer smooth out movement.
The symptoms develop gradually over the years. They are tremors (a rhythmic shaking, usually beginning in the hands), stiffness, and difficulty in walking, balance and co-ordination.
Alzheimer’s Disease
Alzheimer’s disease is the most common form of dementia. Its incidence increases with age.
It shows the loss of cognitive functioning — that is, of thinking, remembering, reasoning and behavioural abilities — to such an extent that it interferes with the person’s daily life and activities.
It occurs due to the loss of cholinergic and other neurons in the CNS, and the accumulation of amyloid proteins in the brain. Cholinergic neurons are those that use acetylcholine as their neurotransmitter — the same transmitter you met at the cholinergic fibres of the parasympathetic system.
There is no cure for Alzheimer’s, but treatment slows down the progression of the disease and may improve the quality of life.
9.9 Endocrine System
Chemical Coordination
Cells and organisms communicate with each other through chemical signals, and these signals are broadly of four types. What separates them is simply how far the message has to travel.
Autocrines — cells release a secretion to stimulate themselves. Paracrines — cells release a secretion to stimulate neighbouring cells. Endocrines — cells release a secretion to stimulate distant cells. Pheromones — organs release secretions to stimulate another organism.
Flowchart
FC 9.14 – Types of chemical signalling by distance travelled: signal released → acts on the same cell (autocrine) → acts on neighbouring cells (paracrine) → carried by blood to distant cells (endocrine) → released outside the body to affect another organism of the same species (pheromone).
Higher animals have a complex body organization. Because of this, in addition to nervous coordination, there is a need for chemical coordination. Chemical coordination is carried out by the secretions of ductless glands, and this chemical coordination system is also called the endocrine system.
Hormones — the Chemical Messengers
The endocrine system controls body activities by means of chemical messengers called hormones. Hormones are released directly into the blood. The hormone is then carried all over the body via the blood.
However, the message is relayed only to the target organs, which are stimulated to carry out a specific process — processes that include activities like growth and development.
Chemical Nature of Hormones
I. Amines — These are simple amines. Catecholamines secreted by the adrenal medulla, epinephrine and non-epinephrine, and melatonin from the pineal gland belong here. Some are modified from amino acids, e.g. thyroxine. II. Peptide hormones — These hormones consist of long or short chains of amino acids, e.g. hormones of the hypothalamus, oxytocin, ADH, GnRH. III. Protein hormones — Insulin, glucagon, TSH, FSH, LTH, GH, relaxin. IV. Fatty acid derivatives — Prostaglandin. V. Steroid hormones — These hormones are lipid soluble and derived from cholesterol and other steroids, e.g. estrogen, testosterone, aldosterone. The action of these hormones is concerned with long-lasting responses. VI. Gas — NO (Nitric Oxide).
Properties of Hormones
Hormones act as chemical messengers and are effective in very low concentration — a few molecules per litre of blood can change the working of a whole organ.
Hormones can function as regulators that inhibit, stimulate or modify specific processes. Some hormones interact with receptors present on the plasma membrane of target cells, whereas some enter the nucleus to interact with genes.
Hypersecretion (too much) or hyposecretion (too little) of hormones leads to various disorders — which is why you will meet a list of disorders beside almost every gland in the next section.
Hormones are metabolised after their function, and thus cannot be reused. This is essential: if a hormone were not destroyed, its message would never end. Hormone secretion is regulated by a positive or negative feedback mechanism.
Mechanism of Hormone Action
Hormones are released in a very small quantity. They produce their effect on the target organs or cells by binding to hormone receptors. The hormone receptors may be on the cell membrane, or may be intracellular.
Once binding occurs, a hormone–receptor complex is formed, and this leads to biochemical changes in the target tissue. There are two entirely different routes by which this happens, and which route a hormone takes depends on one thing only — whether it can cross the lipid plasma membrane.
A. Mode of hormone action through membrane receptors
Hormones like catecholamines, peptide and polypeptide hormones are not lipid soluble. Therefore they cannot enter their target cells through the plasma membrane. They must deliver their message from outside.
These non-steroid, water-soluble hormones interact with a surface receptor, which initiates metabolic activity. Molecules of amino acid derivatives and peptide hormones bind to specific receptor molecules located on the plasma membrane, as shown in Fig. 9.23.
Diagram
Fig. 9.23 – Mechanism of hormone action through a membrane receptor; label hormone (e.g. FSH), receptor on the ovarian cell membrane, hormone–receptor complex, generation of the secondary messenger (cyclic AMP or Ca²⁺), biochemical responses, and physiological responses (e.g. ovarian growth).
The hormone–receptor complex causes the release of the enzyme adenylate cyclase from the receptor site. This enzyme forms cyclic AMP (cAMP) from the ATP of the cell. cAMP then activates enzymatic actions inside the cell.
Notice what has happened: the hormone itself never entered the cell. The hormone acts as the first messenger, and cAMP is the second messenger that carries the instruction onwards inside the cell. Other kinds of second messengers are \(Ca^{2+}\), cGMP and \(IP_3\) (inositol triphosphate).
Flowchart
FC 9.15 – Action through a membrane receptor: water-soluble hormone (first messenger) → binds receptor on plasma membrane → hormone–receptor complex → adenylate cyclase released → ATP converted to cyclic AMP (second messenger) → enzymes activated → biochemical response → physiological response.
B. Mode of action through intracellular receptors
Steroid and thyroid hormones are lipid soluble, and so they easily pass through the plasma membrane of the target cell into the cytoplasm. They need no second messenger, because they can carry the message in themselves.
In the cytoplasm, they bind to specific intracellular receptor proteins, forming a hormone–receptor complex that enters the nucleus, as shown in Fig. 9.24.
Diagram
Fig. 9.24 – Mechanism of hormone action through an intracellular receptor; label hormone (e.g. oestrogen) crossing the uterine cell membrane, receptor–hormone complex, nucleus, genome, mRNA, proteins, and physiological responses (tissue growth and differentiation).
In the nucleus, the hormone–receptor complex binds to a specific regulatory site of DNA. The activated genes transcribe mRNA, which directs protein synthesis and the formation of enzymes in the cytoplasm.
Because this route requires transcription and translation — building new proteins from scratch — the action of lipid-soluble hormones is slower but long lasting. The action of a water-soluble hormone, working through a ready-made enzyme cascade, is fast but short-lived.
Flowchart
FC 9.16 – Action through an intracellular receptor: lipid-soluble hormone (steroid/thyroid) → crosses plasma membrane → binds intracellular receptor protein in cytoplasm → hormone–receptor complex enters nucleus → binds regulatory site on DNA → gene activated → mRNA transcribed → protein/enzyme synthesis → physiological response (slow, long lasting).
9.10 Major Endocrine Glands
Having seen how a hormone works, we now go gland by gland through the endocrine system. As you read, keep asking two questions of every hormone: what does it do, and what happens if there is too much or too little of it? That second question is where most board and NEET questions come from.
Diagram
Fig. 9.33 – The human endocrine system; show the position of the hypothalamus, pituitary, pineal, thyroid, parathyroid, thymus, adrenal glands, pancreas, and the gonads (ovaries in the female, testes in the male), labelled in both a male and a female figure.
A. Hypothalamus
The hypothalamus is ectodermal in origin. It is located at the floor of the diencephalon, as you saw in Section 9.6. Its major function is to maintain homeostasis.
It controls the secretory activity of the pituitary gland by means of releasor (RH) and inhibitor (IH) hormones — one set to switch the pituitary on, the other to switch it off. All the hormones of the hypothalamus are peptide hormones. They are secreted by neurosecretory cells, so they are called neurohormones. The hormones secreted by the hypothalamus are ADH and oxytocin.
The hypothalamic hormones acting on the pituitary are: 1. Adrenocorticotropin Releasing Hormone — It stimulates the release of ACTH by the anterior pituitary gland. 2. Thyrotropin Releasing Hormone — It stimulates the release of TSH by the anterior pituitary gland. 3. Gonadotropin Releasing Hormone (GnRH) — It stimulates the pituitary to secrete gonadotropins. 4. Prolactin Inhibiting Hormone (Prolactostatin) — It inhibits prolactin release by the anterior pituitary gland. 5. Somatostatin — It inhibits the release of growth hormone. 6. Somatotropin — It stimulates the release of growth hormone. 7. Gastrin Releasing Peptide (GRP), and 8. Gastric Inhibitory Polypeptide (GIP).
B. Pituitary Gland (Hypophysis)
The pituitary gland is the smallest gland in the body. It is a pea-sized, reddish-grey coloured gland. It controls almost all the other endocrine glands, and hence it was earlier called the master endocrine gland.
It is located just below the hypothalamus and is attached to it by a stalk called the infundibulum or hypophyseal stalk. The pituitary gland remains lodged in a bony depression called the sella turcica of the sphenoid bone.
The pituitary gland consists of two lobes, called the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). Both the lobes develop from different parts of the embryo, and hence the gland has a dual origin.
The adenohypophysis is an outgrowth from the roof of the buccal cavity. This outgrowth is called Rathke’s pouch, and it grows upward towards the brain. The neurohypophysis grows as a downward extension of the hypothalamus. The two outgrowths together form the pituitary gland. The connection of Rathke’s pouch with the pituitary gland is lost in the embryo. The intermediate lobe (pars intermedia) is a small, reduced part lying in the cleft between the anterior and posterior lobes.
The neurohypophysis is connected directly to the hypothalamus by axon fibres. The adenohypophysis and the intermediate lobe are connected to the hypothalamus through the hypophyseal portal system.
Diagram
Fig. 9.25 – The pituitary gland; label supraoptic nucleus (ADH), paraventricular nucleus (oxytocin), optic chiasma, median eminence, infundibulum, hypophyseal artery, hypophyseal portal vein, hypophyseal vein, pars distalis of adenohypophysis, pars intermedia and pars nervosa of the neurohypophysis.
Adenohypophysis
It is the larger lobe of the pituitary gland. It is a highly cellular and vascular part. It contains various types of epitheloid secretory cells — acidophils, basophils and chromatophores. It is differentiated into three parts: pars distalis, pars intermedia and pars tuberalis.
The hormones of the adenohypophysis are as follows.
1. Somatotropin / Somatotropic Hormone / STH / Growth hormone / GH
This hormone stimulates the growth and development of all tissues, by accelerating protein synthesis and cell division. The highest secretion of GH is seen till puberty, and then its secretion becomes low. However, it is continuously secreted throughout life, for the repair and replacement of body tissues or cells.
Improper secretion of growth hormone produces various disorders. - Hyposecretion of growth hormone since childhood results in stunted physical growth, and the condition is called pituitary dwarfism. - Hypersecretion of growth hormone in childhood causes gigantism, a condition of overgrowth in which the individual attains abnormal height. - When the pituitary gland produces excess growth hormone in middle-aged adults, it results in disproportionate growth, causing disfigurement and enlargement of the bones of the nose, lower jaw, hands, fingers and feet. This condition is called acromegaly.
2. Thyrotropin / Thyroid Stimulating Hormone / TSH — Its primary action is to stimulate the thyroid gland for the secretion of the hormone thyroxine.
3. Adrenocorticotropic hormone / ACTH / Adrenocorticotropin — It stimulates the adrenal cortex to produce and secrete its hormones, and it maintains the functioning of the adrenal cortex.
4. Prolactin / Luteotropin / Mammotropin — Prolactin is unique among pituitary hormones, as it is under predominant inhibitory control from the hypothalamus (recall Prolactin Inhibiting Hormone). Prolactin activates the growth of the breasts during pregnancy (mammotropin), and stimulates milk production and the secretion of milk (lactogenic) by the mammary gland after childbirth.
5. Gonadotropins
a. Follicle stimulating hormone (FSH) — It stimulates the growth of ovarian follicles in females, while in males it is concerned with the development of the seminiferous tubules.
b. Luteinizing Hormone (LH) — In the female, the luteinizing hormone helps in the growth, ripening and discharge of the ovum (ovulation) from the Graafian follicle. FSH and LH together are responsible for the stimulation of the ovaries to produce oestrogen. LH also induces the ruptured follicle to develop into the corpus luteum, for the production of progesterone.
c. Interstitial cell stimulating hormone (ICSH) — In males, ICSH stimulates the testes to produce the androgen called testosterone. Testosterone is responsible for the development of secondary sexual characters.
Neurohypophysis
It is differentiated into three parts: 1. pars nervosa (neural lobe), 2. infundibulum, and 3. median eminence.
The pars nervosa acts as a storage area for the secretions of the hypothalamus. It stores and releases oxytocin and vasopressin — it does not manufacture them.
1. Oxytocin — It stimulates the contraction of the uterus during parturition (childbirth). It also stimulates the contraction of the mammary glands to initiate the ejection or release of milk. It is therefore called the birth hormone or the milk-ejecting hormone.
2. Antidiuretic Hormone (ADH) / Vasopressin — It stimulates the re-absorption of water in the distal convoluted tubule and collecting ducts of the uriniferous tubules of the kidneys. It decreases the loss of water by reducing the urine quantity. It also increases blood pressure by causing vasoconstriction.
Deficiency of ADH reduces water re-absorption and increases urine output. This condition is called diabetes insipidus. Note carefully that no glucose is lost in the urine here — this is not a disorder of sugar at all. The excessive micturition causes excessive thirst, and that condition is called polydipsia.
Pars intermedia — It is poorly developed in human beings. It secretes Melanocyte Stimulating Hormone (MSH) in some lower vertebrates like fish and amphibians. MSH stimulates the dispersion of melanin granules in melanocytes, and is responsible for skin pigmentation.
Diagram
Fig. 9.26 – Hypothalamus, adenohypophysis and neurohypophysis; label neurosecretory cells, hypothalamus, pituitary portal system, anterior pituitary, posterior pituitary, and arrows from the pituitary to its targets — ovaries, testes, bones, tissues, mammary glands, adrenal cortex, thyroid, kidney tubules and smooth muscle.
C. Pineal Gland
The pineal gland is given off from the roof of the diencephalon and is located between the two cerebral hemispheres. The pineal gland is sensitive to the biochemical signals of light.
It secretes a hormone called melatonin, also known as the sleep hormone. Melatonin is derived from tryptophan (an amino acid), and it plays a very important role in the regulation of the Biological Clock — the 24-hour diurnal rhythm of the body.
It helps in maintaining the normal rhythm of the sleep–wake cycle, and it also influences body temperature, metabolism and reproductive cycles.
Diagram
Fig. 9.27 – Pineal gland; show a sagittal section of the brain labelling corpus callosum, thalamus, hypothalamus, pituitary gland, brain stem and the pineal gland, with an inset naming melatonin as the sleep hormone.
D. Thyroid Gland
It is the largest endocrine gland, weighing about 25 gms. This bilobed gland is situated in front of the trachea, just below the larynx. It is richly supplied with blood vessels.
The two lobes of the thyroid gland are connected by a non-secretory band called the isthmus. The thyroid lobes are composed of rounded follicles, held together by interfollicular connective tissue called stroma. The stroma contains blood capillaries and small groups of parafollicular cells or ‘C’ cells (clear cells).
Thyroid follicles are composed of a single layer of cuboidal epithelium resting on a basement membrane, and their lumen is filled with a gelatinous, hormone-rich colloid.
Diagram
Fig. 9.28 – Thyroid gland; (A) whole gland showing larynx, right and left thyroid lobes, isthmus and trachea; (B) histology showing thyroid follicles, follicular cells, colloid, parafollicular (C) cells and blood capillaries.
Thyroid hormones
The thyroid gland is stimulated to secrete its hormones by thyroid stimulating hormone (TSH). The two hormones secreted by the follicular cells are Thyroxine / tetraiodothyronine / \(T_4\) (four atoms of iodine) and Triiodothyronine / \(T_3\) (three atoms of iodine).
Thyroxine is synthesized by attaching iodine to the amino acid tyrosine by enzymatic action. The amino acid tyrosine molecule binds to one atom of iodine to produce Monoiodothyronine (\(T_1\)), or to two atoms of iodine to produce Diiodothyronine (\(T_2\)). \(T_1\) and \(T_2\) molecules bind end to end to make the colloidal mass inside the follicle. They are further metabolised to prepare \(T_3\) and \(T_4\).
Triiodothyronine or \(T_3\) is secreted in small quantity. It is physiologically more active than \(T_4\).
The thyroid gland is the only gland that stores its hormones. \(T_3\) and \(T_4\) hormones are stored before secretion, and are regulated by the thyrotropin of the pituitary gland by a negative feedback mechanism.
Flowchart
FC 9.17 – Synthesis of thyroid hormones: tyrosine + 1 iodine atom → monoiodothyronine (\(T_1\)); tyrosine + 2 iodine atoms → diiodothyronine (\(T_2\)) → \(T_1\) and \(T_2\) bind end to end → colloid stored in the follicle → further metabolised → \(T_3\) (three iodine atoms, more active, secreted in small quantity) and \(T_4\) (four iodine atoms) → released into blood under TSH control → negative feedback on the pituitary.
Functions of thyroxine
Thyroxine regulates the basal metabolic rate (BMR) of the body. It also regulates metabolism by stimulating protein synthesis, and it promotes the growth of body tissues.
It helps in thermoregulation, by increasing heat production. It increases the activity of the neurotransmitters adrenaline and noradrenaline. It also supports the process of RBC production, and the maintenance of water and electrolyte balance. It also regulates reproductive cycles in females.
Calcitonin
Calcitonin is secreted by the parafollicular or ‘C’ cells of the thyroid gland. It regulates the concentration of calcium and phosphorus in the blood.
It is under the feedback control of plasma calcium concentration, and is secreted when the concentration of calcium rises in the blood. It lowers the concentration of calcium and phosphorus in the plasma by decreasing their release from the bones, and by accelerating the uptake of calcium and phosphorus by the bones.
E. Parathyroid Gland
The parathyroid gland is situated on the posterior surface of the lobes of the thyroid gland. Parathyroids are four in number, and are named as the superior and inferior parathyroid glands. The cells of the parathyroid glands are arranged in a compact mass.
Diagram
Fig. 9.31 – Parathyroid glands; label hyoid bone, epiglottis, larynx, thyroid gland, trachea, and the superior and inferior parathyroid glands on the posterior surface of the thyroid.
The parathyroids secrete a peptide hormone called parathormone (PTH). It is also called Collip’s hormone. It regulates the calcium and phosphate balance between the blood and other tissues.
Release of parathormone increases blood calcium level, by three routes at once: by taking calcium from the bones, by increasing calcium absorption in the digestive tract, and by reducing the loss of calcium in the urine. The secretion of parathormone is under the feedback control of the blood calcium level.
The concentration of calcium and phosphate is thus maintained by parathormone and calcitonin together. These two hormones form an antagonistic pair, like insulin and glucagon — one raises blood calcium, the other lowers it.
Disorders - Hyposecretion of parathormone lowers the concentration of calcium in the blood. This increases the excitability of nerves and muscles, causing muscle twitch and spasm. This is called parathyroid tetany or hypocalcaemic tetany. - Hypersecretion of parathormone is responsible for more absorption of calcium from the bones, i.e. demineralization of bones, resulting in softening, bending and fracture of bone. This is called osteoporosis. It is common in women who have reached menopause.
F. Thymus Gland
The thymus gland is located in the upper part of the thorax, on the dorsal side of the heart. It is a soft, pinkish and bilobed mass of lymphoid tissue.
It is a prominent gland at birth, but gets gradually atrophied (shrunken) in the adult, and so it is called a temporary gland.
It secretes the hormone thymosin. Thymosin has an important role in the development of the immune system, by the maturation of T-lymphocytes. It also promotes the production of antibodies, thereby providing humoral immunity.
G. Adrenal Gland / Suprarenal Gland
The adrenal glands have a dual origin, from mesoderm and ectoderm. They are located on the upper border of each kidney. The adrenal glands are small, conical, yellowish glands, and show two distinct regions — an outer cortex and an inner medulla.
Diagram
Fig. 9.32 – Histology of the adrenal gland; label capsule, adrenal cortex with its three zones (zona glomerulosa, zona fasciculata, zona reticularis) and the adrenal medulla.
1. Adrenal cortex
The adrenal cortex is derived from embryonic mesoderm, and it secretes many hormones collectively called corticoids. It is differentiated into three concentric regions.
a. Zona glomerulosa — It is the outer thin part of the adrenal cortex, and it secretes mineralocorticoids. They are released for regulating sodium and potassium ion concentration. They regulate salt–water balance, blood volume and blood pressure. Aldosterone (the salt-retaining hormone) is the main mineralocorticoid. It balances Na–K levels.
b. Zona fasciculata — It is the middle thick part, and is responsible for the secretion of glucocorticoids like cortisol. Glucocorticoids regulate the metabolism of carbohydrates, proteins and lipids.
Cortisol is an important glucocorticoid. It is responsible for an increase in the blood glucose level. It is also immunosuppressive — it suppresses the synthesis of antibodies, and so it is used in the treatment of allergy. It prepares animals to face emergencies in nature.
c. Zona reticularis — It is the inner thin region, responsible for the production of sex corticoids (gonadocorticoids). In males, they have a role in the development and maintenance of external sex characters.
An excess of sex corticoids in a female causes adrenal virilism and hirsutism (excess hair on the face), while in males it causes gynaecomastia, i.e. enlarged breasts. Androgens and estradiols are produced by the adrenal cortex.
2. Adrenal medulla
The adrenal medulla develops from ectoderm. It secretes two hormones — adrenaline (epinephrine) and noradrenaline (norepinephrine).
Adrenaline is also known as the emergency hormone, and is called the 3F hormone — fight, flight and fright. Noradrenaline regulates blood pressure under normal conditions, and also acts as a vasoconstrictor.
Now look back at Section 9.6. The sympathetic nervous system’s postganglionic fibres also release adrenaline and noradrenaline at the effector organs — they are adrenergic fibres. The adrenal medulla is, in effect, a piece of the sympathetic nervous system that has turned into a gland: the nerves deliver the emergency message in a fraction of a second, and the medulla then pours the same chemicals into the blood so that the emergency state is maintained for minutes afterwards.
H. Pancreas
The pancreas develops from endoderm. It is both an exocrine gland (studied in the digestive system) and an endocrine gland — the classic example of a mixed gland.
The endocrine cells of the pancreas form groups of cells called the Islets of Langerhans. There are four types of hormone-producing cells in each islet of Langerhans.
I. Alpha (α) cells (20%) — secrete glucagon. It stimulates the liver for glycogenolysis (the breakdown of stored glycogen), to increase the blood glucose level.
II. Beta (β) cells (70%) — secrete insulin. It stimulates the liver and muscles for glycogenesis (the formation of glycogen). This lowers the blood glucose level.
III. Delta (δ) cells (5%) — secrete somatostatin, which inhibits the secretion of glucagon and insulin. It also decreases gastric secretions, motility and absorption in the digestive tract.
IV. PP cells or F cells (5%) — secrete pancreatic polypeptide (PP). It inhibits the release of pancreatic juice.
I. Gonads
Gonads are the sex organs — the testes and the ovaries. Besides producing gametes, they also produce various hormones.
i. Ovaries
They produce the following hormones.
1. Estrogen — These are secreted by the developing follicle. Estradiol is the main oestrogen. It is responsible for the secondary sexual characters in the female.
2. Progesterone — It is secreted by the corpus luteum of the ovary after ovulation. This hormone is essential for the thickening of the uterine endometrium, thus preparing the uterus for the implantation of the fertilized ovum. It is responsible for the development of the mammary glands during pregnancy. It inhibits uterine contractions during pregnancy — which is why it is called the anti-abortion hormone.
3. Relaxin — It is secreted by the corpus luteum of the ovary at the end of the gestation period. It relaxes the cervix of the pregnant female and the ligaments of the pelvic girdle, for the easy birth of the young one.
4. Inhibin — It is secreted by the corpus luteum. Inhibin inhibits FSH and GnRH production.
ii. Testes
The testes secrete male sex hormones called androgens, such as testosterone.
Testosterone is secreted from the interstitial cells or Leydig cells, under the influence of luteinising hormone (LH). A rise in the testosterone level in the blood above normal inhibits LH secretion — a clear example of negative feedback.
It is also responsible for the appearance of secondary sexual characters, such as facial and pubic hair, deepening of the voice, broadening of the shoulders, male aggressiveness, etc. It also helps in the maintenance of the testes.
J. Diffuse Endocrine Glands
Not every hormone comes from an organ whose only job is to secrete. Several organs with entirely different main functions also contain endocrine cells.
Placenta
The placenta is the intimate connection between the foetus and the uterine wall of the mother, for the physiological exchange of material. The placenta is a temporary endocrine gland.
During pregnancy, the placenta secretes hormones such as estrogen, progesterone, hCG (Human Chorionic Gonadotropin) and human placental progesterone. These hormones check the contraction of the uterine muscles, and also maintain the thickness of the uterine endometrium — thus they help to maintain pregnancy.
Gastrointestinal tract
In the gastro-intestinal mucosa, certain cells are endocrine in function. These cells produce hormones which play a vital role in digestive processes and in the flow of digestive juices.
- Gastrin — It stimulates the gastric glands to produce gastric juice.
- Secretin — It is responsible for the secretion of pancreatic juice and bile, from the pancreas and the liver respectively.
- Cholecystokinin (CCK) and Pancreozymin (PZ) — These hormones stimulate the pancreas to release its enzymes, and also stimulate the gall bladder to release bile.
- Entero-gastrone / Gastric inhibitory peptide (GIP) — It slows gastric contractions and inhibits the secretion of gastric juice.
Kidney
The kidney produces renin, erythropoietin and calcitriol (calcitriol is the active form of the vitamin cholecalciferol, \(D_3\)).
Heart
The heart produces Atrial Natriuretic Hormone (ANH), which increases sodium excretion by the kidneys and reduces blood pressure.
Hormone therapy (HT)
Hormone therapy is the use of hormones in medical treatment. HT is applied in pregnancy, menopause, osteoporosis, growth hormone deficiency, insulin resistance, cancer, etc.
Pheromones are produced by many species of insects. Some pheromones enhance the chances of mating between the sexes; these are called signalling pheromones, and are used to induce a behavioural response. Social insects such as ants make use of signalling pheromones to locate food sources and to warn of danger.
Worker bees are females maintained in a sterile state by a pheromone called the anti-queen factor, produced by the queen. The factor spreads among the workers, preventing the maturation of the ovaries of the workers as long as the queen is present in the beehive. An increase in colony size results in a dilution of the pheromone, and a second queen may then develop.
9.11 Summary
A multicellular body needs both control over its life processes and coordination between its systems, in order to maintain homeostasis and to respond to changes in the external environment. Plants coordinate by chemical signals and movements (phototropic, chemotactic, thigmotactic). Animals use both electrical and chemical signals.
Sponges (Porifera) have no nervous system; they manage by division of labour among specialised cells. In Hydra and Planaria the nervous system alone coordinates the body; in higher animals the work is shared by the nervous (electrical) system and the endocrine (chemical) system.
Hydra (Cnidaria) shows the diffused nervous system — scattered sensory cells and neurons joined by synapses into a nerve net, with two nets in the mesoglea. There are no sense organs, no sensory or motor nerves, and the impulse has no polarity, so it spreads in any direction. This diffuse plan is also seen in Ctenophora and in the enteric (gut wall) system of higher animals, including humans.
Planaria (Platyhelminthes) is the most primitive animal with a central nervous system — an inverted U-shaped cerebral ganglion with nine branches on each side, a pair of ventral nerve cords joined by commissures in a ladder-like manner, a peripheral nerve plexus, and a pair of eyes on the dorsal side of the brain.
Across Hydra, Planaria, cockroach and humans, the trend is: greater specialisation of neurons, a shift from a diffuse to a centralized nervous system, expansion of the brain, centralization of sense organs, and evolution of an efficient signalling network.
Neural tissue has two cell types: neurons and neuroglia. A bundle of axons outside the CNS is a nerve; inside the CNS it is a tract. Nerves may be sensory, motor or mixed.
A multipolar neuron has a cyton (with nucleus, neuroplasm, Nissl’s granules and neurofibrils), many branched dendrons (message towards the cyton) and a single axon arising at the axon hillock, ending in telodendrons (message away from the cyton). Nissl’s granules help synthesise the enzyme needed for neurotransmitter formation; neurofibrils help transmit the impulse.
Non-medullated nerves have only a neurilemma; medullated nerves have a medullary (myelin) sheath as well, and conduct 50 times faster. Nerve coverings, from inside out, are endoneurium → perineurium → epineurium.
Neuroglia are far more numerous than neurons and are ectodermal in origin. In the CNS: oligodendrocytes (myelin), microglia (macrophages), astrocytes (blood–brain barrier, neurotransmitter handling) and ependymal cells (CSF). In the PNS: Schwann cells (myelin) and satellite cells (support).
A synapse is the junction between two nerve cells, with a minute synaptic cleft crossed by a neurotransmitter bridge. Properties of nerve fibres: excitability, conductivity, stimulus (with threshold, subliminal and supraliminal), summation, the all-or-none law, refractory period, synaptic delay (0.3–0.5 ms), synaptic fatigue, and velocity (100–120 m/s in voluntary fibres; 10–20 m/s in autonomic nerves; faster in homeotherms and in medullated fibres).
Electrical synapses (gap about 3.8 nm) are faster and may be bidirectional; they occur where the fastest response is needed. Chemical synapses (gap 20–40 nm) are one-way: the impulse opens voltage-gated \(Ca^{2+}\) channels, \(Ca^{2+}\) enters, synaptic vesicles fuse and release neurotransmitter by exocytosis, the transmitter binds post-synaptic receptors and is either excitatory or inhibitory, and cholinesterase then destroys it. A chemical synapse between a motor neuron and a muscle is a neuromuscular junction.
A resting neuron is polarised: excess \(Na^+\) outside, excess \(K^+\) and negatively charged proteins inside, giving a resting potential of \(-50\) to \(-100\) mV (average \(-70\) mV). The Na–K exchange pump uses ATP and pumps out 3 \(Na^+\) for every 2 \(K^+\) pumped in, and is electrogenic.
On stimulation, voltage-gated \(Na^+\) gates open, \(Na^+\) rushes in, and the membrane depolarises, reaching an action potential of \(+30\) to \(+60\) mV. Then the \(Na^+\) gates close, \(K^+\) gates open, \(K^+\) passes out rapidly, and the membrane repolarises — a self-propagating wave along the fibre.
In medullated fibres, ions can cross only at the nodes of Ranvier, so the impulse leaps node to node — saltatory conduction, at 120 m/second, faster than continuous conduction in the ratio \(50:1\).
The human nervous system has three parts — CNS, PNS and ANS. The CNS (brain and spinal cord) is protected by bone, by three meninges (dura, arachnoid, pia mater) and by CSF (specific gravity 1.005; 100–120 cc), which cushions, maintains pressure, and exchanges nutrients, wastes and oxygen.
The brain has three parts. Forebrain: olfactory lobes (reduced); cerebrum (85% of the brain; two hemispheres joined by the corpus callosum; folded into gyri and sulci; four lobes plus the insula; grey matter outside, white matter inside; frontal = motor, Broca’s area, personality; parietal = somaesthetic sensation and taste; temporal = smell, hearing, speech, emotion; occipital = vision; Wernicke’s area for understanding language; basal nuclei/corpus striatum for subconscious execution); and the diencephalon (epithalamus with the pineal gland; thalamus, the relay centre for all sensations except smell; hypothalamus, which maintains homeostasis, controls hunger, thirst, sleep, temperature, heart rate and B.P., forms part of the limbic system, and links the nervous system with the endocrine system).
Midbrain: cerebral aqueduct; corpora quadrigemina (superior colliculi for visual reflexes, inferior colliculi for auditory reflexes); crura cerebri; red nucleus for posture and muscle tone. Hindbrain: pons varolii; cerebellum (second largest; vermis; arbor vitae; equilibrium, posture, muscle tone, coordination — all involuntary); medulla oblongata (heart beat, respiration, vasomotor activity, peristalsis; coughing, sneezing, swallowing, vomiting, yawning).
The spinal cord is 42–45 cm long and 2.0–2.5 cm broad, tapering into the conus medullaris and filum terminale, with the cauda equina behind. In section, grey matter is inner and H-shaped (six horns) and white matter outer (six funiculi, with ascending sensory and descending motor tracts). The dorsal root (with its ganglion of unipolar sensory neurons) is sensory; the ventral root is motor. The spinal cord is the main centre for reflex actions.
The PNS has 12 pairs of cranial nerves — sensory (I, II, VIII), motor (III, IV, VI, XI, XII) and mixed (V, VII, IX, X) — and 31 pairs of spinal nerves (cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1), all mixed, each formed by a dorsal sensory and a ventral motor root and branching into ramus dorsalis, ramus ventralis and ramus communicans. Afferent fibres carry impulses to the CNS; efferent fibres carry them away.
A reflex action is a sudden, automatic, involuntary response along a reflex arc. Reflexes are classified as cranial or spinal; unconditional or conditional (studied by E. Pavlov); and simple monosynaptic (knee jerk) or complex polysynaptic (cycling, swimming).
The ANS has two neurons in series (preganglionic and postganglionic). The sympathetic system (thoraco-lumbar, T1–L3; 22 pairs of ganglia; short pre- and long post-ganglionic fibres; adrenergic) drives the fight-or-flight response. The parasympathetic system (cranio-sacral; long pre- and short post-ganglionic fibres; cholinergic) is antagonistic to it and acts as the housekeeping system.
Sensory receptors convert a stimulus into an action potential. Exteroceptors: phonoreceptors, statoreceptors, photoreceptors, thermoreceptors, mechanoreceptors and chemoreceptors (gustatory and olfactory). Interoceptors: enteroceptors, proprioceptors and baroreceptors.
The eye has three coats — sclera (with the transparent cornea and the conjunctiva), choroid (choroid proper, ciliary body, iris with the pupil) and retina — plus a biconvex lens dividing the aqueous and vitreous chambers. The retina’s nervous part has rods and cones, bipolar cells and ganglion cells (whose fibres form the optic nerve). Rods contain rhodopsin (a vitamin A derivative) and give dim-light vision; cones, of three types (red, green, blue), give colour and daylight vision, and are densest at the fovea centralis. The blind spot has no photoreceptors.
The ear is the stato-acoustic organ. External ear: pinna, auditory canal, tympanic membrane. Middle ear: malleus, incus and stapes, which amplify vibrations, and the Eustachian tube, which equalises pressure. Internal ear: bony and membranous labyrinth (perilymph and endolymph); the cochlea, with scala vestibuli, scala media and scala tympani, carries the organ of Corti on the basilar membrane, whose hair cells press against the tectorial membrane to generate impulses. The vestibular apparatus — three semicircular canals with cristae (dynamic balance) and the utricle and saccule with maculae and \(CaCO_3\) otoliths (static balance) — maintains equilibrium.
Disorders of the nervous system include psychological disorders (ID, ASD, BD, depression, anxiety disorder, ADHD, stress-related disorders); Parkinson’s disease (degeneration of dopamine-producing neurons → tremors, stiffness, poor balance); and Alzheimer’s disease (loss of cholinergic neurons and accumulation of amyloid proteins → dementia; no cure).
Chemical coordination uses autocrine, paracrine, endocrine and pheromone signals. Hormones are chemical messengers secreted by ductless glands into the blood, effective in very low concentration, acting only on target cells that carry the matching receptor, destroyed after use, and controlled by feedback. Chemically they are amines, peptides, proteins, fatty acid derivatives, steroids or a gas (NO).
Water-soluble hormones bind membrane receptors, release adenylate cyclase, and act through cAMP (or \(Ca^{2+}\), cGMP, \(IP_3\)) as a second messenger — fast but short-lived. Lipid-soluble (steroid and thyroid) hormones enter the cell, bind intracellular receptors, enter the nucleus, bind DNA and switch on genes — slower but long lasting.
Hypothalamus: releasing and inhibiting neurohormones; makes ADH and oxytocin. Pituitary: adenohypophysis secretes GH, TSH, ACTH, prolactin and gonadotropins (FSH, LH, ICSH); neurohypophysis stores and releases oxytocin and ADH; pars intermedia secretes MSH in lower vertebrates. GH disorders: dwarfism, gigantism, acromegaly; ADH deficiency: diabetes insipidus.
Pineal gland: melatonin, the sleep hormone, regulating the biological clock. Thyroid: \(T_3\) and \(T_4\) (iodine + tyrosine) regulate BMR, metabolism, growth and thermoregulation; calcitonin lowers blood calcium. Disorders: hyperthyroidism and Grave’s disease; hypothyroidism, cretinism, myxoedema and simple goitre.
Parathyroid: parathormone (PTH) raises blood calcium — antagonistic to calcitonin. Disorders: parathyroid tetany and osteoporosis. Thymus: thymosin, for maturation of T-lymphocytes.
Adrenal cortex: zona glomerulosa → mineralocorticoids (aldosterone); zona fasciculata → glucocorticoids (cortisol); zona reticularis → sex corticoids. Disorders: Addison’s disease and Cushing’s disease. Adrenal medulla: adrenaline (the 3F hormone) and noradrenaline, working with the sympathetic system.
Pancreas is both exocrine and endocrine. In the Islets of Langerhans: alpha cells → glucagon (raises blood glucose), beta cells → insulin (lowers it), delta cells → somatostatin, PP/F cells → pancreatic polypeptide. Disorder: diabetes mellitus, Type I (IDDM) and Type II (NIDDM, insulin resistance), with glycosuria and ketosis.
Gonads: ovaries → estrogen, progesterone, relaxin, inhibin; testes → testosterone from the Leydig cells. Diffuse endocrine glands: the placenta (estrogen, progesterone, hCG), the GI tract (gastrin, secretin, CCK–PZ, GIP), the kidney (renin, erythropoietin, calcitriol) and the heart (ANH). Hormone therapy uses hormones in medical treatment, and pheromones carry chemical signals between individuals of the same species.
Flowchart
FC 9.18 – The chapter in one line: stimulus → receptor → sensory (afferent) neuron → CNS (spinal cord and brain; integration) → motor (efferent) neuron → effector (muscle or gland) → response; and in parallel: hypothalamus → pituitary → target endocrine gland → hormone in blood → target cell with matching receptor → long-term response → feedback back to the gland.
9.12 Points to Ponder
These questions have no single tidy answer. They are meant to be turned over slowly, discussed, and returned to after you have finished the chapter.
Hydra survives perfectly well with a nerve net and no brain, and it has done so for hundreds of millions of years. If a nerve net is enough, what pressures could have made the far more expensive centralized nervous system worth evolving?
The all-or-none law says a nerve fibre gives either a full impulse or none at all. Yet you can clearly tell the difference between a light touch and a heavy blow. What must the nervous system be doing with many fibres and many impulses to encode something the single impulse cannot?
A synapse is slower than a continuous wire would be, and it can fatigue. Why would evolution keep a junction with these disadvantages, instead of simply fusing neurons into one continuous conducting path?
The rods and cones sit at the back of the retina, so light must pass through layers of nerve cells before reaching them. Is this an inefficient design, a design with hidden advantages, or simply an accident of how the eye developed? What evidence would settle the question?
The blind spot means each eye has a region where it cannot see at all — yet you are never aware of a hole in your visual field. What does this tell you about the difference between what the eye detects and what the brain presents to you as reality?
The medulla oblongata controls heart beat and breathing, and injury to it can be immediately fatal. The cerebrum is the seat of thought, memory and personality, yet a person can survive extensive cerebral damage. What does this say about how “importance” should be measured in the nervous system?
Reflexes are fast because they bypass conscious decision. But conditional reflexes such as cycling begin as voluntary, consciously learned acts. Where exactly does a movement stop being a decision and become a reflex — and can a reflex ever be unlearned?
Every hormone reaches every cell, but only target cells respond. Would the endocrine system be improved if hormones could be delivered only to their targets, as nerve impulses are? What would be lost if that happened?
Thyroxine, insulin and adrenaline all raise or lower a quantity in the blood, and each is opposed by another hormone or system. Why does the body so often build control out of two antagonistic agents rather than a single one that can be turned up and down?
In simple goitre the thyroid gland grows larger while producing less hormone. What does this reveal about a feedback system that has lost the raw material it depends on — and where else in biology might a control loop drive a change that does not solve the problem?
The adrenal medulla releases the very same chemicals that sympathetic nerve endings release. Should we then call it part of the nervous system, part of the endocrine system, or is the distinction between the two systems less sharp than the chapter’s two halves suggest?
Parkinson’s disease follows the loss of one neurotransmitter, and Alzheimer’s the loss of another along with abnormal protein deposits. Both begin quietly and are recognised only years later. What does this delay tell you about how much reserve capacity the brain carries — and what difficulties does it create for anyone hoping to treat these diseases early?
Pheromones let one organism change the physiology of another. If a chemical signal can cross from one body to another, where should we draw the line between an individual organism and the community it lives in — as, for instance, in a beehive?
9.13 Objective Questions
- The nervous system of mammals uses both electrical and chemical means to send signals via neurons. Which part of the neuron receives the impulse?
- Axon
- Dendron
- Nodes of Ranvier
- Neurilemma
- ___________ is a neurotransmitter.
- ADH
- Acetyl CoA
- Acetyl choline
- Inositol
- The supporting cells that produce myelin sheath in the PNS are _________.
- Oligodendrocytes
- Satellite cells
- Astrocytes
- Schwann cells
- A collection of neuron cell bodies located outside the CNS is called _________.
- Tract
- Nucleus
- Nerve
- Ganglion
- Receptors for protein hormones are located
- in cytoplasm
- on cell surface
- in nucleus
- on Golgi complex
- If the parathyroid glands of a man are removed, the specific result will be
- onset of aging
- disturbance of \(Ca^{2+}\)
- onset of myxoedema
- elevation of blood pressure
- The hormones thyroxine, adrenaline and noradrenaline are formed from
- Glycine
- Arginine
- Ornithine
- Tyrosine
- Pheromones are chemical messengers produced by animals and released outside the body. The odour of these substances affects
- skin colour
- excretion
- digestion
- behaviour
- Which one of the following is a set of discrete endocrine glands?
- Salivary, thyroid, adrenal, ovary
- Adrenal, testis, ovary, liver
- Pituitary, thyroid, adrenal, thymus
- Pituitary, pancreas, adrenal, thymus
- After ovulation, the Graafian follicle changes into
- Corpus luteum
- Corpus albicans
- Corpus spongiosum
- Corpus callosum
- Which one of the following pairs correctly matches a hormone with a disease resulting from its deficiency?
- Parathyroid hormone — Diabetes insipidus
- Luteinising hormone — Diabetes mellitus
- Insulin — Hyperglycemia
- Thyroxine — Tetany
- ___________ is in direct contact with the brain in humans.
- Cranium
- Dura mater
- Arachnoid
- Pia mater
- Match the organism with the type of nervous system found in it.
| Column A | Column B |
|---|---|
| a. Neurons | i. Earthworm |
| b. Ladder type | ii. Hydra |
| c. Ganglion | iii. Flatworm |
| d. Nerve net | iv. Human |
- Complete the table.
| Location | Cell Type | Function |
|---|---|---|
| PNS | …………… | Produce myelin sheath |
| PNS | Satellite cells | …………… |
| …………… | Oligodendrocytes | Form myelin sheath around central axon |
| CNS | …………… | Phagocytose pathogens |
| CNS | …………… | Form the epithelial lining of brain cavities and central canal |
9.14 Check Your Concepts
Why is the nerve impulse in Hydra said to show “no polarity”? Explain how this is possible in an animal that lacks sensory and motor nerves, and state one advantage this arrangement gives to Hydra.
Arrange the following in the correct order of increasing centralization of the nervous system, and justify the order you choose: cockroach, Hydra, sponge, human, Planaria.
Assertion (A): The diffused nervous system has not disappeared from higher animals. Reason (R): The gut wall of humans possesses an enteric nerve network organised like a nerve net. Choose whether both A and R are true and R explains A; both true but R does not explain A; A true, R false; or A false, R true — and explain your choice.
What happens to the conduction of impulses if the myelin sheath around an axon is destroyed? Explain your answer with reference to the nodes of Ranvier.
Nissl’s granules are present in the cyton and at the base of dendrons, but are absent in the axon. Suggest a reason based on the function of Nissl’s granules.
Distinguish between a nerve and a tract, and between a ganglion and a nucleus, giving one example of each.
A student writes: “A supraliminal stimulus produces a bigger impulse than a threshold stimulus.” Identify the error and correct it, naming the law involved.
What would happen at a chemical synapse if the enzyme cholinesterase were completely inhibited? Predict the effect on the post-synaptic cell and on the muscle it supplies.
Why is synaptic transmission across a chemical synapse a strictly one-way process, while an electrical synapse may be bidirectional? Relate your answer to the structure of each type.
Explain why the resting potential is negative on the inside, even though the \(Na^+\)–\(K^+\) pump moves ions in both directions across the membrane.
Sequence the following events of impulse generation in the correct order:
- \(K^+\) gates open and \(K^+\) diffuses out
- Membrane becomes more permeable to \(Na^+\)
- Action potential reaches about \(+30\) mV
- Stimulus applied to a resting nerve
- Axoplasm becomes negative and the ECF positive once again
- \(Na^+\) rushes into the axon
Assertion (A): A nerve fails to respond to a second stimulus during the refractory period, however strong that stimulus may be. Reason (R): During this period the \(Na^+\) gates are closed and the \(K^+\) gates are open. State whether A and R are true and whether R correctly explains A.
Meninges and CSF are both described as protective. Explain how each protects the brain, and state two functions of CSF that are not protective.
The surface of the cerebrum is heavily folded into gyri and sulci. Explain why this folding is functionally necessary rather than merely decorative.
A patient can understand everything said to her but is unable to speak fluently. A second patient speaks fluently but produces meaningless sentences. Which area of the cerebrum is likely to be damaged in each case? Give reasons.
The thalamus is called the relay centre of the brain, yet one sensation bypasses it. Name that sensation and state where its impulses go instead.
“The hypothalamus is the bridge between the nervous and the endocrine systems.” Justify this statement with at least three specific pieces of evidence from the chapter.
In the cerebrum, grey matter lies outside and white matter inside; in the spinal cord the arrangement is reversed. Draw a labelled sketch of a T.S. of the spinal cord and use it to explain the position of the dorsal root, ventral root and dorsal root ganglion.
A person steps on a thorn and pulls the foot away before feeling the pain. Explain this observation using the reflex arc, and state why the response is called involuntary.
Classify each of the following into all three schemes of reflex classification given in the chapter (cranial/spinal; conditional/unconditional; monosynaptic/polysynaptic): knee jerk, sneezing, cycling, watering of the mouth at the smell of food.
Match the following:
| Column A | Column B |
|---|---|
| a. Adrenergic fibres | i. Cranio-sacral outflow |
| b. Cholinergic fibres | ii. Long post-ganglionic fibres |
| c. Sympathetic system | iii. Acetylcholine at the effector organ |
| d. Parasympathetic system | iv. Fight-or-flight response |
Explain why the sympathetic nervous system stimulates most organs but inhibits the digestive and excretory organs. What survival advantage does this give?
A person is bright-eyed in daylight but cannot see at all in a dimly lit room. Which photoreceptor cells are likely to be affected, which vitamin is involved, and why is daylight vision unaffected?
Trace the path of light from an object outside the eye up to the point where the image is recognised, naming every structure it crosses in order.
Explain why a person feels dizzy for a few seconds after suddenly stopping a rapid spin. Which receptor and which fluid are responsible?
Distinguish between crista and macula on the basis of location, the fluid around them, and the type of balance each maintains.
Why is the ear called a stato-acoustic organ? Support your answer by naming the receptor responsible for each of its two functions.
Assertion (A): Adrenaline injected into a patient acts within seconds, while a steroid hormone given at the same time takes hours to act. Reason (R): Adrenaline acts through a membrane receptor and a second messenger, whereas steroid hormones act by switching on genes. State whether A and R are true and whether R explains A.
Hormones are carried to every cell of the body but act on only a few. Explain the biological basis of this selectivity.
In simple goitre, the thyroid gland enlarges while the output of thyroxine falls. Explain this apparent contradiction using the negative feedback loop between the thyroid and the pituitary.
Distinguish between gigantism and acromegaly. Both are caused by hypersecretion of the same hormone — what decides which condition develops?
Diabetes insipidus and diabetes mellitus both produce large volumes of urine and great thirst. State the hormone at fault in each, and give one test that would distinguish the two.
What happens to the blood calcium level if the parathyroid glands are surgically removed along with the thyroid? Name the resulting condition and explain the symptoms.
The pancreas is called a mixed gland. Justify this by naming one exocrine and one endocrine function, and identifying the cells responsible for each.
Explain why an untreated diabetic patient loses weight and develops ketosis, even while the blood is rich in glucose.
Give scientific reasons:
- The thymus is called a temporary gland.
- The thyroid is the only gland that stores its hormones.
- Prolactin is unique among the pituitary hormones.
- The adrenal medulla is often described as part of the sympathetic nervous system.
Compare nervous coordination and chemical coordination with respect to the messenger used, the route taken, the speed of action, and the duration of the effect. Give one example of a situation best handled by each.
The chapter says that hormones are “metabolised after their function and cannot be reused.” Explain why this destruction is essential to hormonal control, and predict what would happen if a hormone were never broken down.
What is the function of the red nucleus? (20–30 words)
What is the importance of the corpora quadrigemina? (20–30 words)
What does the cerebellum of the brain control? (20–30 words)
Name the three ear ossicles. (20–30 words)
Name the anti-abortion hormone. (20–30 words)
Name an organ which acts as a temporary endocrine gland. (20–30 words)
Name the type of hormones which bind to DNA and alter gene expression. (20–30 words)
What is the cause of abnormal elongation of the long bones of the arms and legs and of the lower jaw? (20–30 words)
Name the hormone secreted by the pineal gland. (20–30 words)
Which endocrine gland plays an important role in improving immunity? (20–30 words)
Describe the endocrine role of the Islets of Langerhans. (30–40 words)
Mention the functions of testosterone. (30–40 words)
Give the symptoms of the disease caused by hyposecretion of ADH. (30–40 words)
Rakesh got hurt on his head when he fell down from his motorbike. Which inner membranes must have protected his brain? What other roles do they have to play? (80–100 words)
Give reason — injury to the medulla oblongata may prove fatal. (80–100 words)
Distinguish between the sympathetic and parasympathetic nervous system on the basis of the effect they have on: (80–100 words)
- Heart beat
- Urinary bladder
While holding a tea cup, Mr. Kothari’s hands rattle. Which disorder may he be suffering from, and what is the reason for this? (80–100 words)
List the properties of nerve fibres. (80–100 words)
How does the tongue detect the sensation of taste? (80–100 words)
State the site of production and the function of secretin, gastrin and cholecystokinin. (80–100 words)
An adult patient suffers from a low heart rate, low metabolic rate and low body temperature. He also lacks alertness, intelligence and initiative. What can this disease be? What can be its cause and care? (80–100 words)
Where is the pituitary gland located? Enlist the hormones secreted by the anterior pituitary. (80–100 words)
Explain how the adrenal medulla and the sympathetic nervous system function as a closely integrated system. (80–100 words)
Name the secretions of the alpha, beta and delta cells of the Islets of Langerhans. Explain their role. (80–100 words)
What are the two types of goitre? Mention their causes. (80–100 words)
Name the ovarian hormones and give their functions. (80–100 words)
Explain the process of conduction of a nerve impulse up to the development of the action potential. (150–200 words)
Draw neat labelled diagrams of: (150–200 words)
- Human ear
- Sectional view of the human eye
- L.S. of the human brain
- Multipolar neuron
Explain the reflex pathway with the help of a neat labelled diagram. (150–200 words)
Krishna was going to school and on the way he saw a major bus accident. His heartbeat increased and his hands and feet became cold. Name the part of the nervous system that had a role to play in this reaction. (150–200 words)
What will be the effect of thyroid gland atrophy on the human body? (150–200 words)
Write the names of the hormones, and the glands secreting them, for the regulation of the following functions. (150–200 words)
- Growth of the thyroid and secretion of thyroxine.
- Helps in relaxing pubic ligaments to facilitate easy birth of the young one.
- Stimulates intestinal glands to secrete intestinal juice.
- Controls calcium level in the blood.
- Controls tubular absorption of water in the kidneys.
- Urinary elimination of water.
- Sodium and potassium ion metabolism.
- Basal metabolic rate.
- Uterine contraction.
- Heart beat and blood pressure.
- Secretion of growth hormone.
- Maturation of the Graafian follicle.
Explain the role of the hypothalamus and the pituitary as a coordinated unit in maintaining homeostasis. (150–200 words)
What is adenohypophysis? Name the hormones secreted by it. (150–200 words)
Describe in brief an account of the disorders of the adrenal gland. (150–200 words)
Explain the action of steroid hormones and proteinous hormones. (150–200 words)
Describe in brief an account of the disorders of the thyroid. (150–200 words)
9.15 Case-Based Questions
- Answer the following questions after observing the diagram of synaptic transmission (Fig. 9.5). (150–200 words)
- What do the synaptic vesicles contain?
- What process is used to release the neurotransmitter?
- What should be the conditions for the next impulse to be conducted?
- Will the impulse be carried by the post-synaptic membrane even if there is only one pre-synaptic neuron?
- Can you name the channel responsible for their transmission?