Chapter 10: The Nervous System
In the 1780s, Luigi Galvani, a professor of anatomy at Bologna in Italy, kept freshly dissected frogs on his laboratory bench. One day an assistant touched the exposed nerve of a frog’s leg with a metal instrument, just as an electrical machine across the room gave off a spark. The leg kicked, although the animal had been dead for hours.
Galvani could not leave that accident alone. He repeated it again and again. He hung frogs’ legs on metal hooks, brought different metals into contact with the nerve, and even carried his preparations outdoors during thunderstorms. The twitch came each time. Galvani concluded that the tissue itself held a kind of electricity, which he called ‘animal electricity’, and that the nerve carried it to the muscle.
Alessandro Volta, a physicist at Pavia, read the work and disagreed. He argued that two unlike metals touching moist tissue produce electricity by themselves, and that the leg was only detecting it. To prove his point he stacked discs of metal into the first battery, in 1800.
Time has shown that each man held half the answer. Volta was right about the metals. Galvani was right about something larger — nerve and muscle do carry electrical activity of their own, and that activity is how your body carries news from one place to another. Fig. 10.0-1 shows the arrangement on Galvani’s bench.
Diagram
Fig. 10.0-1 — Galvani’s Frog-Leg Experiment Simple line drawing of a laboratory bench: a dissected frog’s hind leg lying on the bench with its nerve exposed, a metal instrument touching the nerve, a hand-cranked electrical machine giving off a spark nearby, and an arrow showing the leg kicking; label exposed nerve, metal instrument, leg muscle, direction of movement, spark from electrical machine.
10.1 Why The Body Needs A Nervous System
A ball flies towards your face and your eyelids shut before you have decided anything. Somebody calls your name in a crowded corridor and your head turns. In each case something outside the body was noticed, and a part of the body was set moving in reply, all within a fraction of a second.
Every organism must be able to do this. It has to find out what is happening around it and then act in a way that keeps it fed, safe and in working order. An animal that fails to notice a predator does not usually get a second chance.
A great deal is also happening inside you that you never notice. Your heart is beating, your stomach is churning food, and your body temperature is being held close to 37 °C (degrees Celsius). All these activities must be timed and matched to one another. This working together of different parts of the body, at the right moment and in the right measure, is called coordination.
Two Systems That Coordinate
In the human body, coordination is looked after by two systems. The nervous system is a network of nerve cells that carries messages as electrical signals, and it acts within fractions of a second. The other is the hormonal system, which sends chemical messengers through the blood; it works more slowly, and you will meet it in the next chapter.
This chapter deals with the nervous system alone — the brain, the spinal cord, and the nerves that run out from them to almost every part of the body, as shown in Fig. 10.1-1.
Diagram
Fig. 10.1-1 — The Human Nervous System In Outline Line drawing of a human figure in outline, front view, showing the brain inside the skull, the spinal cord running down the back, and the main nerves branching out to the arms, trunk and legs; label brain, spinal cord and nerves on straight, non-crossing lead lines.
What The Nervous System Does
The system has four main jobs, and each one tells you something about the way it is built.
It keeps you informed about the world outside. Light, sound, heat, smell and touch are picked up by the sense organs, and news of them is carried to the brain. Without this, you would be sealed off from your surroundings.
It allows you to remember, to think and to reason. Past experience is stored and compared with the present situation, so that you can choose what to do. This is why a child who has once been burnt is careful near a flame.
It controls and harmonises voluntary actions — the muscular activities you carry out by your own will, such as running, or simply holding this book steady while you read. Dozens of muscles must pull in the right order and with the right force, and the nervous system does that timing.
It regulates involuntary actions — the activities that go on without your thinking about them, such as breathing and the beating of the heart. These cannot be left to your attention, or they would stop the moment you fell asleep.
10.2 The Neuron — The Unit Of The Nervous System
The nervous system is built out of the brain, the spinal cord, the sense receptors and a great many nerves. All of these are made of the same kind of cell, so it makes sense to begin with that cell.
A neuron, or nerve cell, is the structural and functional unit of the nervous system. It is the longest cell in the body, and it is shaped for one job — to pick up a message at one end and pass it on at the other. Everything else about a neuron follows from that job.
10.2.1 Structure Of A Neuron
A neuron has three main parts: a cell body, several dendrites and one axon. Their arrangement is shown in Fig. 10.2-1.
Diagram
Fig. 10.2-1 — Generalised Structure Of A Neuron Line drawing of a complete nerve cell, drawn lengthwise: a star-shaped cell body on the left with branching dendrites, one long axon running to the right ending in swollen terminal knobs, and an enlarged inset of a short length of the axon showing its coverings; label dendrites, dendron, cell body (cyton), nucleus, Nissl granules, collateral branch, axon, myelin sheath, neurilemma, node of Ranvier and axon endings on straight, non-crossing lead lines.
10.2.1.1 The Cell Body
The cell body, also called the cyton or perikaryon, is the widest part of the neuron. The name is built from two Greek words — peri, meaning surrounding, and karyon, meaning nucleus. It contains a well-defined nucleus surrounded by granular cytoplasm.
The cell body carries all the usual cell organelles, along with dark granules of its own called Nissl granules. One organelle, however, is missing: the centrosome. A centrosome is needed for cell division, and mature nerve cells have lost the power to divide, so it is not there.
That single missing organelle explains something important. Damaged nerve cells are not replaced by new ones the way skin or blood cells are, which is why injury to nervous tissue is so serious.
10.2.1.2 Dendrites
Dendrites are branched projections of cytoplasm arising from the cell body. Their name comes from dendron, meaning tree, and a neuron under the microscope does look like a bare tree. The thicker main branch is the dendron, and it divides into finer dendrites.
Dendrites carry impulses towards the cell body. Because they are many and much-branched, one neuron can receive messages from a large number of neighbouring neurons at the same time.
10.2.1.3 The Axon
The axon is a single long process arising from the cell body, and it carries the impulse away from it. Axons vary greatly in length — from a few millimetres to more than a metre in the fibres that run from the spinal cord to the toes.
In most neurons the axon is wrapped in a white, fatty, insulating covering called the myelin sheath, or medullary sheath. Over this lies a thin outermost covering, the neurilemma (also written neurolemma). The myelin sheath is not continuous: it is interrupted at regular gaps called the nodes of Ranvier.
Some axons give off side branches along their length, called collaterals. These allow one neuron to deliver its message to more than one destination.
At its far end the axon divides into fine branches whose tips are swollen like tiny bulbs. These swellings store chemicals called neurotransmitters, which are the means by which the message is handed on.
The bulbs lie extremely close to the dendrites of the next neuron, but they never actually join it. The narrow gaps left in between are the synaptic clefts, and how a message crosses them is taken up in Section 10.2.4.
10.2.2 Basic Terms In Nervous Activity
Five terms are used throughout the rest of this chapter, and it is worth fixing their meanings now.
A stimulus is any agent or sudden change, outside the body or inside it, that produces a change in an organism or in any of its parts. Light, sound, heat, pain and hunger are all stimuli.
A response is the change brought about in the organism as a result of that stimulus. Pulling your foot back from a thorn is the response; the prick of the thorn is the stimulus.
An impulse is the message itself — a wave of irritability, that is, of electrical disturbance, sweeping along a nerve fibre. It is not a substance, and nothing material travels from one end of the fibre to the other.
Receptors are the specialised cells that receive a stimulus and set up impulses towards the central nervous system. In some sense organs they are modified epithelial cells; in others they are simply the fine endings of sensory neurons. Photoreceptors respond to light, phonoreceptors to sound, thermoreceptors to temperature and olfactory receptors to smell.
Effectors are the muscles or glands that act on receiving the impulse from the brain or spinal cord. A muscle responds by contracting, a gland by secreting. Between a receptor and an effector lies the whole business of the nervous system.
10.2.3 Transmission Of The Nerve Impulse
In a resting nerve fibre, the outer side of the membrane carries a positive charge and the inner side a negative one. This condition is described as the polarised state, and it exists because there are more sodium ions (\(Na^+\)) outside the axon membrane than inside. Stage A of Fig. 10.2-2 shows a fibre in this resting condition.
Now suppose the fibre is stimulated at one spot — by pressure, by heat, by a chemical or by electricity. At that spot the membrane becomes far more permeable to sodium ions, and they rush inwards. The charge on the two sides is reversed, and the polarisation is lost.
This loss of polarisation is called depolarisation, and the spot where it has happened is the excited region.
The depolarised point now acts as a stimulus for the region of membrane just ahead of it, which depolarises in its turn. In this way the disturbance moves along the fibre, spot by spot.
Meanwhile the region just behind is set right again. Sodium ions are actively pumped back to the outside by what is called the sodium pump, which uses energy supplied by ATP. The restoring of the resting charge is called repolarisation, and it leaves that stretch of fibre ready to conduct once more. FC 10.1 sets out the whole sequence.
Flowchart
FC 10.1 — Conduction Of An Impulse Along A Nerve Fibre Fibre at rest (polarised, more \(Na^+\) outside) → stimulus applied at one spot → membrane there becomes permeable to \(Na^+\) → \(Na^+\) move inwards → depolarisation (excited region) → this point acts as the stimulus for the next region → next region depolarises → sodium pump drives \(Na^+\) out of the region behind → repolarisation → that stretch is polarised again and ready for the next impulse
Diagram
Fig. 10.2-2 — Conduction Of A Nerve Impulse Through A Nerve Fibre Three horizontal strips of nerve fibre one below the other, marked A, B and C, with plus and minus signs shown above and below the membrane in each: A shows a uniformly polarised resting fibre; B shows an excited (depolarised) region at the left end with the rest still resting, and an arrow showing the direction of travel; C shows the excited region shifted to the middle, with a recovering (repolarising) region behind it and a resting region ahead. Label resting region (polarised), excited region (depolarisation), recovery region (repolarisation) and direction of conduction.
Conduction of a nerve impulse is therefore a wave of depolarisation followed by repolarisation, running along the fibre.
It is tempting to compare this with electricity flowing through a wire, but the two are not the same. In a wire, electrons actually travel along its length. In a nerve fibre nothing travels along the fibre at all — no substance, no electrons and no ions. Only the disturbance moves, rather as a ripple crosses a pond while the water itself stays where it is.
That difference shows up plainly in the speeds. Electricity is conducted through a wire at about 150,000 kilometres per second. A nerve impulse travels at a maximum of about 100 metres per second — quick enough for the body, but slower by an enormous margin.
10.2.4 The Synapse
An impulse running along an axon eventually reaches its swollen endings, and there it meets a gap. The junction between the terminal branches of the axon of one neuron and the dendrites of the next is called the synapse — from syn, meaning together, and apse, meaning gap.
At a synapse the impulse cannot simply run across, because the two neurons are not joined. Instead it jumps the gap by a chemical process, shown in Fig. 10.2-3.
As the impulse reaches the terminal end of the axon, a chemical called acetylcholine is released from the swollen bulbs into the gap. It acts on the dendrites of the next neuron and sets up a fresh impulse there. The chemical is then quickly broken down by an enzyme, which clears the gap and leaves the synapse ready for the next transmission.
That last step matters more than it looks. If the chemical were left lying in the gap, the next neuron would keep firing and the message could never be switched off.
Flowchart
FC 10.2 — Transmission Of An Impulse Across A Synapse Impulse arrives at the axon endings → acetylcholine released from the terminal bulbs into the synaptic cleft → chemical acts on the dendrites of the next neuron → a fresh impulse is set up in that neuron → enzyme breaks down the acetylcholine → synapse cleared and ready for the next impulse
Diagram
Fig. 10.2-3 — A Synapse Between Two Neurons Enlarged line drawing of the region where the axon endings of one neuron lie against the dendrites of the next; show the terminal bulbs containing granules of neurotransmitter and the narrow gap between the two cells; label axon ending, terminal bulb, neurotransmitter, synaptic cleft, dendrite of next neuron and direction of transmission.
10.2.5 Types Of Neurons
Neurons are not all alike, because they do not all have the same task. Three kinds are recognised, and Fig. 10.2-4 shows how they are linked.
Sensory neurons convey the impulse from a receptor in a sense organ to the main nervous system, that is, to the brain or the spinal cord.
Motor neurons carry the impulse the other way, from the brain or spinal cord out to an effector — a muscle or a gland.
Association neurons, also called connecting neurons, lie within the brain and the spinal cord. They join sensory neurons to motor neurons, and so decide what response an incoming message will produce.
Diagram
Fig. 10.2-4 — The Three Types Of Neurons And The Synapses Between Them Line drawing showing a sensory neuron on the left receiving dendrites from a receptor, an association neuron in the middle and a motor neuron on the right leading to a muscle; mark the two synapses between them and use arrows to show the direction of conduction throughout; label sensory neuron, association neuron, motor neuron, synapse, dendrites from receptor, to muscle or gland, myelin sheath, node of Ranvier and cell body.
A message therefore passes along a chain: a sensory neuron brings it in, an association neuron passes it on, and a motor neuron carries the order out. This chain is the pattern behind almost everything the nervous system does, and you will meet it again in the reflex arc.
10.3 Nerves
A single axon is far too fine to be seen with the naked eye. What you can see, running through the body, are the whitish thread-like structures called nerves — and each of these is a great many axons travelling together.
Nerves emerge from the brain and the spinal cord and branch out to almost every part of the body. Inside one nerve, the axons belong to entirely separate neurons, and each axon keeps its own message to itself.
A nerve is best pictured as an underground electric cable, as in Fig. 10.3-1. The cable carries dozens of conducting wires side by side, and each wire is insulated so that the currents do not mix. In a nerve, the fatty myelin sheath around each axon is that insulation. Because of it, thousands of different messages can travel through one thin cord at the same time without confusion.
Diagram
Fig. 10.3-1 — A Nerve, Formed Of A Bundle Of Axons Cut-away line drawing of a short length of a nerve with the covering opened at the near end to show the axons inside; the axons are drawn as fine cylinders running lengthwise, each with its own sheath, packed together within an outer covering; label axons, sheath around an individual axon and tubular sheath of the nerve.
10.3.1 The Three Kinds Of Nerves
Nerves are classified by the direction in which their fibres carry impulses. Table 10.2 sets the three kinds side by side.
Sensory nerves contain only sensory fibres. They bring impulses from the receptors in the sense organs to the brain or the spinal cord. The optic nerve, which arises in the eye and ends in the brain, is a sensory nerve.
Motor nerves contain only motor fibres. They carry impulses from the brain or the spinal cord out to the effector organs — muscles or glands — to bring them into action. A nerve arising from the brain and supplying the muscles that rotate the eyeball is a motor nerve.
Mixed nerves contain both sensory and motor fibres, so traffic runs in both directions along the same nerve. A spinal nerve is a mixed nerve.
10.3.2 Ganglia
The cell bodies of neurons are not scattered at random. Outside the brain and the spinal cord they occur in small swollen clusters, and each such cluster is called a ganglion.
A ganglion is therefore a collecting point, not a passage. You will meet one shortly on the pathway of a reflex, where the cell bodies of the sensory neurons lie clustered just outside the spinal cord.
10.4 The Two Major Divisions Of The Nervous System
You have now met the parts from which the system is built — neurons, nerves and ganglia. The next question is how these are arranged in the body, and the arrangement follows a simple plan: one part decides, the other part carries messages to and fro.
The deciding part is the central nervous system (CNS). It consists of the brain, which lies inside the cranium of the skull, and the spinal cord, which lies within the vertebral column. Both are heavily protected by bone, because a decision-making centre cannot be allowed to take knocks.
The carrying part is the peripheral nervous system (PNS). It consists of all the nerves that emerge from the brain and the spinal cord and those that enter into them. These nerves reach out to the skin, the muscles, the sense organs and the internal organs, so nothing in the body is out of touch.
The Two Halves Of The Peripheral System
The nerves of the peripheral nervous system do not all serve the same kind of organ, and so this system is further divided into two.
The somatic nervous system conveys information to the skeletal muscles — the muscles attached to bones, which you move by your own will. When you raise your hand, the order travels through somatic nerves.
The autonomic nervous system (ANS) consists of a pair of chains of ganglia together with their nerves. It controls the involuntary actions of many internal organs, including the smooth muscles of the gut, the muscles of the heart and the glands. Its work goes on whether you attend to it or not. FC 10.3 sets out the whole plan.
Flowchart
FC 10.3 — Divisions Of The Nervous System Nervous system → branches into: (1) central nervous system → brain, and spinal cord; (2) peripheral nervous system → branches into: somatic nervous system (nerves to skeletal muscles) and autonomic nervous system (chains of ganglia and nerves to internal organs)
10.5 The Brain
The brain is the control centre of the whole body. Every message from the sense organs finally arrives here, and almost every order of any importance starts here.
An adult human brain weighs about 1.35 kilograms, which is roughly 2 per cent of the total body weight. It is large in proportion to body size — larger, in proportion, than the brain of most other animals. About three-quarters of its substance is water.
For its size the brain is extraordinarily costly to run. Although it is only one-fiftieth of the body by weight, it uses roughly a fifth of all the oxygen the body takes in. This is why a person becomes unconscious within seconds if the blood supply to the brain is interrupted: nervous tissue cannot store its own fuel and cannot wait.
Seen from outside, the brain shows three main parts — the cerebrum, the cerebellum and the medulla oblongata. Their positions are shown in Fig. 10.5-2, and each is dealt with in turn below.
10.5.1 How The Brain Is Protected
Nervous tissue is soft and cannot repair itself, so the brain is protected more thoroughly than any other organ. It lies inside the bony brain box, or cranium, of the skull.
Inside the bone, the brain is wrapped in three membranous coverings called the meninges, from meninx, meaning membrane. The same three membranes continue backwards over the spinal cord, so brain and cord share one covering. Fig. 10.5-1 shows how they lie.
The outermost is the dura mater (dura: tough, mater: mother), a tough fibrous membrane lying just inside the skull bone. The middle layer is the arachnoid (arachne: spider), a thin delicate membrane that forms a web-like cushion. The innermost is the pia mater (pia: tender), a highly vascular membrane richly supplied with blood, which clings closely to the surface of the brain.
Diagram
Fig. 10.5-1 — The Coverings Of The Brain Enlarged line drawing of a section through the top of the head, drawn as horizontal layers from outside inwards: scalp, bone of the skull, dura mater, arachnoid, a fluid-filled space, pia mater and the folded surface of the brain below; label skull bone, dura mater, arachnoid, cerebrospinal fluid, pia mater and surface of brain on straight, non-crossing lead lines.
The space between the covering membranes is filled with a watery fluid called cerebrospinal fluid (CSF). It works as a shock absorber: the brain floats in it, so a knock on the head is spread out instead of being delivered to the tissue directly.
The same fluid also fills the central spaces of the brain, called the ventricles, and the central canal that runs down the spinal cord. The brain is therefore cushioned from outside and from within.
10.5.2 The Cerebrum
The cerebrum is by far the largest portion of the brain. It is divided into right and left halves called the cerebral hemispheres, and its outer surface is deeply wrinkled with ridges and grooves.
Each hemisphere is hollow inside, and its wall has two regions — an outer part called the cortex and an inner part. The outer cortex contains the cell bodies of the neurons, and because these look grayish it is called the gray matter. The inner part consists mainly of the axons of the neurons, and their white myelin sheaths make it the white matter.
It is the layer of gray matter that is thrown into folds. The raised folds are called the gyri and the grooves between them the sulci, meaning creases. Folding a sheet packs far more of it into the same space, and this is exactly what the folds achieve: a much greater surface area, and so room for a far greater number of nerve cells.
It is believed that the large number of convolutions in the human brain goes with a large number of nerve cells, and hence with our greater intelligence.
The two hemispheres are not independent of each other. A thick sheet of nerve fibres called the corpus callosum, meaning hard body, runs between them and transfers information from one hemisphere to the other, so that both work as one.
Diagram
Fig. 10.5-2 — The Brain In Median (Sagittal) Section Line drawing of the human brain cut along the middle from front to back and seen from the side, with the front of the head to the left; show the folded cerebrum above, the corpus callosum arching below it, the thalamus and hypothalamus in the centre, the pituitary gland hanging below the hypothalamus, the pineal gland behind the thalamus, the midbrain, the pons, the cerebellum with its branching internal white matter, and the medulla oblongata continuing below into the spinal cord; label cerebrum, gyrus, sulcus, corpus callosum, thalamus, hypothalamus, pituitary gland, pineal gland, midbrain, pons, cerebellum, medulla oblongata and spinal cord on straight, non-crossing lead lines.
The highly developed cortex is what allows us to think, to reason, to invent, to plan and to memorise. Taken as a whole, the cerebrum is the seat of intelligence, consciousness and will-power, and it controls all voluntary actions.
Different regions of the cortex look after different tasks, as Fig. 10.5-3 shows. Movement is controlled from the front region and sensation is received just behind it, while separate areas deal with vision, hearing, speech, smell and taste. Memory is not confined to one patch; it involves the cerebrum as a whole.
Diagram
Fig. 10.5-3 — Major Functional Areas Of The Cerebrum Side view line drawing of the whole brain with the front to the left; shade and label distinct areas of the cerebral cortex for movement (anterior), sensation (posterior), speech, hearing, vision (rear), smell and taste; mark the cerebellum below and behind as the area concerned with body balance, and note that memory involves the entire cerebrum.
The cerebrum also holds a great deal that you are not aware of at any moment, which is spoken of as the subconscious or unconscious mind. Older impressions are covered over by more recent ones, which dominate conscious activity; in dreams, some of these older experiences surface again.
10.5.3 The Cerebellum
The cerebellum, whose name means little brain, is a much smaller region lying at the base of the brain, below and behind the large cerebrum. It has no convolutions like the cerebrum, but it does carry numerous fine furrows.
Its arrangement of matter is like the cerebrum’s — an outer cortex of gray matter with white matter within. In a median section, the white matter branches through the cerebellum in a pattern that looks remarkably like a small tree.
The main work of the cerebellum is to maintain the balance of the body and to coordinate muscular activity. Note carefully what this does not mean. The impulse to perform a muscular act originates in the cerebrum, not in the cerebellum.
If you stand up and walk, the decision and the order come from the cerebrum. But walking involves many muscles pulling in turn, and each must contract and relax at exactly the right moment. That timing is the responsibility of the cerebellum.
10.5.4 The Medulla Oblongata
The medulla oblongata is the lowest portion of the brain, lying at the base of the skull. It is roughly triangular in shape and is continued behind as the spinal cord.
Its work is to control the activities of the internal organs. The peristaltic movement of the alimentary canal, the movements of breathing, the beating of the heart and many other involuntary actions are all regulated from here.
Because these actions cannot be suspended even for a short time, injury to the medulla is generally fatal. The small size of this part is no guide to its importance.
10.5.5 The Three Primary Regions Of The Brain
The three parts you have just met are the ones visible from outside. If the brain is considered as a whole, including the parts hidden within, it is described as having three primary regions — the forebrain, the midbrain and the hindbrain. FC 10.4 shows what each region contains.
Flowchart
FC 10.4 — The Three Primary Regions Of The Brain Brain → branches into three regions: (A) Forebrain → cerebrum, and diencephalon → thalamus and hypothalamus; (B) Midbrain → small tubular part; (C) Hindbrain → cerebellum, pons and medulla oblongata
10.5.5.1 Forebrain
The forebrain has two parts. The first is the cerebrum, or the cerebral hemispheres, already described as the seat of intelligence, memory, consciousness and will-power, and the controller of voluntary actions.
The second is the diencephalon, which lies deep in the centre of the brain and itself contains two important structures.
The thalamus relays pain and pressure impulses to the cerebrum. Incoming messages of this kind are passed through it on their way up, so that the cerebrum can act on them.
The hypothalamus lies just below the thalamus, as its name suggests. It controls body temperature, and it also controls the pituitary gland, the small endocrine gland lying just beneath it. Through the pituitary, this small piece of nervous tissue reaches into the hormonal system as well.
10.5.5.2 Midbrain
The midbrain is a small tubular part connecting the forebrain with the hindbrain. It is concerned with reflexes involving the eyes and the ears — for instance, the turning of the head towards a sudden sound.
10.5.5.3 Hindbrain
The hindbrain has three parts. The cerebellum, already described, coordinates muscular activity and maintains the balance of the body.
The pons is located at the base of the brain, just above the medulla oblongata. It carries impulses from one hemisphere of the cerebellum to the other, and in doing so it coordinates muscular movements on both sides of the body. Its name means bridge, which is exactly what it is.
The medulla oblongata, the third part, controls the activities of the internal organs, including heartbeat and breathing.
10.6 The Spinal Cord
The medulla oblongata does not simply stop at the base of the skull. It continues downwards as the spinal cord, so the brain and the cord are one continuous mass of nervous tissue.
The cord runs down almost the whole length of the backbone and ends at the level of the second lumbar vertebra, in the small of the back. Along the way it lies inside the neural canal — the tunnel formed by the ring-shaped part of each vertebra, one behind the other.
10.6.1 Structure Of The Spinal Cord
Cut the cord across and you see the pattern shown in Fig. 10.6-1. Both gray and white matter are present, exactly as in the brain — but their positions are the other way round.
In the spinal cord the gray matter lies on the inside, arranged in a shape rather like a butterfly with its wings spread. Its projecting corners are called horns. This gray matter contains the cell bodies of the motor neurons and of the association neurons.
The white matter lies outside it, forming the rest of the cord. It contains axons running lengthwise, carrying traffic to and from the brain, and some of these fibres cross from one side of the cord to the other.
Diagram
Fig. 10.6-1 — Transverse Section Of The Spinal Cord T.S. line drawing of the spinal cord: a butterfly-shaped mass of gray matter in the centre with projecting dorsal and ventral horns, surrounded by white matter, and a small central canal at the very centre where the two halves of the gray matter meet; a narrow groove indents the front surface. Label gray matter, dorsal horn, ventral horn, white matter, central canal, dorsal side and ventral side on straight, non-crossing lead lines.
Exactly at the centre of the cord is a fine central canal, which runs its entire length. This canal is continuous with the ventricles of the brain and is filled with the same cerebrospinal fluid.
The fluid does two jobs here. It cushions the cord against shocks, and it serves as the medium through which food materials, waste products and respiratory gases are exchanged with the neurons.
Externally, the cord is covered by the same three membranes as the brain — dura mater, arachnoid and pia mater — running on without a break from those of the brain. Bone outside, membranes within, and fluid innermost: the cord is protected on the same plan as the brain.
10.6.2 Functions Of The Spinal Cord
The spinal cord does three things.
It brings about reflexes below the neck. These are dealt with on the spot, without troubling the brain, which is why the response is so quick. You will see how in Section 10.9.
It conducts sensory impulses from the skin and the muscles up to the brain, so that you become aware of what is happening in the body.
It conducts motor responses from the brain down to the muscles of the trunk and the limbs, so that the brain’s orders reach them.
The last two functions make the cord the main highway between the brain and the body below the neck. The first makes it a decision-making centre in its own right, though for a limited class of actions.
10.7 The Peripheral Nervous System
The peripheral nervous system is the wiring of the body. Its nerves carry impulses to the central nervous system from the sense organs, and away from it to the muscles and glands. Its two halves — somatic and autonomic — are now taken in turn.
10.7.1 The Somatic Nervous System
The somatic nervous system consists of two sets of nerves, named according to the part of the central nervous system they arise from: the cranial nerves and the spinal nerves.
10.7.1.1 Cranial Nerves
Cranial nerves emerge directly from the brain. There are twelve pairs of them, and between them they serve the head, the neck and some of the organs of the chest and abdomen.
They are not all of one kind. Some are sensory, such as the olfactory nerve for the nose, the optic nerve for the eyes and the auditory nerve for the ears. Some are motor, such as the nerves going to the muscles that move the eyeball. Others are mixed, such as those going to and coming from the face and the tongue.
The pattern is worth noticing. Most of the sense organs are crowded into the head, so the nerves serving them are short and leave the brain directly, without travelling down the spinal cord and back.
10.7.1.2 Spinal Nerves
Spinal nerves emerge from the spinal cord and pass out between the vertebrae. There are thirty-one pairs, distributed along the length of the backbone: 8 pairs in the neck region, 12 pairs in the thorax, 5 pairs in the lumbar region, 5 pairs in the sacral region and 1 pair in the coccygeal region.
Every spinal nerve is a mixed nerve, carrying both sensory and motor fibres. Follow one back towards the cord, however, and it does not join the cord as a single trunk. Close to the cord it divides into two roots — a dorsal root at the back and a ventral root at the front, as shown in Fig. 10.7-1.
At that junction the two kinds of fibre part company. The sensory fibres continue into the dorsal root, and the motor fibres into the ventral root. The dorsal root also carries a small oval swelling, the dorsal ganglion, which holds the cell bodies of those sensory neurons.
Both roots then enter the gray matter of the spinal cord and end in the dorsal and ventral horns respectively. The result is a tidy arrangement: everything coming in enters at the back, and everything going out leaves at the front.
Diagram
Fig. 10.7-1 — A Spinal Nerve And Its Two Roots T.S. line drawing of the spinal cord with one spinal nerve shown on the right side: the butterfly-shaped gray matter with dorsal and ventral horns, white matter around it, the dorsal root leaving the dorsal horn and bearing an oval dorsal ganglion, the ventral root leaving the ventral horn, and the two joining to form a single spinal nerve; use arrows to show sensory fibres passing inwards through the dorsal root and motor fibres passing outwards through the ventral root. Label gray matter, white matter, central canal, dorsal root, dorsal ganglion, ventral root and spinal nerve on straight, non-crossing lead lines.
10.7.2 The Autonomic Nervous System
The autonomic nervous system consists of a pair of chains of nerves and ganglia lying on either side of the backbone, with branches running to the internal organs. It controls their involuntary actions, and it does so without waiting for any decision from you.
10.7.2.1 Sympathetic And Parasympathetic Divisions
The system has two parts, the sympathetic and the parasympathetic, and their arrangement in the body is different.
The nerves of the sympathetic system arise from the spinal cord in the region between the neck and the waist. The parasympathetic system is located at two places instead — one anteriorly, in the head and neck region, and the other posteriorly, in the sacral region.
Almost every internal organ receives fibres from both. This matters, because the two parts are in general opposite to each other in their action, as Fig. 10.7-2 and Table 10.6 show. One speeds an organ up while the other slows it down, and the balance between them settles what the organ actually does.
Diagram
Fig. 10.7-2 — The Two Parts Of The Autonomic Nervous System Line drawing with the brain and spinal cord down the centre and the same set of internal organs drawn on both sides — eye, salivary gland, heart, lungs, stomach and pancreas, liver, adrenal gland, intestine and urinary bladder. On the left show the sympathetic chain of ganglia lying close to the spinal cord, with nerves running to each organ and its effect written beside it (pupil dilates, salivation decreases, heart rate increases, bronchi dilate, gastric and pancreatic activity inhibited, glycogen converted into glucose in the liver, adrenaline released, peristalsis inhibited, bladder relaxes). On the right show the parasympathetic nerves arising from the brain and from the sacral region, with the opposite effect written beside each organ. Label sympathetic (prepares the body for action), parasympathetic (prepares the body for relaxation), spinal cord and chain of ganglia lying close to the spinal cord.
Read the last row of the table first, and the other nine fall into place. The sympathetic system prepares the body for violent action against abnormal conditions. Every one of its effects fits that purpose — more blood pumped, wider air passages, more light entering the eye, and digestion set aside for the moment.
The parasympathetic system is concerned with re-establishing normal conditions once the emergency is over. It slows the heart, narrows the air passages again and lets digestion resume.
The sympathetic system does not work alone. Its effects are reinforced by adrenaline, a hormone secreted by the adrenal glands, which sit one on top of each kidney. Nerve and hormone act hand in hand, which is why a state of alarm comes on within a second and then lingers for some minutes. FC 10.5 follows the sequence through.
Flowchart
FC 10.5 — What Happens When You Are Startled Sudden alarming stimulus → brain alerts the sympathetic system → adrenaline released from the adrenal glands into the blood → heart beats faster, bronchi dilate, pupils dilate, liver releases glucose, peristalsis slows → body ready for immediate action → danger passes → parasympathetic system takes over → heart slows, breathing eases, digestion resumes → normal condition restored
10.7.2.2 Emotions And The Autonomic Nervous System
The autonomic nervous system is strongly influenced by the emotions. Grief, anger and fear all act on it, which is why a strong feeling brings a dry mouth, a hammering heart or a loss of appetite. You cannot order these changes, and you cannot easily order them to stop.
Short bursts of this kind do no harm. Emotional stress that continues over a long period is a different matter, and it can contribute to raised blood pressure and to digestive complaints. This is one of the clearest places in the body where the state of the mind and the state of the organs meet.
10.8 Voluntary And Involuntary Actions
You have now seen the whole machinery — neurons, nerves, brain, spinal cord and the two peripheral divisions. The rest of the chapter is about what that machinery actually does, and every action it produces falls into one of two classes.
An action is voluntary if it is performed consciously, by your own will. It is involuntary if it is performed unconsciously, without your deciding anything at all.
The Stimulus Comes First
No action of either kind starts by itself. Every one of them begins with a stimulus, and stimuli come in several forms.
- Physical or mechanical — touch, a prick, pressure.
- Chemical — a substance acting on a receptor, as in taste and smell.
- Radiant — light falling on the eye.
- Thermal — heat or cold.
- Electrical — a current applied to a nerve or muscle.
Whatever its form, the stimulus is picked up by a receptor and converted into an impulse. From that point on the nervous system deals in impulses only, which is why one system can handle light, sound, heat and pressure with the same kind of cell.
10.8.1 Voluntary Actions
A voluntary action begins with a wish. You want to watch a programme, so you switch on the television and press the remote for a particular channel. You pick up an apple and eat it. You want to know the time, so you raise your arm and look at the watch on your wrist.
In each case the command originates in the cerebrum, travels down through the spinal cord and out along somatic nerves to the skeletal muscles. You can begin such an action, alter it midway, or stop it, because the decision remains with you throughout.
10.8.2 Involuntary Actions
Involuntary actions go on without your knowledge and often without your consent. Your heart beats without a pause, your chest rises and falls while you sleep, swallowed food is pushed steadily along the alimentary canal, you shiver when it is very cold and sweat when it is very hot.
Notice what these have in common. Every one of them either protects the body or keeps it running, and every one of them would be done badly if it had to wait for your attention.
Most involuntary actions are reflexes, which are taken up in the next section. But the two words are not interchangeable, and it is worth being exact about this.
All reflexes are involuntary, since none of them is under the control of the will. Not all involuntary actions are reflexes, though: the steady beating of the heart, for instance, continues on its own and is not a response called forth by a particular stimulus from outside.
10.9 Reflex Action
The word reflex comes from the Latin reflexus, meaning reflected or directed back. The name is well chosen. In a reflex, a message travelling in is turned around at once and sent straight back out, rather like light bouncing off a mirror.
Two examples show the range. If a particle of dust falls into your eye, tears flush out at once to wash it away — a glandular secretion. If your hand touches a hot iron, it is snatched back before you can think — a muscular movement, shown in Fig. 10.9-1.
Every reflex follows this pattern. It is set off by some kind of sensory stimulation, and it ends either in a muscular action or in a glandular secretion. Nothing else is possible, because muscles and glands are the only effectors the body has.
Diagram
Fig. 10.9-1 — Withdrawal Of The Hand On Touching A Hot Iron Line drawing of a person seen from the side, with one arm outstretched and the fingers touching a hot iron; show the sensory nerve running from the fingers up the arm to the spinal cord, the spinal cord within the vertebral column, and the motor nerve running back from the cord to the muscle of the upper arm, which is drawn contracting; use arrows on each nerve to show the direction of the impulse. Label receptor in the skin, sensory nerve, spinal cord, motor nerve and effector muscle on straight, non-crossing lead lines.
10.9.1 Reflexes Compared With Voluntary Actions
A reflex and a voluntary action may end in the same movement, yet they are quite different events. Table 10.7 sets out where they differ.
The third row is the one that explains all the others. A command that need not travel to the brain and back is issued far more quickly, and speed is exactly what a protective action requires.
10.9.2 Types Of Reflexes
Reflexes are of two kinds — natural, or inborn, reflexes and conditioned, or acquired, reflexes. The difference lies in whether the reflex was there from birth or had to be built up during life.
10.9.2.1 Natural (Inborn) Reflexes
A natural reflex is one for which no previous experience or learning is required. It is inborn, that is, inherited from the parents, and a newborn baby already has it.
Some natural reflexes are protective. Blinking, coughing and sneezing all keep something harmful away from a delicate surface.
Others provide functional efficiency, that is, they keep the ordinary business of the body running smoothly. Salivation, swallowing and peristalsis are of this kind.
10.9.2.2 Conditioned (Acquired) Reflexes
A conditioned reflex is one that develops during a lifetime, through experience or learning. It is still involuntary and automatic when it finally works, but it had to be acquired first.
Think of what happens when you catch the smell of a dish you know and like: your mouth waters at once. Now notice the catch. If you had never eaten that dish before, nothing would happen. The response depends on a memory.
Salivation can also be a natural reflex — saliva flows when food is actually in the mouth and you begin to chew. In the case just described, however, the food had not been tasted at all. The sight or the smell was enough, because the brain had stored the taste and now acted on it without your knowledge.
A conditioned reflex, then, is an involuntary and automatic response brought about by a previously learned experience. The stimulus that triggers it need have no natural connection with the response at all.
Diagram
Fig. 10.9-2 — Pavlov’s Experiment To Demonstrate A Conditioned Reflex Four small line-drawing panels arranged in a block and marked A, B, C and D. A: a bowl of food in front of a dog, with saliva dripping — caption ‘food, dog salivates’. B: a bell being rung, dog unaffected — caption ‘ringing of bell, dog does not salivate’. C: bell being rung while food is presented together, dog salivating — caption ‘ringing of bell along with presentation of food, dog salivates’. D: bell being rung alone after training, dog salivating — caption ‘ringing the bell, not followed by food, dog salivates’.
Conditioned reflexes are common in other animals too. A dog that has been stoned will run off at the mere sight of a person bending down as if to pick something up, and the strange acts performed by trained circus animals are largely conditioned reflexes.
There is one more thing worth noticing. A voluntary action, repeated consciously enough times with the same stimulus, can itself turn into an involuntary one. Actions that become automatic in this way are exactly what conditioned reflexes are.
10.9.2.3 Natural And Conditioned Reflexes Compared
10.9.3 Common Reflexes In Humans
Natural (Inborn) Reflexes
- Knee-jerk — sit with one leg resting across the other and give a sharp tap just below the knee-cap of the relaxed leg; the lower leg swings forward on its own.
- Quick closing of the eyelids when an object suddenly approaches the eye, or when a strong beam of light is flashed across it.
- Withdrawal of the hand, or of any part of the body, when it is suddenly pricked.
- Peristaltic reflexes — chyme is driven along the small intestine by waves of peristalsis. When a portion of the intestine is distended with chyme, the stretching of its wall is the stimulus, and the contraction that pushes the food along is the response.
- The coughing reflex, when swallowed food enters the wind pipe.
- The sneezing reflex, when an irritant enters the nose.
Conditioned (Acquired) Reflexes
Most of the daily habits and skills you have picked up are conditioned reflexes.
- Standing up when the teacher enters the classroom.
- Tying your shoe-laces while talking, without knowing which lace you crossed over which.
- Playing a musical instrument.
- Using the keys of a keyboard while working on a computer.
- Giving a hand signal automatically as you turn your cycle to the right.
- Applying the brakes at once if someone steps in front of your bicycle.
Each of these had to be learnt with full attention at first. Practice has made them automatic, which frees your attention for something else — you can talk while your fingers tie the laces.
10.9.4 The Nervous Pathway In A Reflex
A reflex is of value only if it is fast. A hand that leaves a hot surface half a second late has already been burnt. Speed, therefore, decides how the pathway is built: it must be as short as possible.
In a simple reflex, the impulses are not conducted up and down the spinal cord at all. They leave the cord at the same level at which they entered it. The pathway may involve two neurons, or three.
Where two neurons are involved, these are a sensory neuron, whose cell body lies in the dorsal root, and a motor neuron, whose cell body lies in the ventral part of the gray matter and whose fibre runs out in the ventral root.
The impulse from the sense receptor travels in along the sensory fibres of the dorsal root — sensory fibres are also called afferent, meaning carrying towards. As soon as it reaches the motor neuron in the cord, a response impulse is flashed back out through the same spinal nerve to the effector muscle, which reacts.
Often a third neuron is included. This is the intermediate, connecting, association or relay neuron, and it lies inside the spinal cord between the sensory fibre and the motor neuron.
That last step explains a common experience. After the instant response, the sensation is carried to the brain through the long interconnecting fibres running lengthwise in the cord. The brain is not left out — it is simply informed afterwards.
10.9.5 The Reflex Arc
The route just traced has a name.
The path may be written out in full as: stimulus → receptor in the sense organ → afferent (sensory) nerve fibre → central nervous system, that is the spinal cord or brain → efferent (motor) nerve fibre → muscle, which contracts, or gland, which secretes. FC 10.6 sets out the same path in the form used in Fig. 10.9-3.
Flowchart
FC 10.6 — Path Of A Reflex Arc Stimulus → receptor → sensory (afferent) neuron → spinal cord or brain, with an association neuron in the centre → motor (efferent) neuron → effector — muscle contracts or gland secretes → response
The parts named in that path are as follows.
- Receptor — a group of specialised cells, in contact with the terminal endings of nerve cells, which respond to the stimulus and convert it into an impulse in a sensory neuron.
- Sensory neuron — the neuron that receives the impulse through its endings, which are in contact with a receptor cell, and carries it to the spinal cord.
- Central nervous system — the region in the spinal cord or brain where an incoming sensory impulse generates an outgoing motor impulse. In the centre of the arc there may be an association neuron between the sensory and the motor neuron.
- Motor neuron — carries the impulse generated in the central nervous system out to the effector organ.
- Effector — the organ, a muscle or a gland, that responds to the motor impulse.
Diagram
Fig. 10.9-3 — The Nervous Pathway In A Spinal Reflex Line drawing of a transverse section of the spinal cord with the pathway of a reflex drawn across it: at the right, a block of skin containing a receptor with a sharp object touching it; a sensory neuron running from the receptor through the dorsal root, its cell body in the dorsal root ganglion, into the dorsal horn of the gray matter; an association neuron within the gray matter; a motor neuron running from the ventral horn out through the ventral root and along the spinal nerve to a block of muscle drawn at the right as the effector. Show the butterfly-shaped gray matter, the surrounding white matter and the central canal, and mark the direction of the impulse with arrows at every stage. Label white matter, gray matter, central canal, dorsal root, dorsal root ganglion, sensory neuron, association neuron, motor neuron, ventral root, spinal nerve, receptor in skin and effector muscle on straight, non-crossing lead lines.
10.9.6 Complex Reflex Action
Not every reflex is settled at a single level of the cord. A complex reflex action involves neurons at several different levels of the spinal cord at once.
Suppose you are walking along a road in the dark and your foot comes down on a coiled piece of rope. You leap aside instantly, in the fear that it is a snake. Almost every skeletal muscle in the body takes part in that single movement — legs, trunk, arms and neck all act together.
The response is still automatic and still involuntary. It is simply that the arc, instead of running across one level of the cord, has spread over many.
10.10 Summary
Coordination is the working together of the different parts of the body at the right moment and in the right measure. In humans it is carried out by two systems — the nervous system, which acts through electrical impulses and works fast, and the hormonal system, which acts through chemicals in the blood and works more slowly.
The nervous system keeps us informed about the outside world through the sense organs, enables us to remember, think and reason, controls and harmonises voluntary muscular activities, and regulates involuntary activities such as breathing and the beating of the heart.
The neuron, or nerve cell, is the structural and functional unit of the nervous system. Its three main parts are the cell body, the dendrites and the axon.
The cell body, or cyton, has a well-defined nucleus in granular cytoplasm along with Nissl granules. The centrosome is absent, and mature nerve cells therefore cannot divide.
Dendrites are branched and carry impulses towards the cell body. The axon is single and long and carries the impulse away from it.
In most neurons the axon is covered by an insulating myelin sheath, over which lies the thin neurilemma. The gaps in the myelin sheath are the nodes of Ranvier, and side branches of the axon are collaterals.
A stimulus is any change that produces a change in the body; the resulting change in the organism is the response. Receptors receive stimuli and set up impulses; effectors — muscles and glands — act on receiving them.
A resting nerve fibre is polarised, with more sodium ions outside the membrane. On stimulation, sodium ions move in and the region is depolarised; the sodium pump then drives them out again, using energy from ATP, and the region is repolarised.
Conduction of a nerve impulse is therefore a wave of depolarisation followed by repolarisation. Nothing travels along the fibre — neither substance, nor electrons, nor ions.
Electricity passes through a wire at about 150,000 kilometres per second, but a nerve impulse travels at a maximum of about 100 metres per second.
A synapse is the point of contact between the axon endings of one neuron and the dendrites of another, separated by a fine gap. The impulse crosses it chemically: acetylcholine is released, sets up a new impulse in the next neuron, and is then destroyed by an enzyme.
Neurons are of three types — sensory neurons carrying impulses from receptors to the central nervous system, motor neurons carrying them out to effectors, and association neurons connecting the two inside the brain and spinal cord.
A nerve is a bundle of axons of separate neurons enclosed in a tubular sheath. Nerves are of three kinds: sensory, motor and mixed. A ganglion is an aggregate of the cell bodies of nerve cells.
The nervous system has two major divisions — the central nervous system, consisting of the brain and the spinal cord, and the peripheral nervous system, consisting of the nerves that emerge from and enter into them. The peripheral system is further divided into the somatic nervous system, serving the skeletal muscles, and the autonomic nervous system, serving the internal organs.
The adult brain weighs about 1.35 kg, roughly 2 per cent of the body weight. It lies within the cranium and is wrapped in three meninges — the outer dura mater, the middle arachnoid and the inner pia mater.
Cerebrospinal fluid fills the space between the membranes, the ventricles of the brain and the central canal of the spinal cord. It cushions the brain and cord against shocks.
The cerebrum is the largest part of the brain, divided into two hemispheres joined by the corpus callosum. Its outer cortex is gray matter folded into gyri and sulci, which increases surface area and accommodates more nerve cells; the inner region is white matter.
The cerebrum is the seat of intelligence, memory, consciousness and will-power, and it controls all voluntary actions.
The cerebellum maintains the balance of the body and coordinates muscular activity. The impulse to perform a muscular act arises in the cerebrum, not in the cerebellum.
The medulla oblongata controls the activities of the internal organs — heartbeat, breathing, peristalsis and other involuntary actions. Injury to it is generally fatal.
Taken as a whole, the brain has three primary regions. The forebrain contains the cerebrum and the diencephalon, in which the thalamus relays pain and pressure impulses to the cerebrum and the hypothalamus controls body temperature and the pituitary gland. The midbrain is a small tubular part concerned with reflexes of the eyes and ears. The hindbrain contains the cerebellum, the pons, which carries impulses between the two hemispheres of the cerebellum, and the medulla oblongata.
The spinal cord runs from the medulla down to the second lumbar vertebra within the neural canal. Here the arrangement of matter is the reverse of that in the brain: gray matter lies inside and white matter outside.
The spinal cord brings about reflexes below the neck, conducts sensory impulses from the skin and muscles to the brain, and conducts motor responses from the brain to the muscles of the trunk and limbs.
Twelve pairs of cranial nerves arise from the brain and thirty-one pairs of spinal nerves from the spinal cord — 8 in the neck, 12 in the thorax, 5 lumbar, 5 sacral and 1 coccygeal.
Every spinal nerve is a mixed nerve joined to the cord by two roots: the dorsal root, which carries sensory fibres and bears the dorsal root ganglion, and the ventral root, which carries motor fibres.
The autonomic nervous system has two parts of opposite action. The sympathetic system prepares the body for violent action against abnormal conditions and is also stimulated by adrenaline from the adrenal glands; the parasympathetic system re-establishes normal conditions afterwards. The autonomic system is strongly influenced by the emotions.
Voluntary actions are initiated by a willing thought, are commanded from the brain and involve only muscles. Reflexes are initiated by a stimulus, are commanded mostly from the spinal cord and the autonomic nervous system, and involve muscles as well as glands.
All reflexes are involuntary, but not all involuntary actions are reflexes.
Reflexes are of two kinds. Natural, or inborn, reflexes are inherited and need no learning — blinking, coughing, sneezing, salivation, swallowing and peristalsis. Conditioned, or acquired, reflexes are built during life by experience, as Pavlov showed with a dog that came to salivate at the sound of a bell alone.
A reflex arc is the shortest route an impulse can take from a receptor to an effector: stimulus → receptor → sensory neuron → central nervous system → motor neuron → effector → response. In a simple reflex the impulse leaves the cord at the level at which it entered, and the sensation reaches the brain only afterwards.
A complex reflex action involves neurons at several different levels of the spinal cord, so that many muscles act together.
Flowchart
FC 10.7 — Components Of The Nervous System At A Glance Nervous system → branches into two: (A) Central nervous system → brain → cerebrum (conscious and unconscious mind, intelligence, memory, reason), cerebellum (balance of the body), medulla oblongata (heartbeat, breathing, peristalsis); and spinal cord (reflexes below the neck, conducts impulses to and from the brain). (B) Peripheral nervous system → branches into two: somatic nervous system → cranial nerves (12 pairs) and spinal nerves (31 pairs); autonomic nervous system (chains of ganglia beside the backbone) → sympathetic (nerves between neck and waist; accelerates heart beat, dilates pupil, constricts blood vessels, releases glucose from liver) and parasympathetic (from head and neck region and below the sacrum; restores normal body condition)
10.11 Key Terms
- Acetylcholine — the chemical released at an axon ending that carries the impulse across a synapse to the next neuron.
- Adrenaline — the hormone from the adrenal glands that stimulates the sympathetic nervous system and prepares the body for action.
- Arachnoid — the thin, delicate middle meninx, which forms a web-like cushion around the brain and spinal cord.
- Association neuron — a neuron inside the brain or spinal cord that connects a sensory neuron to a motor neuron.
- Autonomic nervous system (ANS) — the part of the peripheral nervous system that controls the involuntary actions of internal organs.
- Axon — the single long process of a neuron, which carries the impulse away from the cell body.
- Cell body (cyton) — the widest part of a neuron, containing the nucleus and granular cytoplasm.
- Central canal — the fine canal running the length of the spinal cord, filled with cerebrospinal fluid.
- Central nervous system (CNS) — the brain and the spinal cord together.
- Cerebellum — the part of the brain below and behind the cerebrum that maintains balance and coordinates muscular activity.
- Cerebral hemispheres — the right and left halves into which the cerebrum is divided.
- Cerebrospinal fluid (CSF) — the watery fluid around and inside the brain and spinal cord that cushions them against shocks.
- Cerebrum — the largest part of the brain, the seat of intelligence, memory, consciousness and will-power.
- Collaterals — side branches given off by some axons along their length.
- Conditioned (acquired) reflex — an automatic, involuntary response built up during life through experience or learning.
- Coordination — the working together of the different parts of the body at the right moment and in the right measure.
- Corpus callosum — the sheet of nerve fibres that connects the two cerebral hemispheres and transfers information between them.
- Cranial nerves — the twelve pairs of nerves that emerge directly from the brain.
- Dendrites — the branched projections of the cell body that carry impulses towards it.
- Depolarisation — the loss of the resting charge on a nerve membrane as sodium ions move inwards.
- Diencephalon — the part of the forebrain lying deep in the centre of the brain, containing the thalamus and the hypothalamus.
- Dorsal root — the root at the back by which the sensory fibres of a spinal nerve enter the spinal cord.
- Dorsal root ganglion — the oval swelling on the dorsal root that holds the cell bodies of the sensory neurons.
- Dura mater — the tough, fibrous outermost meninx.
- Effectors — the muscles and glands that act on receiving an impulse, by contracting or by secreting.
- Forebrain — the primary region of the brain containing the cerebrum and the diencephalon.
- Ganglion — an aggregate of the cell bodies of nerve cells, from which nerve fibres arise or into which they enter.
- Gray matter — nervous tissue made up mainly of the cell bodies of neurons.
- Gyri — the raised folds on the surface of the cerebrum.
- Hindbrain — the primary region of the brain containing the cerebellum, the pons and the medulla oblongata.
- Hypothalamus — the part of the diencephalon that controls body temperature and the pituitary gland.
- Impulse — a wave of electrical disturbance travelling along a nerve fibre.
- Involuntary actions — actions performed unconsciously, without any decision by the will.
- Knee-jerk — the natural reflex in which the lower leg swings forward when the relaxed leg is tapped below the knee-cap.
- Medulla oblongata — the lowest part of the brain, which controls heartbeat, breathing and other activities of the internal organs.
- Meninges — the three membranes that cover the brain and the spinal cord.
- Midbrain — the small tubular primary region of the brain concerned with reflexes of the eyes and ears.
- Mixed nerves — nerves containing both sensory and motor fibres.
- Motor nerves — nerves containing only motor fibres, carrying impulses from the central nervous system to effectors.
- Motor neuron — a neuron that carries impulses from the central nervous system out to a muscle or gland.
- Myelin sheath — the white, fatty, insulating covering around most axons.
- Natural (inborn) reflex — an automatic response present from birth, needing no previous experience or learning.
- Nerve — a bundle of axons of separate neurons enclosed in a tubular sheath.
- Nervous system — the system of nerve cells that coordinates the body by carrying messages as electrical impulses.
- Neurilemma — the thin outermost covering lying over the myelin sheath of an axon.
- Neuron — the nerve cell; the structural and functional unit of the nervous system.
- Neurotransmitters — the chemicals stored in the swollen axon endings, by which an impulse is passed across a synapse.
- Nissl granules — the dark granules found in the cytoplasm of the cell body of a neuron.
- Nodes of Ranvier — the regular gaps in the myelin sheath along an axon.
- Parasympathetic nervous system — the part of the autonomic nervous system that re-establishes normal conditions after an emergency.
- Peripheral nervous system (PNS) — all the nerves that emerge from and enter into the brain and spinal cord.
- Pia mater — the innermost meninx, richly supplied with blood, clinging to the surface of the brain and cord.
- Polarised state — the resting condition of a nerve fibre, with a positive charge outside the membrane and more sodium ions there.
- Pons — the part of the hindbrain above the medulla that carries impulses between the two hemispheres of the cerebellum.
- Receptors — the specialised cells that receive a stimulus and set up an impulse in a sensory neuron.
- Reflex action — an automatic, immediate and involuntary action brought about by a stimulus.
- Reflex arc — the shortest route taken by an impulse from a receptor to an effector.
- Repolarisation — the restoring of the resting charge on a nerve membrane after an impulse has passed.
- Response — the change produced in an organism as a result of a stimulus.
- Sensory nerves — nerves containing only sensory fibres, bringing impulses from receptors to the central nervous system.
- Sensory neuron — a neuron that carries impulses from a receptor to the central nervous system.
- Sodium pump — the mechanism that drives sodium ions back out of a nerve fibre, using energy from ATP.
- Somatic nervous system — the part of the peripheral nervous system that conveys information to the skeletal muscles.
- Spinal nerves — the thirty-one pairs of mixed nerves that emerge from the spinal cord.
- Stimulus — any agent or change, outside or inside the body, that produces a change in an organism or its parts.
- Sulci — the grooves between the folds on the surface of the cerebrum.
- Sympathetic nervous system — the part of the autonomic nervous system that prepares the body for violent action.
- Synapse — the point of contact between the axon endings of one neuron and the dendrites of another, separated by a fine gap.
- Synaptic cleft — the narrow gap at a synapse, across which the neurotransmitter passes.
- Thalamus — the part of the diencephalon that relays pain and pressure impulses to the cerebrum.
- Ventral root — the root at the front by which the motor fibres of a spinal nerve leave the spinal cord.
- Ventricles — the fluid-filled central spaces of the brain.
- Voluntary actions — actions performed consciously, by one’s own will.
- White matter — nervous tissue made up mainly of the axons of neurons, white because of their myelin sheaths.
10.12 Points To Ponder
Mature nerve cells have lost the power to divide, while skin and blood cells divide throughout life. What might a nervous system gain from cells that never replace themselves, if what you remember depends on the connections those particular cells have made?
In a spinal reflex the response is over before the sensation reaches the brain. What would be lost if the brain were never informed at all, and the arc simply ended with the muscle?
In the brain the gray matter lies outside and the white matter inside; in the spinal cord the arrangement is reversed. Given what each kind of matter does, can you suggest why the two organs might be built in opposite ways?
Almost every internal organ receives two sets of nerves that act against each other. Why might a pair of opposing controls suit the body better than a single control that is simply turned up or down?
A conditioned reflex frees your attention for something else, which is why you can talk while your hands work. Can you think of a situation in which a well-learnt automatic response would be exactly the wrong thing to do?
A nerve impulse travels at a maximum of about 100 metres per second. What difficulties would this speed create for a very large animal, and how might such an animal’s nervous system be arranged to cope with them?
10.13 Objective Questions
- The structural and functional unit of the nervous system is the: (1 mark)
- nephron
- neuron
- ganglion
- nerve
- The organelle absent from a mature nerve cell is the: (1 mark)
- nucleus
- mitochondrion
- centrosome
- ribosome
- The regular gaps along the myelin sheath of an axon are called: (1 mark)
- synaptic clefts
- sulci
- nodes of Ranvier
- ventricles
- The number of pairs of spinal nerves in a human being is: (1 mark)
- 12
- 21
- 31
- 33
- Body temperature is controlled by the: (1 mark)
- thalamus
- hypothalamus
- pons
- medulla oblongata
- In a transverse section of the spinal cord: (1 mark)
- gray matter lies outside and white matter inside
- gray matter lies inside and white matter outside
- only white matter is present
- only gray matter is present
- The chemical released at the axon endings during transmission across a synapse is: (1 mark)
- adrenaline
- acetylcholine
- ATP
- cerebrospinal fluid
The following are assertion–reason questions. Choose your answer from the codes given below. (1) A is True and R is False (2) A is False and R is True (3) Both A and R are True (4) Both A and R are False
Assertion (A): All reflexes are involuntary actions. Reason (R): A reflex is set off by a stimulus and not by a willing thought. (1 mark)
Assertion (A): The cerebellum is the largest part of the human brain. Reason (R): The cerebellum maintains the balance of the body and coordinates muscular activity. (1 mark)
Assertion (A): A nerve impulse travels far more slowly than electricity in a wire. Reason (R): In a nerve fibre, electrons move along the length of the axon. (1 mark)
Match each structure in Column A with its correct description in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Corpus callosum | i. Middle layer of the meninges |
| b. Thalamus | ii. Carries impulses between the two hemispheres of the cerebellum |
| c. Arachnoid | iii. Connects the two cerebral hemispheres |
| d. Pons | iv. Holds the cell bodies of sensory neurons |
| e. Dorsal root ganglion | v. Relays pain and pressure impulses to the cerebrum |
- Fill in each blank with a suitable word. (4 marks)
- The gaps between the folds on the surface of the cerebrum are called …………… .
- Sodium ions are driven back out of a nerve fibre by the …………… , using energy from ATP.
- The membranes covering the brain and the spinal cord are together called the …………… .
- Nerves that contain both sensory and motor fibres are called …………… nerves.
- State whether each statement is true or false. (4 marks)
- The impulse to perform a muscular act originates in the cerebellum.
- Cerebrospinal fluid fills the ventricles of the brain and the central canal of the spinal cord.
- Every spinal nerve is a sensory nerve.
- The parasympathetic nervous system slows down the heart beat.
- Name the following. (5 marks)
- The gap between the axon ending of one neuron and the dendrite of the next.
- The bony case that encloses the brain.
- The part of the brain whose injury generally results in death.
- The root of a spinal nerve that carries motor fibres.
- The hormone that stimulates the sympathetic nervous system.
- Choose the odd one out in each set and give a reason for your choice. (4 marks)
- Dura mater, pia mater, gray matter, arachnoid
- Cerebrum, cerebellum, medulla oblongata, spinal cord
- Sneezing, coughing, blinking, cycling
- Dendrite, axon, cyton, ganglion
10.14 Check Your Concepts
Define a nerve impulse. (2 marks)
Name the three meninges in order from the outside inwards. (2 marks)
State two functions of cerebrospinal fluid. (2 marks)
Give a reason: a nerve cell that has been destroyed is not replaced by a new one. (2 marks)
Give a reason: the folded surface of the cerebrum is an advantage to us. (2 marks)
Differentiate between a sensory nerve and a motor nerve, stating the basis of difference. (3 marks)
State the three functions of the spinal cord. (3 marks)
Explain what happens at a synapse when an impulse reaches the end of an axon. (3 marks)
Differentiate between the sympathetic and the parasympathetic nervous systems, stating the basis of difference, with reference to any two organs. (4 marks)
Explain with an example how a conditioned reflex is developed during a lifetime. (4 marks)
What would happen if the enzyme that destroys the neurotransmitter at a synapse stopped working? (4 marks)
Why does a person who has drunk a large amount of alcohol walk unsteadily? Name the part of the brain involved and state its normal function. (4 marks)
Describe the conduction of a nerve impulse along a nerve fibre, from the resting state to the return of the resting state. (5 marks)
What is the difference between a reflex action and a voluntary action? Give four points of difference, with one example of each. (5 marks)
10.15 Figure-Based Questions
- Study Fig. 10.2-1, which shows the generalised structure of a neuron. (5 marks)
- Name the parts marked in the figure.
- State one function of the myelin sheath.
- In which direction does the impulse travel through this cell?
- Redraw the figure and label the cyton, the axon, the dendrites, the myelin sheath and a node of Ranvier.
- Fig. 10.9-3 shows the nervous pathway in a spinal reflex. (5 marks)
- Name the phenomenon depicted and define it.
- Name the three neurons taking part, in the order in which the impulse passes through them.
- Give the technical term for the whole route shown.
- Why is this route so short, and what advantage does its shortness give?
- Fig. 10.5-2 shows the human brain in median (sagittal) section. (4 marks)
- Identify the largest part shown and state two of its functions.
- Name the structure that connects the two cerebral hemispheres.
- Name the part that lies just above the medulla oblongata and state its function.
- Name the part of the brain that continues below as the spinal cord.
- Fig. 10.6-1 shows a transverse section of the spinal cord. (3 marks)
- Name the matter that lies in the centre and state what it contains.
- How does this arrangement differ from that in the cerebrum?
- Name the small canal at the very centre and state what fills it.
- Fig. 10.7-1 shows a spinal nerve joined to the spinal cord by its two roots. (3 marks)
- Name the root that bears a ganglion and state what the ganglion contains.
- Name the fibres carried by the other root.
- Why is every spinal nerve described as a mixed nerve?