Chapter 16: Skeleton and Movements
In the 1780s, in the Italian city of Bologna, a physician named Luigi Galvani was working with the dissected hind legs of frogs. The legs were dead. The frog was dead. Nothing about them should have moved again. Yet whenever a metal instrument touched the exposed nerve while a spark or a second metal was involved, the leg muscles gave a sharp twitch, as though the animal were about to leap off the table.
Galvani repeated the experiment many times, and other scientists across Europe repeated it too. He concluded that some kind of electricity was carried by the nerve into the muscle, and he called it “animal electricity.” His interpretation was later corrected by others, but the central observation held: a muscle can be made to contract by an electrical signal reaching it, and contraction is the one thing a muscle really does.
That twitching frog leg is the starting point of this chapter. Every time you lift a bag, blink, swallow, or stand still on a moving bus, thousands of tiny protein machines inside your muscle fibres are pulling on one another, and hard bones and carefully shaped joints are turning that pull into useful movement. In this chapter you will follow that pull from the arrival of a nerve impulse right down to the sliding of filaments, and then outwards to the 206 bones and the joints that decide exactly how your body is allowed to move.
Diagram
Fig. 16.1 – Opening illustration: an athlete mid-sprint with the arm and leg muscles and underlying bones shown in cutaway, and an inset magnified view of a single muscle fibre with its striations
Keep these questions in mind as you read. Each one of them is answered somewhere in this chapter, and you will find the answer marked for you at the exact place where the explanation appears.
16.1 Movements and Locomotion
Look closely at any living organism and you will find that something is always moving. Movement is one of the most obvious signs of life, and it appears in an enormous range of forms — from the slow streaming of protoplasm inside a single cell, to the wave-like peristalsis that pushes food down the alimentary canal, to a person walking, running or swimming.
All of these count as movement. But not all of them shift the organism from one place to another. A movement may or may not end up in locomotion, that is, in the displacement of the whole organism. This simple distinction is where our study of the muscular and skeletal systems begins.
Internal and External Movements
Movements may be internal or external. Internal movements take place inside the body and are usually invisible from outside — the beating of the heart, the churning of the stomach, the flow of blood through vessels, the streaming of cytoplasm within a cell.
External movements involve parts of the body that we can see moving — the swing of an arm, the blinking of an eyelid, the movement of the jaw while chewing, or the turning of the eyeball as you read this line. Try sorting the examples in the first paragraph into these two groups yourself, and add a few of your own.
Voluntary and Involuntary Movements
Movements may also be classified by whether we control them consciously. A voluntary movement is one you decide to make, such as raising your hand. An involuntary movement happens without your conscious decision — you cannot choose to stop your intestines from pushing food along.
In human beings, three types of muscle tissue bring about these movements, and each type matches the kind of control the body needs over it.
Smooth muscles bring about involuntary movements such as the peristaltic movements in the alimentary canal, and the constriction and dilation of blood vessels.
Contraction and relaxation of the heart is controlled by cardiac muscles.
Voluntary movements of limbs, head, trunk, eyes and similar parts are controlled by striated muscles.
What Locomotion Means
Locomotion is a special kind of movement. It is the change in locus — the position — of the whole body of a living organism from one place to another place.
An organism does not move about without reason. Locomotion is undertaken in the search for food, shelter, a mate or a breeding ground, and to escape from an enemy. Each of these is a survival need, which is why the ability to move the whole body has been so strongly favoured throughout the animal kingdom.
Four Types of Locomotory Movement
There are four basic types of locomotory movements found throughout the animal kingdom. They differ in the cell structure that generates the force, and the sequence from the simplest to the most complex is set out in FC 16.1.
Flowchart
FC 16.1 – Types of locomotory movement: Locomotory movement → branches into (i) Amoeboid movement, by pseudopodia, e.g. leucocytes; (ii) Ciliary movement, by cilia, e.g. ciliated epithelium; (iii) Whorling movement, by flagella, e.g. sperms; (iv) Muscular movement, by muscles with the help of bones and joints
Amoeboid movement is performed by pseudopodia, e.g. leucocytes. Pseudopodia are temporary finger-like extensions of the cell that flow forward and drag the rest of the cell along.
Ciliary movement is performed by cilia, e.g. ciliated epithelium. Cilia are short, numerous, hair-like projections that beat in a coordinated rhythm. In Paramecium, cilia help in the passage of food through the cytopharynx.
Whorling movement is performed by flagella, e.g. sperms. A flagellum is longer than a cilium and usually occurs singly, lashing the fluid behind the cell to drive it forward.
Muscular movement is performed by muscles, with the help of bones and joints. This is the type used by human beings and by most large animals, and it is the type this chapter will study in the greatest detail.
The three different cell-level mechanisms are compared side by side in Fig. 16.5.
Diagram
Fig. 16.5 – Three cellular locomotory mechanisms side by side: an amoeboid cell extending a pseudopodium, a ciliated cell with rows of short cilia, and a sperm cell with a single long flagellum
Notice that all four of these are locomotory mechanisms, but the body performs a great many movements that move nothing anywhere. This gives us a rule worth remembering: all locomotions are movements, but all movements are not locomotion.
How Muscles Reach the Bones
A skeletal muscle cannot move a bone unless it is firmly joined to it. Skeletal muscles are attached to the bones by tendons and help in the movement of the parts of the skeleton.
Tendons are inelastic thick bands of collagen fibres. The word “inelastic” is the key to their function. If tendons stretched like rubber, part of every contraction would be wasted in stretching the tendon instead of moving the bone. Because they hardly stretch at all, the full pull of the muscle is transmitted straight to the bone.
Movement and locomotion, then, are never the work of muscles alone. They are the combined action of bones, joints and skeletal muscles: the bones act as rigid levers, the joints decide the direction and range of the movement, and the muscles supply the force.
For exams, this section is usually tested through short “distinguish between” questions (movement and locomotion; voluntary and involuntary) and through match-the-following items pairing a locomotory type with its structure and example. Learn the four types along with both their structure and their example, since questions often supply one and ask for the other.
16.2 Location and Structure of Skeletal Muscles
Where a Muscle Actually Lies
A muscle that moves a bone is usually not placed on that bone at all. The major part of the muscle which moves a bone usually does not lie on the same bone, but is located on the bone atop — that is, on the bone lying above it in the limb.
The biceps and triceps are a good example. Both of them move the forearm, yet both are located in the upper arm. Only their tendons run down past the elbow to grip the forearm bones.
At any joint, two types of bones are present: a stationary bone and a movable bone. When the muscle contracts, it cannot pull both bones equally — one bone is held firm by other muscles and by the body’s weight, so the pull is expressed as movement of the other.
Origin, Insertion and Belly
The two ends of a skeletal muscle are therefore not equivalent, and each is given its own name. The end of the muscle attached to the stationary bone is called the origin, while the opposite end attached to the movable bone is called the insertion. The middle thick part of the muscle is called the belly. These three parts are marked in Fig. 16.44.
Diagram
Fig. 16.44 – A biceps muscle of the upper arm shown attached to bone, labelled: origin (on the stationary bone), belly (thick middle part), insertion (on the movable forearm bone), and tendon at each end
Why Large Muscles Are Spindle-Shaped
All the fibres in a muscle do not extend from end to end. Instead, the number of fibres is greatest in the middle of the muscle and falls off towards the two ends. This is why the belly is thick while the ends taper.
As a result, large muscles are most often fusiform in shape — that is, spindle-shaped, thick in the middle and narrow at both ends. The shape is not accidental. A wide middle packs in the maximum number of contractile fibres, giving the muscle its power, while the narrow tapering ends converge neatly into a tendon that can be anchored to a small area of bone.
Three Roles a Muscle Can Play
A single movement is rarely produced by one muscle working alone. On the basis of the part they play in bringing about a movement, striated muscles are of three types, as summarised in FC 16.2.
Flowchart
FC 16.2 – Types of skeletal muscles on the basis of movement: Striated muscles → branches into (i) Agonists (prime movers): bring initial movement of the part, e.g. biceps; (ii) Antagonists: bring the action opposite to that of prime movers, e.g. triceps; (iii) Synergists: assist prime movers, e.g. brachialis assists biceps
Agonists, also called prime movers, bring about the initial movement of a part. When you bend your elbow, the biceps is the agonist.
Antagonists bring about the action opposite to that of the prime movers. For the bending of the elbow, the triceps is the antagonist, because its action straightens the same joint.
Synergists assist the prime movers. The brachialis, for instance, assists the biceps. A synergist adds force to the main movement and steadies the joint so that the pull of the prime mover is not wasted in unwanted directions.
The same muscle can change its role depending on the movement being performed. When you straighten the elbow instead of bending it, the triceps becomes the agonist and the biceps becomes the antagonist. The names describe a role in a particular movement, not a fixed property of the muscle.
Questions from this section commonly ask you to identify the origin and insertion of a named muscle, or give a movement and ask which muscle is the agonist and which the antagonist. Reason it out from which bone moves, rather than memorising pairs blindly.
16.3 Working of Skeletal Muscles
Muscles Work in Pairs
Generally muscles work in pairs and produce opposite actions. The biceps, which are flexors, bring about flexion or folding of the elbow joint, while the triceps, which are extensors, bring about extension of the same joint.
The muscles which bring about opposite actions are called antagonistic muscles. If one member of a pair is capable of bending the joint by pulling on the bones, the other member is capable of straightening the same joint, also by pulling. Thus the triceps and biceps of the upper arm are antagonistic to each other. Their opposing action across the elbow is shown in Fig. 16.45.
Diagram
Fig. 16.45 – The elbow in two positions: on the left, biceps contracted and triceps relaxed producing flexion; on the right, triceps contracted and biceps relaxed producing extension; label biceps, triceps, humerus, radius and ulna
The two members of an antagonistic pair are not equally powerful. In an antagonistic pair of muscles, one member is much stronger than the other — for example, the biceps is stronger than the triceps. This makes sense functionally, because bending the arm to lift a load demands far more force than straightening it, where the weight of the limb itself gives some help.
Why a Muscle Can Only Pull
The fundamental characteristic of muscle is contraction. A muscle can shorten and it can then return to its resting length, but it cannot actively lengthen itself and drive a bone away. Therefore, a muscle can only pull and not push the bone.
This single limitation explains why antagonistic pairs exist at all. If a muscle could push, one muscle would be enough to work a joint in both directions. Since it can only pull, a second muscle must be placed on the opposite side of the joint to pull the bone back again.
Some Important Antagonistic Muscles
Antagonistic pairs are found all over the body, and each pair is named after the kind of opposite movement it produces.
Flexor and Extensor: A flexor muscle on contraction results in bending or flexion of a joint, e.g. biceps. An extensor on contraction results in straightening or extension of a joint, e.g. triceps.
Abductor and Adductor: An abductor muscle moves a body part away from the body axis, e.g. the deltoid muscle of the shoulder moves the arm away from the body. An adductor moves a body part towards the body axis, e.g. the latissimus dorsi of the shoulder moves the arm near to the body.
Pronator and Supinator: A pronator turns the palm downward. A supinator turns the palm upward.
Levator and Depressor: A levator raises a body part. A depressor lowers the body part.
Protractor and Retractor: A protractor moves a part forward. A retractor moves a part backward.
Sphincters: These are circular muscles present in the inner wall of the anus, stomach and similar openings, and they work for closure and opening. A sphincter is unusual in that it does not have a separate antagonist muscle — when the ring of muscle contracts the opening closes, and when it relaxes the opening widens on its own.
In board and NEET papers this section usually appears as a “which of the following is NOT an antagonistic pair” question, or as an assertion–reason item on why muscles occur in pairs. Both are answered by the same idea: a muscle only pulls, so every movement needs an opposite puller.
16.4 Mechanism of Muscle Contraction
Where Contraction Actually Happens
You are aware that both flexion and extension take place by contraction of skeletal muscles. But how do these muscles contract and bring about movement and locomotion? Striated muscles are specifically designed to bring about vigorous contractions, and the design lies deep inside the fibre.
Recall the ultra-microscopic structure of skeletal muscle you have studied in animal tissues. A muscle is made of muscle fibres, each fibre contains long myofibrils, and each myofibril is divided along its length into repeating units. The contractile unit of muscles is called the sarcomere, and it contains the contractile proteins actin and myosin.
Everything a muscle does, from lifting a weight to holding your head upright, is the sum of what happens inside millions of these sarcomeres at the same instant. So the two proteins inside the sarcomere must be examined before the mechanism can be understood.
Myosin Filament
Each myosin filament is a polymerized protein. Many monomeric proteins called meromyosins constitute one thick filament, packed together so that the filament is thick along its whole length.
A myosin molecule consists of two heavy chains — heavy meromyosin, or HMM — coiled around each other forming a double helix. One end of each of these chains is projected outwardly, away from the body of the filament. This projecting end is known as the cross bridge, and it is folded into a globular protein mass called the myosin head.
Two light chains are associated with each head, making a total of four light chains, also called light meromyosin or LMM. The myosin head has a special ATPase activity — that is, it carries an enzyme site that can split ATP to produce energy. Myosin contributes 55% of muscle proteins.
The arrangement of the tails and heads within one thick filament is shown in Fig. 16.2. Notice that the myosin tails are arranged to point towards the centre of the sarcomere, while the heads point to the sides of the myofilament band. Because of this arrangement, the heads on the two halves of the filament pull in opposite directions and draw both ends of the sarcomere inwards together.
Diagram
Fig. 16.2 – Myosin filament: label tail, heads and a single myosin head, showing that tails point towards the centre of the sarcomere while heads project towards the sides of the myofilament band
Actin Filament
Actin is also a complex type of contractile protein. Unlike myosin, it is not a single kind of molecule: it consists of three different components, all visible in Fig. 16.3.
Diagram
Fig. 16.3 – Actin filament: label the double-stranded F actin backbone made of G actin beads, the two tropomyosin strands winding along it, and the troponin complexes spaced along the tropomyosin
i. F actin: It forms the backbone of the actin filament. It is a double stranded protein. Each strand is composed of polymerized G actin molecules, so the backbone looks like two strings of beads twisted around each other. One ADP molecule is attached to each G actin molecule, and it is at these points that the myosin heads will later bind.
ii. Tropomyosin: The actin filament contains two additional protein strands that are polymers of tropomyosin molecules. Each strand is loosely attached to an F actin. In the resting stage, tropomyosin physically covers the active binding sites for myosin of the actin strand. This is the reason a resting muscle does not contract even though actin and myosin lie side by side within the sarcomere.
iii. Troponin: It is a complex of three globular proteins, which are attached approximately \(\frac{2}{3}\) rd distance along each tropomyosin molecule. It has affinity for actin, tropomyosin and calcium ions. The troponin complex is believed to attach the tropomyosin to the actin. The strong affinity of troponin for calcium ions is believed to initiate the contraction process.
The Sliding Filament Theory
The sliding filament theory was put forth by H.E. Huxley and A.F. Huxley. It is also called the walk along theory or the Ratchet theory, because the myosin head appears to walk step by step along the actin filament, gripping and releasing like a ratchet.
According to this theory, the interaction between actin and myosin is the basic cause of muscular contraction. Actin filaments are interdigitated with myosin filaments — that is, the two sets of filaments overlap and lie between each other, like the crossing of the fingers of two hands.
The head of the myosin is joined to the actin backbone by a cross bridge, forming a hinge joint. The head is not free to swing about in any direction at this hinge; its movement is limited to a tilt. This movement is an active process which requires the use of ATP.
The Cross-Bridge Cycle
The myosin head contains ATPase activity. It can derive energy by the breakdown of the ATP molecule, and this energy can be used for the movement of myosin heads.
During the contraction process, the myosin heads get attached to the active site of the actin filaments and pull them inwardly, so that the actin filaments slide over the myosin filaments. This results in the contraction of the muscle fibre.
This attaching, pulling and releasing is not a single event but a repeating cycle, and one turn of the cycle moves the actin only a very short distance. The cycle repeats many times over, so the filaments are drawn in step by step, which is why it is called a ratchet mechanism. The four stages of the cycle are shown in Fig. 16.4 and listed in order in FC 16.3.
Diagram
Fig. 16.4 – Cyclic events in muscle contraction: four stages around a circle, showing (1) myosin head carrying ADP and Pi attached to actin, (2) head tilting and releasing ADP as it pulls actin, (3) fresh ATP binding and the head detaching, (4) ATP hydrolysis re-cocking the head ready for the next cycle
Flowchart
FC 16.3 – Cross-bridge cycle: myosin head carrying ADP and Pi binds to the uncovered active site of actin → head tilts and pulls the actin filament inwards, ADP and Pi released → a fresh ATP molecule binds to the head → head detaches from actin → ATPase of the head hydrolyses ATP to ADP and Pi, re-cocking the head → back to the start, if calcium ions are still present
Note carefully that ATP is needed at two separate points — to energise the head before it pulls, and to detach the head afterwards. Remembering the second of these will explain a great deal about muscle fatigue, and about what happens to muscles after death.
This section is one of the most heavily tested in the chapter. Expect diagram-labelling on the actin and myosin filaments, sequence-ordering questions on the cross-bridge cycle, and assertion–reason items on whether the filaments shorten during contraction.
16.5 Physiology of Muscle Relaxation
The Muscle at Rest
When the muscle is relaxed, the active sites remain covered with the tropomyosin and troponin complex. Due to this, myosin cannot interact with the active site of actin, and therefore contraction cannot occur.
Relaxation, then, is not simply the absence of activity. It is a locked state, actively maintained by proteins sitting over the binding sites. Something must arrive from outside the muscle to unlock it.
Before following that signal, three names are worth fixing in mind. The sarcolemma is the plasma membrane of the muscle fibre; the sarcoplasm is the cytoplasm within the fibre; and the sarcoplasmic reticulum is the specialised endoplasmic reticulum of the fibre, which stores calcium ions.
Arrival of the Impulse
When an action potential, that is an impulse, comes to the muscle through the motor end plate, it spreads throughout the sarcolemma of the myofibril. The motor end plate is the point where the branch of a motor nerve meets the muscle fibre, so this is the place where a decision made in the nervous system is handed over to the muscle.
The transverse tubules of the sarcoplasmic reticulum then release a large number of calcium ions into the sarcoplasm. These transverse tubules, usually called T tubules, are inward folds of the sarcolemma that dip deep into the fibre.
Calcium Uncovers the Active Sites
These calcium ions interact with troponin molecules. This interaction inactivates the troponin–tropomyosin complex, which leads to a change in the structure of tropomyosin.
As a result, tropomyosin gets detached from the active site of the actin (F actin) filament. Thus the active site becomes uncovered. This is the moment at which the lock is opened, and it explains why troponin’s strong affinity for calcium ions is described as initiating the whole contraction process.
Now the head of the myosin cleaves the ATP and derives energy.
Formation of the Actomyosin Complex
Using this energy, myosin gets attached to the uncovered active site of actin, and this results in the formation of the actomyosin complex.
The myosin heads are now tilted backwards and pull the attached actin filament inwardly. This results in the contraction of the muscle fibres, and the sarcomere is seen to shorten as the thin filaments move deeper in between the thick ones. The difference between the relaxed and the contracted sarcomere is shown in Fig. 16.6.
Diagram
Fig. 16.6 – Relaxation and contraction of muscle: one sarcomere drawn twice, relaxed above and contracted below, labelling sarcomere, thick filament (myosin filament), thin filament (actin filament) and the two Z lines moving closer together
The complete pathway, from the nerve impulse to the sliding of the filaments, is summarised in FC 16.4. Learn this sequence in order, since the steps are meaningless if rearranged — calcium must be released before troponin can change, and the sites must be uncovered before any head can bind.
Flowchart
FC 16.4 – Mechanism of muscle contraction: Impulse from motor nerve → action potential develops in T tubule → T tubule releases \(Ca^{++}\) ions into sarcoplasm → conformational change in troponin–tropomyosin complex → active sites of actin filaments become uncovered → myosin head breaks up ATP to derive energy → myosin head gets attached to active sites of actin, forming actomyosin complex → myosin head tilts and pulls actin filaments towards them → actin filaments slide over myosin and contraction occurs
Sequence-ordering questions on this pathway are common in NEET, and a favourite multiple-choice item asks for the precise role of calcium in contraction — the correct answer is that it binds to troponin and changes its shape so that the actin binding sites are exposed, not that it breaks cross bridges or carries the impulse across the junction.
16.6 Relaxation of Muscle Fibres
Every Step Runs Backwards
During relaxation, all the events occur in the reverse direction. Nothing new is invented — the same structures simply return to the state they were in before the impulse arrived.
When stimulation is terminated, the actomyosin complex is broken down and the myosin head gets detached from the actin filaments. This process involves the use of ATP. At the same time, calcium ions return back into the store from which they came.
This is also an active process that uses energy. The calcium ions do not drift back on their own; they are pumped back into the sarcoplasmic reticulum against a concentration gradient, and pumping always costs ATP.
Due to the disappearance of calcium ions from the sarcoplasm, the troponin–tropomyosin complex is restored again. This complex covers the active sites of the actin filament.
Due to this, the interaction between actin and myosin ceases to occur, and the actin filaments return back to their original position. This results in muscular relaxation. The complete reverse sequence is set out in FC 16.5.
Flowchart
FC 16.5 – Relaxation of a muscle fibre: stimulation stops → actomyosin complex breaks down, using ATP → myosin heads detach from actin → \(Ca^{++}\) ions pumped back into the sarcoplasmic reticulum, using ATP → troponin–tropomyosin complex restored → active sites of actin covered again → actin and myosin can no longer interact → actin filaments slide back to their original position → muscle relaxes
Relaxation Is Not Simply Switching Off
Like contraction, relaxation is also an active process. This point is easy to miss and is very often examined.
There are two separate places where relaxation spends ATP. One molecule of ATP must bind to each myosin head before the head will let go of the actin. Further ATP is spent by the pumps that carry the calcium ions back into the sarcoplasmic reticulum.
Two examination points follow directly from this section. First, remember that ATP is needed for relaxation as well as for contraction — assertion–reason questions frequently test exactly this. Second, remember that rigor mortis is stiffening caused by a lack of ATP, not by an excess of stimulation.
16.7 Properties of Muscles on Electrical Stimulation
Much of what is known about how muscles behave comes from experiments in which a muscle is given brief electrical shocks and its response is recorded. By changing the strength of the shock, or the interval between successive shocks, several properties of muscle can be brought out one by one.
A. Single Muscle Twitch
A muscle contraction initiated by a single brief stimulation is called a single muscle twitch. It is the simplest possible response a muscle can give.
A twitch occurs in 3 stages: a latent period of no contraction, a contraction period, and a relaxation period. The record of these three stages is shown in Fig. 16.46.
Diagram
Fig. 16.46 – Myogram of a single muscle twitch: tension plotted against time, with the moment of stimulus marked and the three stages labelled — latent period, contraction period, relaxation period
B. Summation
If the muscle is stimulated before the end of the twitch, it generates greater tension. That is, summation or addition of effect takes place, because the second contraction begins while some tension from the first is still present.
Repeated stimuli will therefore produce increasing strength of contraction. This stepwise rise is known as the staircase phenomenon.
C. Tetanus
If stimulation is very rapid and frequent, the muscle does not have time to relax at all between one stimulus and the next. It remains in a state of sustained contraction called tetanus.
D. Refractory Period
Immediately after one stimulus, the muscle fibre cannot respond to another stimulus, however strong that stimulus may be. This resting or refractory period is about 0.02 seconds.
The refractory period sets an upper limit on how rapidly a fibre can be made to respond, and it gives the fibre a brief moment in which to restore its membrane before the next impulse arrives.
E. Threshold Stimulus
For a muscle fibre to contract, a certain minimum strength or intensity of stimulus is required. This is called the threshold stimulus.
F. All or None Principle
A stimulus below threshold will not result in contraction. A threshold stimulus will result in contraction, and this contraction leads to maximum force. A higher stimulus will not increase the force of contraction. In other words, a muscle fibre contracts either fully or not at all. This is the “all or none” principle, and all types of muscle fibres and nerve fibres obey this law.
G. Oxygen Debt
During strenuous exercise, the muscle’s oxygen supply rapidly becomes insufficient to maintain oxidative phosphorylation of the respiratory substrate. Oxidative phosphorylation is the oxygen-requiring stage of respiration in which most of the ATP is normally produced, so when oxygen runs short, this route can no longer keep pace with demand.
At this stage, muscles contract anaerobically and accumulate lactic acid produced by anaerobic glycolysis. In this pathway, glucose is broken down only as far as lactic acid:
\[C_6H_{12}O_6 \rightarrow 2C_3H_6O_3\]
Lactic acid produces less ATP and is toxic. It causes tiredness, pain and muscle cramps, which is why hard exercise leaves the muscles aching and prone to cramping.
During recovery, oxygen consumption of the muscle far exceeds that in the resting state. This extra oxygen consumed during recovery is called the oxygen debt of the muscle. The whole sequence is summarised in FC 16.6.
Flowchart
FC 16.6 – Oxygen debt: strenuous exercise → oxygen supply becomes insufficient → oxidative phosphorylation cannot be maintained → anaerobic glycolysis takes over → lactic acid accumulates → tiredness, pain and cramps → exercise stops → extra oxygen consumed during recovery (oxygen debt) → lactic acid oxidised aerobically and creatine phosphate and ATP restored → muscle returns to resting state
For examinations, fix the numerical value of the refractory period, keep the three stages of the twitch in their correct order, and be ready to state the all-or-none principle precisely — the common error is to say that a stronger stimulus gives a stronger contraction of the same fibre, which the principle expressly denies.
16.8 Skeletal System
Bone, Joint and Muscle as a Lever
Before leaving muscles behind, it is worth looking at the bone–joint–muscle unit as a simple machine. We can compare this unit with a lever, where the joint acts as the fulcrum and the respective muscle generates the force required to move the bone associated with the joint.
In the language of levers, three things must be identified: the fulcrum, which is the fixed pivot; the effort, which is the force applied; and the resistance, which is the load to be moved. All three classes of lever occur in the human body, and they are compared in Fig. 16.47.
Diagram
Fig. 16.47 – The three classes of lever in the human body: (a) Class I at the joint between the skull and the first vertebra, (b) Class II with the body raised on the toes, (c) Class III at the elbow during flexion of the forearm; mark Resistance (R), Fulcrum (F) and Effort (E) on each
Class I lever: The joint between the first vertebra and the occipital condyle of the skull is an example of a first class lever. The force is directed towards the joint, which is the fulcrum; contraction of the back muscles provides the force, while the part of the head that is raised acts as the resistance.
Class II lever: The human body raised on the toes is an example of a second class lever. The toe acts as the fulcrum, the contracting calf muscles provide the force, and the raised body acts as the resistance.
Class III lever: Flexion of the forearm at the elbow exhibits a lever of class III. The elbow joint acts as the fulcrum, the radius and ulna provide the resistance, and the contracting biceps muscle provides the force for the movement.
Class III levers are by far the commonest arrangement in the body. They do not multiply force — in fact the muscle must pull harder than the load it lifts — but they multiply speed and range of movement, so a small shortening of the biceps swings the hand through a wide arc very quickly.
Exoskeleton and Endoskeleton
Multicellular animals need support to maintain body structure. Without a firm framework, a large soft body would collapse under its own weight and would have nothing for its muscles to pull against.
Various groups of organisms show various supportive structures, placed either inside the body, or outside the body, or both inside as well as outside. You have studied that these skeletal structures are called exoskeleton when present on the outer surface of the body, and endoskeleton when they are present inside the body.
Bones and cartilage form the major endoskeletal components. Exoskeletal components change from lower to higher groups of animals. These include chitinous structures, nails, horns, hooves, scales, hair and many others; you may add to the list from animals you have observed.
What the Skeleton Is Made Of
Can you imagine life without a skeletal system? Our skeletal system is made up of cartilage and bone, which together form the framework of the body.
The two differ in an important way. Cartilage is slightly pliable, so it can bend a little without breaking, while bones have a hard matrix that resists bending altogether. The body uses each where its property is needed — a rigid matrix where load must be carried, and a pliable one where a little give is useful, as at the tip of the nose or the ends of the ribs.
What Bones Do for the Body
Bones form the framework of our body and thus provide the shape which gives us our identity. They protect delicate organs, thus helping in the smooth functioning of the body — the skull encloses the brain and the thoracic cage shields the heart and lungs.
Joints between bones help in movement and locomotion, and bones provide a firm surface for the attachment of muscles. Without such a firm anchorage, the pull of a contracting muscle would have nothing to act upon.
Bones also do two jobs that have nothing to do with support. They are reservoirs of calcium, from which the body withdraws calcium when the level in the blood falls, and they are one important site for haemopoiesis, that is, the formation of blood cells.
The Two Divisions of the Human Skeleton
The endoskeleton of an adult human consists of 206 bones, which can be grouped into two principal divisions: the axial skeleton and the appendicular skeleton. The whole framework is shown in Fig. 16.7.
Diagram
Fig. 16.7 – Human skeleton in anterior and posterior view, with the bones of the axial skeleton and the appendicular skeleton distinguished and the major bones labelled
The bones of the axial skeleton lie along the longitudinal axis of the human body — that is, along the central line running from the head down to the hip.
The bones of the appendicular skeleton include the bones of the fore limb, the hind limb and the girdles. Girdles are the bones that connect the limbs to the axial skeleton, so they are the link between the two divisions. This basic division is set out in FC 16.7.
Flowchart
FC 16.7 – Divisions of the human skeleton: Human endoskeleton (206 bones) → branches into Axial skeleton (bones lying along the longitudinal axis of the body) and Appendicular skeleton → the appendicular skeleton further branches into girdles (which connect the limbs to the axial skeleton), fore limb bones and hind limb bones
Questions from this section usually ask for the functions of the skeleton, for the difference between exoskeleton and endoskeleton with examples, or for the total number of bones in an adult human — a figure worth remembering exactly.
16.9 Group of Skeleton
The 206 bones of the adult human body are not a random collection. They fall into two clear groups, and within each group the bones are arranged in an order that can be followed from the head downwards. This section takes each group in turn.
A. Axial Skeleton
The axial skeleton is the central column of the body. It includes the skull, the hyoid bone, the ear ossicles, the vertebral column and the thorax, all of which are grouped in FC 16.8.
Flowchart
FC 16.8 – Axial skeleton: Axial skeleton → Skull (cranium and face) → Hyoid → Ear ossicles → Vertebral column → Thorax (sternum and ribs)
The Skull
Made up of 22 bones, the skull is located at the superior end of the vertebral column — that is, at the top of the backbone.
It consists of two main sets of bones: the cranium, also called the brain box, and the facial bones. The bones are joined by fixed or immovable joints, except for the lower jaw.
This is worth pausing over. A box built of separate plates locked together is far stronger than a single sheet of the same weight, because a crack in one plate is stopped at the join instead of running across the whole vault. The one exception is the lower jaw, which must move for chewing and speaking, and so is left free.
Cranium
The cranium is made up of four median bones and two paired bones.
Frontal bone: This is a median, unpaired bone that forms the forehead, the roof of the orbit — the eye socket — and most of the anterior part of the cranium. It is connected to the two parietals, the sphenoid and the ethmoid bone.
Parietal bones: These are paired bones. They form the roof of the cranium and a greater portion of the sides of the cranium. The lateral view in Fig. 16.8 shows how they meet the frontal bone in front and the occipital behind.
Diagram
Fig. 16.8 – Lateral view of the skull, labelling frontal bone, sphenoid bone, nasal bone, ethmoid bone, lacrimal bone, zygomatic bone, maxilla, mandible, parietal bone, temporal bone, occipital bone and mastoid process
Temporal bones: These are paired bones situated laterally, just above the ear on either side. Each temporal bone gives out a zygomatic process that joins the zygomatic bone to form the zygomatic arch.
Just at the base of the zygomatic process is the mandibular fossa, a depression for the mandible, that is the lower jaw bone. This forms the only movable joint of the skull. This bone also harbours the ear canal, which directs sound waves into the ear.
The processes of the temporal bones provide points for the attachment of various muscles of the neck and tongue.
Occipital bone: Present at the back of the head, this single bone forms the posterior part and most of the base of the cranium.
The inferior part of this bone shows the foramen magnum, the large opening through which the medulla oblongata connects with the spinal cord. On either side of the foramen magnum are two prominent protuberances called the occipital condyles. These fit into the corresponding depressions present in the 1st vertebra, and they are visible in the ventral view of the skull in Fig. 16.9.
Diagram
Fig. 16.9 – Ventral view of the skull, labelling maxilla, palatine, zygomatic, sphenoid, vomer, temporal, parietal, occipital, condyle and foramen magnum
Sphenoid bone: This is a median bone present at the base of the skull that articulates with all the other cranial bones and holds them together, rather like a keystone.
This butterfly-shaped bone has a saddle-shaped region called the sella turcica. In this hypophyseal fossa, the pituitary gland is lodged. You will read more about the sella turcica in the chapter on hormonal co-ordination.
Ethmoid bone: This median bone is spongy in appearance. It is located anterior to the sphenoid and posterior to the nasal bones. It contributes to the formation of the nasal septum and is the major supporting structure of the nasal cavity.
Sutures of the Skull
In this chapter you are going to learn about sutures, a type of immovable joint. In the skull there are many sutures present; four prominent ones are given below.
- Coronal suture: Joins the frontal bone with the parietals.
- Sagittal suture: Joins the two parietal bones.
- Lambdiodal suture: Joins the two parietal bones with the occipital bone.
- Lateral or squamous sutures: Join the parietal and temporal bones on the lateral side.
Facial Bones
Fourteen facial bones give the characteristic shape to the face. Growth of the face stops at the age of 16. The facial bones are as under, and all of them can be traced in the anterior view of the skull in Fig. 16.11.
Diagram
Fig. 16.11 – Anterior view of the skull, labelling frontal bone, sphenoid bone, temporal bone, inferior nasal concha, maxilla, mandible, parietal bone, nasal bone, ethmoid bone, lacrimal bone, zygomatic bone and vomer
Nasals: Paired bones that form the bridge of the nose.
Maxillae: The upper jaw bones. These are paired bones that join with all the facial bones except the mandible. The upper row of teeth is lodged in these.
Palatines: Paired bones that form the roof of the buccal cavity, which is the same thing as the floor of the nasal cavity.
Zygomatic bones: Commonly called the cheek bones. You have read about the zygomatic arch earlier in this chapter.
Lacrimal bones: The smallest of the facial bones. These bones form the medial wall of each orbit. They have a lacrimal fossa that houses the lacrimal sacs. These sacs gather tears and send them to the nasal cavity — which is why a person with watering eyes also develops a running nose.
Inferior nasal conchae: They form part of the lateral wall of the nasal cavity. These help swirl and filter the air before it passes to the lungs.
Vomer: A median, roughly triangular bone that forms the inferior portion of the nasal septum.
Mandible: A median bone that forms the lower jaw. It is the largest and strongest facial bone, and the only movable bone of the skull. It has a curved horizontal body and two perpendicular branches, the rami. These help in the attachment of muscles. The lower row of teeth is lodged in it.
Hyoid Bone
The hyoid bone is a ‘U’ shaped bone that does not articulate with any other bone. It is suspended from the temporal bone by ligaments and muscles.
It is located between the mandible and the larynx. It has a horizontal body and paired projections called horns, as shown in Fig. 16.12. It provides a site for the attachment of some tongue muscles and the muscles of the neck and pharynx.
Diagram
Fig. 16.12 – Hyoid bone, labelling greater horns (greater cornua), lesser horns (lesser cornua) and body, shown in position above the thyroid cartilage and trachea
Ear Ossicles
Three tiny bones, namely the malleus, incus and stapes, together called the ear ossicles, are present in each middle ear. Their shapes and the way they link one to the next are shown in Fig. 16.13.
Diagram
Fig. 16.13 – Ear ossicles, labelling malleus (head, neck, handle, anterior process, lateral process), incus (body, short limb, long limb, articulation surfaces) and stapes (head, anterior limb, posterior limb, base)
Vertebral Column
The human backbone or vertebral column is made up of a chain of irregular bones called vertebrae.
It consists of 33 vertebrae during childhood. In adults, five sacral vertebrae fuse to form one sacrum and four coccygeal vertebrae fuse to form a single coccyx, and thus the total number of bones is 26.
There are five types of vertebrae in the human spine, namely 7 cervical (neck), 12 thoracic (chest), 5 lumbar (abdominal), 5 sacral (hip region, fused in adults forming 1 sacrum) and 4 coccygeal (fused to form the vestigial tail bone called the coccyx).
Though the vertebrae vary in size, shape and processes from one region to another, they exhibit a similar basic plan, which is described a little further on.
Curvatures of the Spine
Feel your own spine. Is it straight or curved?
There are four curvatures in the human spine. The cervical and lumbar curves are secondary and convex, whereas the thoracic and sacral curvatures are primary and concave. All four are marked in the lateral view in Fig. 16.14.
Diagram
Fig. 16.14 – Vertebral column, lateral view, labelling 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, sacrum, coccyx, intervertebral discs, intervertebral foramina, and the cervical (secondary), thoracic (primary), lumbar (secondary) and sacral (primary) curves
The curvatures help in balancing the body in an upright position, absorb shocks while walking, and also protect the vertebrae from fracture. A straight rod would transmit every jolt of a footstep directly up to the skull; a gently curved column flexes slightly at each step and takes the shock out of it.
Typical Vertebra
Each vertebra has a prominent central body called the centrum. The centra of human vertebrae are flat in the antero-posterior aspect. Hence human vertebrae are said to be amphiplatyan.
From either side of the centrum are given out two short, thick processes which unite to form an arch-like structure called the neural arch, lying posterior to the centrum.
The neural arch forms the vertebral foramen, which surrounds the spinal cord. The vertebral foramina of all the vertebrae together form a continuous neural canal. The spinal cord, along with blood vessels and a protective fatty covering, passes through this neural canal.
At the point where the two processes of the centrum meet, the neural arch is drawn out into a spinous process called the neural spine.
From the base of the neural arch, two articulating processes called zygapophyses are given out on either side. The anterior ones are called superior and the posterior ones inferior zygapophyses.
In a stack of vertebrae, the inferior zygapophyses of one vertebra articulate with the superior zygapophyses of the next vertebra. This allows slight movement of the vertebrae without allowing them to slip off — a compromise between flexibility and safety that is the whole design problem of a backbone.
At the junction of the zygapophyses, a small opening is formed on either side of the vertebra, called the intervertebral foramen, that allows the passage of a spinal nerve.
From the base of the neural arch, lateral processes are given out, called transverse processes. The neural arch, neural spine and transverse processes are meant for the attachment of muscles. All these parts are marked in the two views in Fig. 16.15.
Diagram
Fig. 16.15 – Basic plan of a vertebra in posterior (end) view and lateral view, labelling centrum (body), pedicle, lamina, neural arch, vertebral foramen (neural canal), spinous process (spine), transverse process, superior articular process and inferior articular process
Let us now study the modifications seen in the vertebrae in different regions of the vertebral column.
Atlas Vertebra
This is the ring-like 1st cervical vertebra. It consists of anterior and posterior arches. It does not have a centrum or a spinous process.
Its transverse processes and transverse foramina are large. The vertebral foramen is large and is divided into two parts by a transverse ligament. The spinal cord passes through the anterior compartment.
The anterior zygapophyses are replaced by facets for attachment with the occipital condyles of the skull, and this articulation forms the ‘Yes joint’ — the joint that lets you nod. Its ring-like form is clear in Fig. 16.16.
Diagram
Fig. 16.16 – Atlas vertebra, labelling anterior arch, anterior portion, posterior portion, facet for articulation with the occipital condyle, facet for articulation with the odontoid process of the axis, foramen for vertebral artery and posterior tubercle representing the spine
Axis Vertebra
This is the second cervical vertebra. The centrum of this vertebra gives out a tooth-like odontoid process.
This process fits into the anterior portion of the vertebral foramen of the atlas vertebra, forming a pivot joint, also called the ‘No joint’ — the joint that lets you shake your head sideways. Fig. 16.17 shows the odontoid process projecting upwards.
Diagram
Fig. 16.17 – Axis vertebra, labelling odontoid process, facet for the transverse ligament, facet for atlas, foramen for vertebral artery, pedicle, transverse process, body, lamina and spinous process
Cervical Vertebrae
Vertebrae number 3 to 6 are called typical cervical vertebrae. They show a short centrum and a bifid spinous process — that is, a spine split into two at its tip.
The transverse processes of these vertebrae are reduced, each having a large vertebrarterial canal at its base for the passage of the vertebral artery. This canal is a feature you will not find in vertebrae of any other region.
The 7th cervical vertebra is called the vertebra prominens. It is the largest cervical vertebra, and its neural spine is straight. You can feel it as the bump at the base of the back of your neck.
Thoracic Vertebrae
These are twelve in number and are found in the chest region. The centrum of a thoracic vertebra is heart shaped, and all its processes are well developed.
Except for vertebrae number 11 and 12, the transverse processes of the other thoracic vertebrae show facets for attachment with the ribs. These facets, marked in Fig. 16.18, are the surest way of recognising a thoracic vertebra.
Diagram
Fig. 16.18 – Thoracic vertebra, labelling spine (spinous process), lamina, transverse process, facet for tubercle of rib, vertebral foramen, pedicle, facet for head of rib and heart-shaped centrum
Lumbar Vertebrae
There are five lumbar vertebrae. These are well developed vertebrae that exhibit all the characters of a typical vertebra. The centrum is kidney shaped, and it is markedly larger than that of the vertebrae above, because these vertebrae carry the greatest share of the body’s weight.
Diagram
Fig. 16.19 – Lumbar vertebra, labelling body, pedicle, vertebral foramen, transverse process, lamina, spinous process and superior articular process
Sacrum and Coccyx
The sacrum is a triangular bone formed by the fusion of five sacral vertebrae. It is located in the pelvic cavity between the two hip bones.
The anterior end of the sacrum is broad and the posterior end is narrow. The vertebral foramina that are formed by the fusion of the vertebrae can be seen on it, and reduced neural spines can be observed projecting from its dorsal aspect. It gives strength to the pelvic girdle.
The coccyx is formed by the fusion of four coccygeal vertebrae. It is reduced and does not show vertebral foramina or spinous processes. The transverse processes of the coccygeal vertebrae are also reduced. It is a triangular bone. Both bones are shown together in Fig. 16.20.
Diagram
Fig. 16.20 – Sacrum and coccyx, labelling promontory, articular process for L5, articulating surface for the intervertebral disc (body), surface for articulation with the left ilium, foramina for the passage of nerves, the fused segments S1 to S5, and the coccygeal segments C1 to C4
Thoracic Cage
The thoracic cage consists of the twelve thoracic vertebrae, which have already been discussed, twelve pairs of ribs, and the breast bone, the sternum. Together these enclose the space in which the heart and lungs lie, as seen in Fig. 16.21.
Diagram
Fig. 16.21 – Thoracic cage in anterior view, labelling sternum with its manubrium, body and xiphoid process, the ribs, and the vertebral column behind
Sternum
The sternum is a flat, narrow bone, around 15 cms in length. It is placed medially in the anterior thoracic wall, that is, the chest region. It consists of three distinct parts — the manubrium, the body and the xiphoid process.
The manubrium shows two notches on the antero-lateral side for attachment with the clavicle of each side. It also shows two notches on each of the lateral sides for the attachment of the first two pairs of ribs.
The body of the sternum is a flat bone that shows five notches on the lateral aspect, which are meant for the direct or indirect attachment of ribs. The ribs are attached to the sternum by means of cartilaginous extensions called costal cartilages.
The xiphoid process is the lowermost part of the sternum. It is cartilaginous initially and gets ossified in adults. It provides space for the attachment of the diaphragm and the abdominal muscles. The three parts are labelled in Fig. 16.22.
Diagram
Fig. 16.22 – Sternum in anterior view, labelling suprasternal or jugular notch, clavicular notch, manubrium, sternal angle, body, xiphoid process and the attached ribs
Ribs
A rib is a ‘C’ shaped bone that is attached to the respective thoracic vertebra on the dorsal side. Twelve pairs of ribs are attached to the twelve thoracic vertebrae.
For this attachment, the posterior ends of the ribs have two protuberances, namely the head and the tubercle. The head of the rib attaches to the facet formed by the demifacets of adjacent thoracic vertebrae at the base of the transverse processes. The tips of the transverse processes of these vertebrae also have facets for the attachment of ribs, where the tubercles of the ribs are attached. These features are marked in Fig. 16.23.
Diagram
Fig. 16.23 – A rib, labelling head (articulates with vertebral bodies), neck, tubercle (facet articulates with transverse process of vertebra), angle, shaft, costal groove, inferior border and depression for costal cartilage
On the ventral side, the ribs may or may not attach to the sternum. Depending on their attachment, ribs are classified into three types, as set out in FC 16.9.
Flowchart
FC 16.9 – Classification of ribs by ventral attachment: Twelve pairs of ribs → (i) True ribs: first seven pairs, attached directly to the sternum by their costal cartilages; (ii) False ribs: pairs 8, 9 and 10, whose costal cartilages attach to rib number 7 on either side and not directly to the sternum; (iii) Floating ribs: last two pairs, with no ventral connection
i. True ribs: The first seven pairs of ribs are attached directly to the sternum by means of their costal cartilages.
ii. False ribs: The costal cartilages of ribs number 8, 9 and 10 are attached to rib number 7 on either side, and not directly to the sternum. Hence these are called false ribs.
iii. Floating ribs: The last two pairs of ribs have no ventral connection. Hence they are called floating ribs.
The space between the ribs is called the intercostal space. The ribs provide space for the attachment of the intercostal muscles, and it is the contraction of these muscles that swings the ribs during breathing.
B. Appendicular Skeleton
As mentioned earlier in this chapter, the appendicular skeleton consists of the bones of the limbs and the girdles. Its parts are set out in FC 16.10.
Flowchart
FC 16.10 – Appendicular skeleton: Appendicular skeleton → Pectoral girdle (clavicle, scapula) → Bones of forelimb (humerus → radius and ulna → carpals → metacarpals → phalanges); and Pelvic girdle (hip bones) → Bones of lower limb (femur, patella → tibia and fibula → tarsals → metatarsals → phalanges)
Pectoral Girdle
The pectoral girdle, also called the shoulder girdle, attaches the forelimb skeleton to the axial skeleton. There are two pectoral girdles, and each consists of a shoulder blade or scapula and a collar bone or clavicle, as seen in Fig. 16.24.
Diagram
Fig. 16.24 – Pectoral girdle, labelling clavicle, acromion, coracoid process, glenoid cavity, scapula, body of scapula and head of humerus
Clavicle: It is an ‘s’ shaped slender bone. One end of the clavicle is attached to the acromion process of the scapula. The other rounded end, called the sternal end, attaches to the manubrium of the sternum. This connects the upper arm skeleton to the axial skeleton.
Scapula: It is a large, flat, triangular bone that occupies the posterior chest wall, extending from the second to the seventh ribs. It is attached to the axial skeleton by muscles and tendons.
At its lateral angle, the scapula bears a concave socket called the glenoid cavity. The head of the humerus, the upper arm bone, fits into the glenoid cavity.
Two processes arise from the scapula: a beak-like coracoid process that projects from the lateral angle of the scapula, and the acromion process, easily felt as the high point of the shoulder. Both are meant for the attachment of muscles.
Bones of the Forelimb
The forelimb consists of the humerus; the radius and ulna, together forming the forearm bones; the bones of the wrist, the carpals; the bones of the palm, the metacarpals; and the bones of the digits, the phalanges — together making 30 bones.
Humerus
This is the bone of the upper arm. It has a hemispherical head at its proximal end, which fits into the glenoid cavity to form a ball and socket joint at the shoulder.
On either side of the head of the humerus are present a pair of projections termed the greater and lesser tubercles. There is a deep groove between the tubercles called the bicipital groove, where a tendon of the biceps muscle is attached.
The shaft of the humerus shows the deltoid tuberosity, the roughened patch to which the deltoid muscle is anchored. The distal end of the humerus shows a pulley-like part called the trochlea, that articulates with the ulna. These features are labelled in Fig. 16.25.
Diagram
Fig. 16.25 – Humerus, anterior aspect of the right humerus, labelling head, greater tubercle, lesser tubercle, bicipital groove, radial fossa, coronoid fossa, lateral epicondyle, medial epicondyle, capitulum and trochlea
Radius and Ulna
The radius is located laterally, on the thumb side of the forearm. The proximal end of the radius has a disc-like head that articulates with the humerus bone. The shaft of the radius widens distally to form the styloid process.
The ulna is located medially, on the little finger side of the forearm. At the proximal end of the ulna there is a prominent process called the olecranon process, that forms the elbow joint with the humerus bone. This is the bony point you feel when you rest your elbow on a table.
On the lateral side, near the upper end of the ulna, is present the radial notch, into which the side of the head of the radius is fixed.
The radius and ulna articulate with each other at the upper and lower extremities by the superior and inferior radio-ulnar joints. In between the shafts of the two bones, an interosseous membrane is present. Both bones are shown together in Fig. 16.26.
Diagram
Fig. 16.26 – Radius and ulna, anterior aspect, labelling olecranon, coronoid process, head, radius, ulna, styloid process of radius and styloid process of ulna
Carpals, Metacarpals and Phalanges
Carpals: These are the bones of the wrist, arranged in two rows of four each.
Metacarpals: Five elongated metacarpals form the bones of the palm. Their proximal ends join with the carpals and their distal ends form the knuckles.
Phalanges: These are the bones of the fingers and thumb. Four fingers have three phalanges each and the thumb has two, thus making it fourteen phalanges in each hand. The complete arrangement is shown in Fig. 16.27.
Diagram
Fig. 16.27 – Bones of the hand, labelling carpals, metacarpals and phalanx 1, phalanx 2 and phalanx 3 of a finger
Pelvic Girdle
The pelvic or hip girdle connects the hind limb skeleton with the axial skeleton. It is made up of two hip bones called coxal bones. They unite posteriorly with the sacrum.
Each large, irregularly shaped coxal bone is made up of three parts — the ilium, ischium and pubis. At the point of fusion of these three bones, a cavity called the acetabulum is present, that forms a ball and socket joint with the thigh bone.
The two pubis bones are joined medially by a cartilaginous joint called the pubic symphysis. The pubis and ischium together form a ring of bone that encloses a space called the obturator foramen, marked in Fig. 16.28.
Diagram
Fig. 16.28 – Pelvic girdle in anterior view, labelling sacrum, acetabulum, superior ramus of pelvis, inferior ramus of pelvis and obturator foramen
Bones of the Lower Limb
Femur: The thigh bone is the longest bone in the body. Its head is joined to the shaft at an angle by a short neck. It forms a ball and socket joint with the acetabulum cavity of the coxal bone.
The lower one third region of the shaft is a triangular flattened area called the popliteal surface. The distal end has two condyles that articulate with the tibia and fibula. The parts of the femur are labelled in Fig. 16.29.
Diagram
Fig. 16.29 – Femur, labelling head, neck, greater trochanter, lesser trochanter, shaft, lateral condyle and medial condyle
Patella: Also called the knee cap, it is a sesamoid bone — a bone that develops within a tendon. It is a flat, rounded bone with a pointed lower end, as shown in Fig. 16.30.
Diagram
Fig. 16.30 – Patella, labelling base and apex
Tibia and Fibula
These are the two long bones of the shank or lower leg. The two are connected to each other at the extremities. In between the two bones an interosseous membrane is present.
Tibia: It is much thicker and stronger than the fibula. Its broad and expanded upper end articulates with the femur. Its lower end articulates with the talus, a tarsal bone.
Fibula: It is a long slender bone on the lateral side of the tibia. Both bones are shown together in Fig. 16.31.
Diagram
Fig. 16.31 – Tibia and fibula, labelling lateral condyle, medial condyle, facet for head of fibula, fibula, tibia, lateral malleolus and medial malleolus
Tarsals, Metatarsals and Phalanges
Tarsals: These are the bones of the ankle. Seven tarsals are arranged in three rows — two proximal, one intermediate and four distal.
Metatarsals: Five metatarsal bones support the sole region of the foot. Proximally they attach with the distal row of tarsals. Distally the metatarsals articulate with the phalanges.
Phalanges: These are the bones of the toes. Except the big toe, which has two phalanges, the rest of the four toes have three phalanges each. The bones of the foot are shown in Fig. 16.32.
Diagram
Fig. 16.32 – Bones of the foot, labelling calcaneum (heel bone), talus, tarsals, metatarsals with base, shaft and head, phalanx 1, phalanx 2, phalanx 3 and the big toe
Counting the Bones
The full count of the 206 bones, division by division, is given in Table 16.10. Add the numbers yourself and check that the subtotals and the grand total agree.
Table 16.10 – Details of Human Skeleton
| Division | Bone | Number |
|---|---|---|
| Axial skeleton — Skull | Cranium | 8 |
| Face | 14 | |
| Hyoid | 1 | |
| Ear ossicles | 6 | |
| Vertebral column | Vertebral column | 26 |
| Thorax | Sternum | 1 |
| Ribs | 24 | |
| Subtotal | 80 | |
| Appendicular skeleton — Pectoral girdle | Clavicle | 2 |
| Scapula | 2 | |
| Upper limbs | Humerus | 2 |
| Radius | 2 | |
| Ulna | 2 | |
| Carpals | 16 | |
| Metacarpals | 10 | |
| Phalanges | 28 | |
| Pelvic girdle | Hip bone | 2 |
| Lower limbs | Femur | 2 |
| Patella | 2 | |
| Tibia | 2 | |
| Fibula | 2 | |
| Tarsals | 14 | |
| Metatarsals | 10 | |
| Phalanges | 28 | |
| Subtotal | 126 | |
| Total | 206 |
This section is examined mainly through diagram labelling and through match-the-following items pairing a bone with its region or with a distinctive feature. The numbers in Table 16.10 are worth learning as a whole, because a favourite question gives one subtotal and asks for the other.
16.10 Types of Joints
You have studied about joints in a previous standard. Without joints, the various movements of the body would not be possible at all — a skeleton of rigid bones fused into one piece would be a suit of armour welded shut.
A point where two or more bones get articulated is called a joint, or an articulation, or an arthrosis. The study of joints is called arthrology.
Though bones are rigid, the ligaments that cover the bones, forming a joint, render slight flexibility to the bones. A ligament is a tough band of connective tissue running from bone to bone, and it decides how far a joint may travel before it is checked.
How Joints Are Classified
Joints are classified based on the degree of flexibility or movement they permit, into three types: fibrous joints, which are also known as synarthroses or immovable joints; cartilaginous or slightly movable joints, also called amphiarthroses; and lastly synovial or freely movable joints, of the diarthroses type. The full classification is set out in FC 16.11.
Flowchart
FC 16.11 – Classification of joints: Joints → (i) Fibrous joints (synarthroses, immovable) → sutures, syndesmoses, gomphoses; (ii) Cartilaginous joints (amphiarthroses, slightly movable) → synchondroses, symphysis; (iii) Synovial joints (diarthroses, freely movable) → pivot, ball and socket, hinge, condyloid, gliding, saddle
The degree of movement allowed at joints in various parts of the body is remarkably well matched to what each part must do. We must always appreciate this design in our own body — a skull that could flex would be useless, and a shoulder that could not would be equally useless.
Fibrous Joints — Synarthroses
In this joint, the articulating bones are held together by means of fibrous connective tissue. The bones do not exhibit movement. Hence it is an immovable or fixed type of joint.
Fibrous joints are further classified into sutures, syndesmoses and gomphoses.
Sutures
A suture is composed of a thin layer of dense fibrous connective tissue, as shown in Fig. 16.33.
Diagram
Fig. 16.33 – Structure of a suture, showing two skull bones with the thin layer of fibrous tissue between their interlocking edges, and the position of such sutures marked on a whole skull
Sutures are places of growth. They remain open till growth is complete, and on completion of growth they tend to ossify. This is the reason they are found in the skull, where the bones must keep enlarging as the brain grows.
Sutures may permit some moulding during childhood. Sutures are further classified into different types according to the shape of the meeting edges, as shown in Table 16.34.
Table 16.34 – Types of Sutures
| Type of suture | Character | Example |
|---|---|---|
| Butt joint | Square edged | Two nasal bones |
| Scarf joint | Tapering | Various skull bones |
| Lap joint | Overlapping | Temporal and parietal bone |
| Serrate joint | Irregular / interlocking | Various skull bones |
Syndesmoses
A syndesmosis is present where there is a greater distance between the articulating bones. At such locations, the fibrous connective tissue is arranged as a sheet or a bundle.
Examples include the distal tibiofibular ligament, the interosseous membrane between the tibia and fibula, and the corresponding membrane between the radius and ulna, as shown in Fig. 16.35.
Diagram
Fig. 16.35 – Syndesmoses: the interosseous ligament stretched between tibia and fibula, and between radius and ulna, labelling tibia, fibula, radius, ulna and interosseous ligament
Gomphoses
In this type of joint, a cone-shaped bone fits into a socket provided by another bone, e.g. the tooth and the jaw bones. The tooth is held in its socket by the periodontal ligament, as seen in Fig. 16.36.
Diagram
Fig. 16.36 – Gomphosis, showing the root of a tooth held in its bony socket by the periodontal ligament, labelling socket, gomphosis, periodontal ligament and root of tooth
A. Cartilaginous or Slightly Movable Joints
These are also called amphiarthroses. These joints are neither fixed nor freely movable. The articulating bones are held together by hyaline cartilage or by fibrocartilage. They are further classified as follows.
a. Synchondroses: The two bones are held together by hyaline cartilage. They are meant for growth. On completion of growth, the joint gets ossified. Examples are the epiphyseal plate found between the epiphysis and diaphysis of a long bone, and the rib–sternum junction. The epiphyseal plate is shown in Fig. 16.37.
Diagram
Fig. 16.37 – Synchondrosis, showing the upper end of a long bone with the epiphyseal plates marked between epiphysis and diaphysis, labelling epiphyseal plates, epiphysis and diaphysis
b. Symphysis: In this type of joint, a broad flat disc of fibrocartilage connects two bones. These occur in the midline of the body. One example of this type of joint is the intervertebral discs. Can you write another example? Look back at the pelvic girdle for one.
B. Synovial Joints or Freely Movable Joints
These are also called diarthroses.
A synovial joint is characterised by the presence of a space called the synovial cavity between the articulating bones, and it is this space that renders free movement at the joint.
The articulating surfaces of the bones at a synovial joint are covered by a layer of hyaline cartilage, so that the bones do not touch each other. This reduces friction during movement and helps to absorb shock.
The synovial cavity is lined by a synovial membrane that forms the synovial capsule. The synovial membrane secretes synovial fluid. All these parts are labelled in Fig. 16.38.
Diagram
Fig. 16.38 – Synovial joint in section, labelling ligament, synovial cavity containing synovial fluid, articular (hyaline) cartilage, fibrous layer, articular capsule and synovial membrane
Synovial Fluid
Synovial fluid is a clear, viscous, straw-coloured fluid similar to lymph. It is viscous due to hyaluronic acid.
The fluid also contains nutrients, mucus and phagocytic cells to remove microbes.
Synovial fluid does four distinct jobs. It lubricates the joint, absorbs shocks, nourishes the hyaline cartilage, and removes waste materials from the hyaline cartilage cells. This last pair of functions is essential because cartilage is avascular, that is, it has no blood supply of its own, so the fluid must serve as its delivery and drainage system. The phagocytic cells destroy microbes and the cellular debris formed by the wear and tear of the joint.
If the joint is immobile for a while, the synovial fluid becomes viscous, and as joint movement starts, it becomes less viscous.
The joint is provided with a capsular ligament and numerous accessory ligaments. The fibrous capsule is attached to the periosteum of the articulating bones. The ligaments help in avoiding dislocation of the joint.
Let us now study the types of synovial joints. Note that any type of synovial joint will show all the components mentioned above; the types differ only in the shape of the articulating surfaces and therefore in the directions of movement allowed.
Pivot Joint
Here, the rounded or pointed surface of one bone articulates with a ring formed partly by another bone and partly by a ligament.
Rotation only around its own longitudinal axis is possible. For example, in the joint between the atlas and axis vertebrae, the head turns sideways to form the ‘NO’ joint, as shown in Fig. 16.39.
Diagram
Fig. 16.39 – Pivot joint between atlas and axis, labelling odontoid process, transverse ligament, atlas and axis, with arrows showing rotation
Ball and Socket Joint
The ball-like surface of one bone fits into the cup-like depression of another bone, forming a movable joint.
Multiaxial movements are possible. This type of joint allows movements along all three axes and in all directions. Examples are the shoulder and hip joint, shown in Fig. 16.40.
Diagram
Fig. 16.40 – Ball and socket joint at the hip, labelling pelvis, cartilage, head of femur and neck of femur
Hinge Joint
In a hinge joint, the convex surface of one bone fits into the concave surface of another bone. In most hinge joints, one bone remains stationary and the other moves.
Angular, opening and closing motion, like that of a hinge, is possible. In this joint only monoaxial movement takes place, such as flexion and extension. Examples are the elbow and knee joint, shown in Fig. 16.41.
Diagram
Fig. 16.41 – Hinge joint at the elbow, labelling humerus, biceps, triceps, cartilage, joint capsule with synovial fluid, radius and ulna
Condyloid Joint
It is an ellipsoid joint. The convex oval-shaped projection of one bone fits into an oval-shaped depression in another bone.
It is a biaxial joint, because it permits movement along two axes. Flexion, extension, abduction, adduction and circumduction are possible. An example is the metacarpophalyngeal joint — the joint at the base of a finger, which lets the finger bend and also spread sideways. A condyloid arrangement between the radius and the carpals is shown in Fig. 16.42.
Diagram
Fig. 16.42 – Condyloid joint between radius and carpals, labelling radius, ulna, articular cartilage, synovial membrane, ligament, synovial cavity and carpals
Gliding Joint
A gliding joint is a planar joint, where the articulating surfaces of the bones are flat or slightly curved.
These joints are non-axial, because the motion they allow does not occur along an axis or a plane. Examples are the intercarpal and intertarsal joints. Individually each such joint moves very little, but many of them working together give the wrist and ankle their overall suppleness.
Saddle Joint
This joint is a characteristic of Homo sapiens. Here, the articular surface of one bone is saddle-shaped, and that of the other bone fits into such a saddle as a sitting rider would sit. That is, each bone has both concave and convex areas.
It is a modified condyloid joint in which movement is somewhat more free. It is a biaxial joint that allows flexion, extension, abduction, adduction and circumduction. The example is the carpometacarpal joint between the trapezium carpal and the metacarpal of the thumb, shown in Fig. 16.43.
Diagram
Fig. 16.43 – Saddle joint of the thumb, labelling carpal (trapezium) and metacarpal #1, with arrows showing the two axes of movement
Joints are a favourite topic for match-the-following and for “which type of joint is present at …” questions. Learn each type together with its axis of movement and one clear example, and use the shape of the articulating surfaces to reason out the answer rather than memorising lists.
16.13 Summary
Movements and locomotion
- Movements range from the streaming of protoplasm inside a cell to peristalsis, walking and running. They may be internal or external, and voluntary or involuntary.
- Three types of muscle bring about movement in human beings: smooth muscles for involuntary movements such as peristalsis and the constriction and dilation of blood vessels, cardiac muscles for the contraction and relaxation of the heart, and striated muscles for voluntary movements of the limbs, head, trunk and eyes.
- Locomotion is the change in locus of the whole body of an organism from one place to another. It is undertaken in search of food, shelter, a mate or a breeding ground, and to escape from enemies.
- There are four basic types of locomotory movement: amoeboid, by pseudopodia, e.g. leucocytes; ciliary, by cilia, e.g. ciliated epithelium and the cytopharynx of Paramecium; whorling, by flagella, e.g. sperms; and muscular, by muscles with the help of bones and joints.
- All locomotions are movements, but all movements are not locomotion.
- Skeletal muscles are attached to bones by tendons, which are inelastic thick bands of collagen fibres. Movement and locomotion are the combined action of bones, joints and skeletal muscles.
Location and structure of skeletal muscles
- The major part of a muscle that moves a bone lies not on that bone but on the bone above it; the biceps and triceps that move the forearm lie in the upper arm.
- At any joint there is a stationary bone and a movable bone. The end of a muscle attached to the stationary bone is the origin, the end attached to the movable bone is the insertion, and the thick middle part is the belly.
- All fibres do not run from end to end, and their number is greatest in the middle, so large muscles are most often fusiform in shape.
- On the basis of movement, striated muscles are of three types: agonists or prime movers, e.g. biceps; antagonists, which produce the opposite action, e.g. triceps; and synergists, which assist the prime movers, e.g. brachialis assists the biceps.
Working of skeletal muscles
- Muscles generally work in pairs producing opposite actions. Such muscles are called antagonistic, e.g. biceps and triceps of the upper arm. In an antagonistic pair one member is much stronger than the other; the biceps is stronger than the triceps.
- The fundamental characteristic of muscle is contraction, so a muscle can only pull and never push a bone. This is why antagonistic pairs are necessary.
- Important antagonistic pairs are flexor and extensor, abductor and adductor, pronator and supinator, levator and depressor, and protractor and retractor. Sphincters are circular muscles in the inner wall of the anus, stomach and similar openings, meant for closure and opening.
Structure of the contractile proteins
- The contractile unit of muscle is the sarcomere, which contains the contractile proteins actin and myosin.
- Each myosin filament is a polymerized protein made of meromyosins. A myosin molecule has two heavy chains (HMM) coiled into a double helix, with one end projecting outwards as the cross bridge, folded into a globular myosin head. Four light chains (LMM) are associated with the heads. The head has ATPase activity and can split ATP. Myosin contributes 55% of muscle proteins.
- The actin filament has three components: F actin, a double-stranded backbone of polymerized G actin molecules each carrying one ADP molecule; tropomyosin, two strands loosely attached to F actin that cover the active binding sites in the resting stage; and troponin, a complex of three globular proteins attached about \(\frac{2}{3}\) rd of the way along each tropomyosin, having affinity for actin, tropomyosin and calcium ions.
Mechanism of contraction and relaxation
- The sliding filament theory was put forth by H.E. Huxley and A.F. Huxley, and is also called the walk along theory or the Ratchet theory. Actin filaments are interdigitated with myosin filaments, and the interaction between actin and myosin is the basic cause of contraction. The filaments themselves do not shorten — they slide.
- In a relaxed muscle the active sites are covered by the tropomyosin–troponin complex, so myosin cannot bind and contraction cannot occur.
- When an action potential reaches the muscle through the motor end plate, it spreads over the sarcolemma, and the transverse tubules of the sarcoplasmic reticulum release calcium ions into the sarcoplasm.
- Calcium ions bind troponin, inactivating the troponin–tropomyosin complex; tropomyosin changes shape and detaches from the active site of F actin, uncovering it. The myosin head cleaves ATP, binds the uncovered site to form the actomyosin complex, tilts backwards and pulls the actin filament inwards, and the fibre contracts.
- During relaxation all events occur in reverse. The actomyosin complex breaks down and the heads detach, using ATP; calcium ions return to their store; the troponin–tropomyosin complex is restored and covers the active sites; actin filaments return to their original position. Like contraction, relaxation is also an active process.
- Rigor mortis is the stiffening of muscles some hours after death. No fresh ATP can be supplied, so once the local store is finished the myosin cannot detach from actin, leaving the muscle permanently contracted. It helps in fixing the hours since death after a murder.
Flowchart
FC 16.15 – The chapter in one sequence: nerve impulse arrives at motor end plate → action potential spreads over sarcolemma and into T tubules → \(Ca^{++}\) released into sarcoplasm → troponin–tropomyosin complex inactivated → active sites of actin uncovered → myosin head splits ATP and binds actin (actomyosin complex) → head tilts and pulls actin inwards → filaments slide, sarcomere shortens, muscle contracts → muscle pulls on tendon → tendon pulls the movable bone → bone turns about the joint acting as a fulcrum → movement, and if the whole body is displaced, locomotion → stimulus ends, \(Ca^{++}\) pumped back, active sites covered, muscle relaxes
Properties of muscles on electrical stimulation
- A single muscle twitch is a contraction produced by one brief stimulation, occurring in three stages: a latent period of no contraction, a contraction period and a relaxation period.
- Summation occurs when a muscle is stimulated before the end of a twitch, generating greater tension; repeated stimuli give increasing strength of contraction, the staircase phenomenon.
- Tetanus is the sustained state of contraction produced when stimulation is so rapid and frequent that the muscle has no time to relax.
- The refractory period, during which the fibre cannot respond to another stimulus, is about 0.02 seconds.
- The threshold stimulus is the minimum strength of stimulus needed to make a fibre contract.
- By the all or none principle, a stimulus below threshold produces no contraction, a threshold stimulus produces maximum force, and a stronger stimulus produces no extra force — a fibre contracts fully or not at all. All muscle fibres and nerve fibres obey this law.
- During strenuous exercise the oxygen supply becomes insufficient for oxidative phosphorylation, so muscles contract anaerobically and accumulate lactic acid from anaerobic glycolysis. Lactic acid yields less ATP and is toxic, causing tiredness, pain and cramps. The extra oxygen consumed during recovery is the oxygen debt, which is used to oxidise the accumulated lactic acid aerobically and to restore the depleted creatine phosphate and ATP.
Bones, levers and the skeletal system
- The bone–joint–muscle unit works as a lever, with the joint as the fulcrum and the muscle supplying the force. Class I is seen at the joint between the first vertebra and the occipital condyle, class II when the body is raised on the toes, and class III in flexion of the forearm at the elbow.
- Skeletal structures on the outer surface of the body form the exoskeleton, and those inside the body the endoskeleton. Bones and cartilage are the major endoskeletal components; chitinous structures, nails, horns, hooves, scales and hair are exoskeletal.
- Bones give the body shape, protect delicate organs, permit movement at joints, provide a firm surface for muscle attachment, act as reservoirs of calcium and serve as an important site for haemopoiesis. Cartilage is slightly pliable while bone has a hard matrix.
- The adult human endoskeleton has 206 bones in two divisions: the axial skeleton, lying along the longitudinal axis, and the appendicular skeleton, comprising the fore limb, hind limb and girdles. Girdles connect the limbs to the axial skeleton.
The axial skeleton
- The skull has 22 bones, made of the cranium and the facial bones, joined by immovable joints except for the lower jaw. The cranium has four median bones — frontal, occipital, sphenoid and ethmoid — and two paired bones, the parietals and temporals.
- The occipital bone bears the foramen magnum, through which the medulla oblongata connects with the spinal cord, and the occipital condyles that fit into the 1st vertebra. The sphenoid articulates with all other cranial bones and its sella turcica lodges the pituitary gland. The ethmoid contributes to the nasal septum.
- Four prominent sutures are coronal, sagittal, lambdiodal and lateral or squamous. Sutures are places of growth that ossify when growth is complete, and are classified as butt, scarf, lap and serrate joints.
- Fourteen facial bones give the face its shape and complete their growth by the age of 16: nasals, maxillae, palatines, zygomatics, lacrimals, inferior nasal conchae, vomer and mandible. The mandible is the largest, strongest and only movable bone of the skull.
- The hyoid is a U-shaped bone that articulates with no other bone and gives attachment to tongue, neck and pharyngeal muscles. Three ear ossicles — malleus, incus and stapes — lie in each middle ear.
- The vertebral column has 33 vertebrae in childhood and 26 in the adult, after five sacral vertebrae fuse into one sacrum and four coccygeal vertebrae into one coccyx. It has 7 cervical, 12 thoracic, 5 lumbar, 5 sacral and 4 coccygeal vertebrae.
- Its four curvatures — cervical and lumbar, which are secondary and convex, and thoracic and sacral, which are primary and concave — help in balancing the upright body, absorb shocks and protect the vertebrae from fracture.
- A typical vertebra has a centrum, flat in the antero-posterior aspect so that human vertebrae are amphiplatyan; a neural arch enclosing the vertebral foramen, which together form the neural canal; a neural spine; superior and inferior zygapophyses; intervertebral foramina for spinal nerves; and transverse processes for muscle attachment.
- The atlas is a ring-like first cervical vertebra without a centrum or spinous process, forming the ‘Yes joint’ with the occipital condyles. The axis is the second cervical vertebra and its odontoid process forms a pivot joint with the atlas, the ‘No joint’.
- Typical cervical vertebrae are numbers 3 to 6, with a short centrum, a bifid spine and a vertebrarterial canal; the 7th is the vertebra prominens. Thoracic vertebrae are twelve, with a heart-shaped centrum and facets for ribs except on numbers 11 and 12. The five lumbar vertebrae have a kidney-shaped centrum. The sacrum strengthens the pelvic girdle and the coccyx is a reduced triangular bone.
- The thoracic cage consists of twelve thoracic vertebrae, twelve pairs of ribs and the sternum. The sternum is about 15 cms long and has three parts: manubrium, body and xiphoid process.
- A rib is a ‘C’ shaped bone with a head and a tubercle for attachment to the thoracic vertebrae. Ribs are of three types: seven pairs of true ribs attached directly to the sternum by costal cartilages, three pairs of false ribs (8, 9 and 10) attached to rib 7, and two pairs of floating ribs with no ventral connection. The space between ribs is the intercostal space.
The appendicular skeleton
- The pectoral girdle attaches the forelimb to the axial skeleton and consists of a scapula and a clavicle on each side. The scapula bears the glenoid cavity, the coracoid process and the acromion process.
- The forelimb has 30 bones: humerus, radius and ulna, carpals, metacarpals and phalanges. The humerus has a hemispherical head, greater and lesser tubercles, a bicipital groove, deltoid tuberosity and trochlea. The radius lies on the thumb side and the ulna on the little finger side, bearing the olecranon process and the radial notch, and the two articulate at superior and inferior radio-ulnar joints with an interosseous membrane between them.
- Each hand has eight carpals in two rows of four, five metacarpals, and fourteen phalanges — three in each finger and two in the thumb.
- The pelvic girdle is made of two coxal bones, each formed of ilium, ischium and pubis, meeting at the acetabulum. The two pubis bones join at the pubic symphysis, and the pubis and ischium enclose the obturator foramen.
- The lower limb has the femur, the longest bone of the body, forming a ball and socket joint at the acetabulum; the patella, a sesamoid bone; the tibia, thicker and stronger, and the slender fibula; seven tarsals in three rows; five metatarsals; and fourteen phalanges, with two in the big toe and three in each of the other toes.
Joints
- A joint, articulation or arthrosis is a point where two or more bones are articulated; their study is arthrology. Ligaments hold the bones of a joint together and give slight flexibility.
- Joints are classified by the movement they permit into fibrous or synarthroses (immovable), cartilaginous or amphiarthroses (slightly movable), and synovial or diarthroses (freely movable).
- Fibrous joints are of three kinds: sutures, which are places of growth that later ossify; syndesmoses, where a sheet or bundle of fibrous tissue bridges a wider gap, as between tibia and fibula or radius and ulna; and gomphoses, where a cone-shaped bone fits a socket, as in the tooth and jaw.
- Cartilaginous joints are of two kinds: synchondroses, joined by hyaline cartilage and meant for growth, as at the epiphyseal plate and the rib–sternum junction; and symphysis, joined by a broad flat disc of fibrocartilage in the midline, as in the intervertebral discs.
- A synovial joint has a synovial cavity between the articulating bones, whose surfaces are covered with hyaline cartilage so the bones do not touch. The synovial membrane forms the capsule and secretes synovial fluid, a clear, viscous, straw-coloured fluid made viscous by hyaluronic acid, which lubricates the joint, absorbs shocks, nourishes the avascular hyaline cartilage, removes its wastes and contains phagocytic cells that destroy microbes and debris. Capsular and accessory ligaments prevent dislocation.
- Six types of synovial joint occur: pivot, allowing rotation about the longitudinal axis, as between atlas and axis; ball and socket, multiaxial, as in the shoulder and hip; hinge, monoaxial, allowing flexion and extension, as at the elbow and knee; condyloid, biaxial and ellipsoid, as in the metacarpophalyngeal joint; gliding, non-axial with flat surfaces, as in intercarpal and intertarsal joints; and saddle, a biaxial modified condyloid joint characteristic of Homo sapiens, as in the carpometacarpal joint of the thumb.
Disorders
- Muscular dystrophy is a genetically inherited wasting disease of skeletal muscles; internal muscles such as the diaphragm are spared. The Duchenne type usually occurs in boys and affects the lower limbs, while limb girdle dystrophy affects the shoulders or hips and begins at 20–35 years. No treatment appears to cure it.
- Myasthenia gravis is an autoimmune disorder in which antibodies bind to the acetylcholine receptors of the neuromuscular junction, blocking transmission of nerve impulses to muscle fibres. It causes progressive weakness, with ptosis, diplopia and difficulty in swallowing, chewing and speech.
- Arthritis is the inflammation of joints and is of three types: osteoarthritis, in which joint cartilage degenerates through ageing, obesity or muscle weakness, affecting hands, knees and spine; gouty arthritis, in which uric acid accumulates as sodium urate and degenerates cartilage, usually in the joints of the feet; and rheumatoid arthritis, an autoimmune disorder in which the synovial membrane swells, secretes extra fluid and may form pannus that erodes cartilage and leads to stiffness.
- Osteoporosis makes bones porous and brittle. With age, bone resorption outpaces bone formation and more calcium is lost than gained. It is more common in women, and may follow decreased estrogen secretion after menopause, vitamin D deficiency, a low calcium diet, and decreased secretion of sex hormones and thyrocalcitonin. It leads to fractures, shrinkage of vertebrae, height loss, hunched back and bone pain, and is best prevented by adequate calcium and exercise at a young age.
16.14 Points to Ponder
A movement need not carry an organism anywhere, yet the body spends energy on movement every second of the day. What would be lost if only those movements that produce locomotion were retained?
Amoeboid, ciliary, whorling and muscular movement all achieve the same end by very different means. Why should so many separate solutions to the same problem have persisted in the animal kingdom instead of one best method replacing the rest?
Muscles that move the fingers lie in the forearm, and muscles that move the forearm lie in the upper arm. Follow this pattern outwards along a limb and consider what it would cost an animal if each muscle sat directly on the bone it moves.
A muscle can only pull. Almost every joint in your body is therefore worked by at least two muscles. Can you think of any structure in the body, or in a machine you know, that gets around this limitation in some other way?
Why should the shivering that keeps us warm in winter be a muscular activity at all? What does this tell you about where the energy of contraction actually goes?
During contraction, neither the actin nor the myosin filament changes its length. Before this was discovered, most scientists assumed the filaments must shorten. Why is a sliding mechanism a better engineering solution than a shortening one?
ATP is needed both to make a muscle contract and to let it relax. If a cell had to give up one of these two uses of ATP, which loss would be more serious for the organism, and why?
Rigor mortis is a normal biological process taking place in a body that is no longer alive. What does that tell you about the difference between a process being “living” and a process being “chemical”?
A single muscle fibre obeys the all or none principle, yet a whole muscle can produce a gentle or a powerful pull at will. Think through how the same set of fibres can give such finely graded force.
Oxygen debt allows an animal to keep working after its oxygen supply has fallen short. What kind of situations in the life of a wild animal would make such a borrowing arrangement worth its later cost?
The body uses all three classes of lever, but class III levers, which demand more force from the muscle than the load being lifted, are the commonest. What is gained in exchange for that extra force?
Long bones are slightly bent rather than perfectly straight, and the vertebral column is curved rather than upright. Is there a common principle behind both of these designs?
Bone is a rigid supporting structure, a store of calcium and a site of blood cell formation, all at once. Does combining such different roles in one organ create any conflict between them?
The skull is built of many separate bones locked together, while the mandible is left free to move. Suppose the skull had been a single fused bone from birth — list the consequences you can think of.
Every joint in the body is a compromise between mobility and stability. Choose the shoulder, the elbow and a suture, and decide where each of them sits on that scale and why the body has chosen that particular balance in that place.
Cartilage has no blood supply of its own, and depends on synovial fluid for its nourishment. Why might a tissue that is constantly compressed be better off without blood vessels running through it?
In myasthenia gravis, in rheumatoid arthritis and in some other conditions you have met, the body’s own immune system attacks its own tissues. What difficulty does this create for treatment, compared with a disease caused by an outside organism?
Osteoporosis is treated mainly by prevention, and the prevention has to begin decades before the disease appears. What makes such a disorder particularly hard to control in a population, and what could be done about it?
The hyoid bone articulates with no other bone, and the patella develops within a tendon. Both are exceptions to the usual plan of the skeleton. What do such exceptions suggest about how the skeleton has taken its present form?
Human beings can hold and manipulate an object far better than most other animals, and much of this depends on a single saddle joint at the base of the thumb. How far can a whole way of life be said to rest on one anatomical detail?
16.15 Check Your Concepts
Distinguish between movement and locomotion, and explain why the statement “all locomotions are movements but all movements are not locomotion” is true.
Why would a muscle be unable to move a bone if its tendons were elastic instead of inelastic?
A muscle has its origin on the scapula and its insertion on the humerus. Which of the two bones will move when the muscle contracts, and why?
A sphincter has no antagonistic partner muscle, yet the opening it guards can be both closed and opened. Explain how this is possible.
Why are large muscles most often fusiform in shape? Relate your answer to the distribution of fibres within the muscle.
Name the agonist, the antagonist and a synergist involved when the elbow is flexed, and state what would happen if the antagonist failed to relax at that moment.
Draw a labelled diagram of an actin filament and mark F actin, tropomyosin and troponin. Against each label, write its role in contraction.
What happens if the tropomyosin of a muscle fibre fails to move away from the active sites of actin even after calcium ions are released?
Arrange the following in the correct sequence: myosin head tilts and pulls actin; T tubules release calcium ions; impulse arrives at the motor end plate; active sites of actin are uncovered; actomyosin complex is formed; conformational change occurs in the troponin–tropomyosin complex.
The sliding filament theory is also called the walk along theory or the ratchet theory. Justify both of these alternative names using the events of the cross-bridge cycle.
Why is ATP required for relaxation of a muscle, when relaxation appears to be nothing more than the switching off of contraction? Name the two specific steps at which it is used.
Explain rigor mortis in terms of the cross-bridge cycle, and state how this knowledge is useful in forensic investigation.
Distinguish between summation and tetanus in a stimulated muscle.
A student argues that if a stronger stimulus is applied to a single muscle fibre, it will contract more forcefully. Is the student correct? Support your answer with the appropriate principle.
Draw a myogram of a single muscle twitch, label its three stages, and explain what is happening inside the fibre during the latent period.
What happens to the muscles of an athlete who sprints at full speed for a longer time than the oxygen supply allows? Trace the sequence of events up to the repayment of the oxygen debt.
Why does a muscle continue to consume extra oxygen for several minutes after strenuous exercise has stopped? What is this extra oxygen used for?
Identify the class of lever in each case and justify your answer: (i) raising the body on the toes, (ii) nodding the head, (iii) lifting a load by flexing the forearm.
Differentiate between exoskeleton and endoskeleton, giving two examples of each from different groups of animals.
List any five functions of bones, and for each one name a bone or a part of the skeleton that illustrates it.
Why are the bones of the skull joined by immovable joints, while the mandible is joined by a movable one? What would be the consequences if this arrangement were reversed?
What are fontanelles? Explain their importance during birth and during infancy.
Draw a labelled diagram of a typical vertebra and mark the centrum, neural arch, vertebral foramen, neural spine, transverse process and zygapophyses.
Why is the joint between the skull and the atlas called the ‘Yes joint’ and that between the atlas and the axis the ‘No joint’? Name the type of joint in each case.
How would you identify a thoracic vertebra, a cervical vertebra and a lumbar vertebra if all three were placed before you?
Why does the vertebral column of a child contain 33 vertebrae while that of an adult contains only 26 bones? Which vertebrae account for the difference?
What would happen to a person’s ability to walk upright and absorb shock if the vertebral column were perfectly straight?
Classify ribs on the basis of their ventral attachment and state which pairs fall into each class.
Give the total number of bones in the axial skeleton and in the appendicular skeleton, and show how these add up to the total number of bones in an adult human.
Compare the skeleton of the hand with that of the foot, and explain how the differences suit their different functions.
Classify joints on the basis of the movement they permit, and present the classification in the form of a chart with one example of each type.
Distinguish between a synchondrosis and a symphysis, giving one example of each.
What makes a synovial joint freely movable? Name the four functions of synovial fluid and explain why cartilage depends upon it.
Why can the shoulder move in all directions while the elbow can only flex and extend? Name the type of joint in each case and relate the movement to the shape of the articulating surfaces.
The saddle joint is described as a characteristic of Homo sapiens. Explain what makes this joint special and how it contributes to the human ability to handle objects.
Differentiate between muscular dystrophy and myasthenia gravis with respect to cause, site of the defect and symptoms.
Explain why a patient with myasthenia gravis often notices a drooping eyelid before any other symptom.
A patient complains of a painful, swollen big toe, and a blood test shows a raised uric acid level. Name the likely disorder and explain how the raised uric acid damages the joint.
Distinguish between osteoarthritis and rheumatoid arthritis on the basis of cause and the changes produced in the joint.
Why is osteoporosis more common in women than in men, and why is prevention considered better than treatment in this disorder?
What happens to the skeleton if the parathyroid gland secretes an excess of parathormone over a long period? Explain the link between the hormone and the condition of the bones.
What kind of contraction occurs in your neck muscles while you are reading your class assignment?
Observe the given diagram of a vertebra and enlist the importance of the parts marked ‘A’, ‘B’ and ‘C’.
Raju intends to train his biceps. While exercising using dumbbells, which joints should remain stationary and which should move?
In a road accident, Moses fractured his leg. One of the passers by tied a wooden plank to the fractured leg while Moses was rushed to the hospital. Was this essential? Why?
A sprain is more painful than a fracture. Why?
Why can a red muscle work for a prolonged period whereas a white muscle fibre suffers from fatigue after a shorter period of work? (Refer to the chapter on animal tissues.)
How is the structure of the sarcomere suitable for the contractility of the muscle? Explain its function according to the sliding filament theory. (Refer to the chapter on animal tissues.)
Ragini, a 50 year old office goer, suffered hair-line cracks in her right and left foot at short intervals of time. She was worried that minor jerks were leading to hair-line cracks in her bones. The doctor explained to her why this must be happening and prescribed medicines. What must be the cause of Ragini’s problem? Why has it occurred? What precautions should she have taken earlier? What care should she take in future?
How does the structure of actin and myosin help muscle contraction?
Justify the structure of the atlas and axis vertebrae with respect to their position and function.
Observe the blood report given below and diagnose the possible disorder.
| Examination of Blood | Result | Normal value |
|---|---|---|
| Uric Acid | 9.2 | 2.5 - 7.0 mg/l |
| Blood Urea Nitrogen (BUN) | 24 | 10 - 20 mg/dl |
- Write short notes on the following points.
- Actin filament
- Myosin filament
- Role of calcium ions in contraction and relaxation of muscles
- Draw labelled diagrams.
- Synovial joint
- Different cartilagenous joints
16.16 Objective Questions
The functional unit of striated muscle is …………..
- cross bridges
- myofibril
- sarcomere
- z-band
A person slips from the staircase and breaks his ankle bone. Which bones are involved?
- Carpals
- Tarsal
- Metacarpals
- Metatarsals
Muscle fatigue is due to accumulation of ……..
- pyruvic acid
- lactic acid
- malic acid
- succinic acid
Which one of the following is NOT an antagonistic muscle pair?
- Flexo-extensor
- Adductor-abductor
- Levator-depressor
- Sphinetro-suprinater
Swelling of a sprained foot is reduced by soaking it in hot water containing a large amount of common salt,
- due to osmosis
- due to plasmolysis
- due to electrolysis
- due to photolysis
The role of calcium in muscle contraction is ……….
- to break the cross bridges as a cofactor in the hydrolysis of ATP
- to bind with troponin, changing its shape so that the actin filament is exposed
- to transmit the action potential across the neuromuscular junction
- to re-establish the polarisation of the plasma membrane following an action potential
Hyper-secretion of parathormone can cause which of the following disorders?
- Gout
- Rheumatoid arthritis
- Osteoporosis
- Gull’s disease
Select the correct option showing the type of suture present between the two nasal bones, from the four suture diagrams marked a, b, c and d.
Assertion: Skeletal muscles occur in antagonistic pairs. Reason: A muscle can only pull and never push a bone. Decide whether both statements are true, and whether the reason correctly explains the assertion.
Assertion: The length of the actin and myosin filaments remains unchanged during muscle contraction. Reason: The sarcomere shortens during contraction. Comment on both statements and on the relationship between them.
Assertion: A warming-up session is essential before beginning vigorous exercise. Reason: Synovial fluid becomes less viscous once joint movement begins. State whether both are true and whether the reason explains the assertion.
Match the following:
| Column A | Column B |
|---|---|
| a. Amoeboid movement | i. Flagella |
| b. Ciliary movement | ii. Muscles, bones and joints |
| c. Whorling movement | iii. Pseudopodia |
| d. Muscular movement | iv. Cilia |
- Match the following:
| Column A | Column B |
|---|---|
| a. Sella turcica | i. Passage of the medulla oblongata |
| b. Foramen magnum | ii. Lodges the pituitary gland |
| c. Glenoid cavity | iii. Receives the head of the femur |
| d. Acetabulum | iv. Receives the head of the humerus |