Locomotion and Movement: Class 11 Biology · Chapter 17

Watch a fast bowler on match day. The run-up carries the whole body twenty-odd metres down the pitch. At the crease the feet plant and the bowling arm whips over the shoulder, and all the while the bowler is blinking, breathing and swallowing. Every one of these is a movement, yet only the run-up takes the bowler somewhere new.

Biologists give that kind of movement its own name. When an animal, by its own effort, shifts its whole body from one place to another, it shows locomotion (गमन). Walking, running, climbing, swimming and flying all count as locomotion. A bowler's arm swing, a frog's blinking eye and the beating of your heart are movements that are not.

Look closely and almost nothing alive is perfectly still. Your eyelids blink, your jaw works as you chew, your tongue shapes every word you speak and your heart never stops. Among single cells, an Amoeba creeps along as its cytoplasm flows into a forward bulge, and a great many protists and small animals beat hair-like cilia or lash whip-like flagella, or wave tentacles, as a Hydra does.

Do animals keep separate machinery for travelling? Usually not. Your legs carry you to school, yet the same legs let you squat, kneel or stand on tiptoe without going anywhere. A Hydra catches its prey with its tentacles and also uses them to somersault across a surface. In a Paramoecium a single set of cilia rows the cell through the water and sweeps food particles down the oral groove into its cytopharynx. Since one set of equipment so often serves both purposes, movement and locomotion are best studied together, with locomotion treated as one special kind of movement.

Think of a few journeys. A chital bolts the instant it senses a tiger. Lakhs of olive ridley turtles crawl ashore at Gahirmatha in Odisha to lay their eggs. Amur falcons stop over in Nagaland on their long flight from eastern Asia to southern Africa, escaping the northern winter. In general animals travel to escape predators, to reach breeding grounds, to find a better climate, and to look for food, shelter and mates. How an animal travels depends on where it lives and on what the moment demands.

17.1 Types of Movement

Zoom in from the whole body to single cells and you find that human cells move in three basic ways: amoeboid, ciliary and muscular (Fig. 17.1).

Amoeboid movement. Graze your knee and within hours white blood cells are crawling out of nearby capillaries towards the bacteria in the wound. These leucocytes, and the macrophages that patrol our tissues, travel the way an Amoeba does. Microfilaments of the cytoskeleton push the cytoplasm forward into a temporary foot, a pseudopodium, and the rest of the cell flows in behind it.

Ciliary movement. Breathe in on a dusty road and some of the dust reaches your trachea, but it does not stay there. The cells lining the trachea carry cilia that beat in coordinated waves, like a stadium crowd doing a wave, and they keep a film of mucus creeping up towards the throat; trapped dust and germs ride up with it and are swallowed or coughed out. The same kind of lining, ciliated epithelium, covers the inside of many tube-shaped organs. In the oviduct its beating helps an ovum along towards the uterus.

Muscular movement. The third kind, and the subject of most of this chapter, depends on muscle cells that shorten with force. Bending an arm, chewing or moving the tongue all need it, and humans and most other multicellular animals rely on it for locomotion too. Think of a puppet show. The strings pull, the wooden frame gives the pull something rigid to act on, and the puppeteer decides which string to pull and when. In the body the muscles pull, the skeleton provides the rigid levers and the nervous system gives the orders, so locomotion needs all three working in step. We take the muscles first, then the skeleton and its joints.

17.2 Muscle

Flagella deserve a word before we turn to muscle. You met them in Chapter 8 as outgrowths of the cell membrane, built like cilia but longer and fewer. Euglena swims by lashing its flagellum; the collar cells of a sponge beat theirs to keep water moving through its canal system; and a human sperm is driven forward by its own flagellum.

Nearly half of your body weight, about 40–50 per cent in an adult, is muscle (पेशी). This tissue develops from the mesoderm, the middle germ layer of the embryo, and it is specialised for a single job, pulling. Four properties make that possible:

• Excitability: it responds to a stimulus, usually a nerve impulse, with an electrical change in its membrane.
• Contractility: it can shorten and so pull on whatever it is attached to.
• Extensibility: it can be stretched, as your hamstrings are when you touch your toes.
• Elasticity: after being stretched or contracted it returns to its resting length.

Three questions sort muscles out. Where is it? Does it look striped under a microscope? Can you control it at will? The first question gives three types, and the other two describe them (Fig. 17.2).

By location. Skeletal muscles are attached to bones and other parts of the skeleton. Visceral muscles lie in the inner walls of hollow organs such as the alimentary canal and the reproductive tract. Cardiac muscle is found only in the heart, where its cells branch and link up into a network.

By appearance. Skeletal and cardiac fibres show regular cross stripes, so both are striated. Visceral muscle shows none, so it is called smooth (non-striated) muscle. By control. Only skeletal muscle obeys your will, so it is voluntary; visceral and cardiac muscle are involuntary. Nerves can make the heart beat faster or slower, but they do not order each beat.

Each type suits its job. Skeletal muscles hold your posture and power locomotion. The slow squeezing waves of visceral muscle push food along the gut and gametes along the genital tract. Cardiac muscle contracts rhythmically for a lifetime to pump blood.

Inside a skeletal muscle

A skeletal muscle such as your biceps is built in levels, the way a thick rope is made of thinner ropes (Fig. 17.3). The whole muscle sits inside a sheet of collagen-rich connective tissue, the fascia, which also binds together the bundles within it. Each bundle is a fascicle, and each fascicle holds many muscle fibres.

A muscle fibre is a single cell, but an unusual one: it can be several centimetres long and contains many nuclei in one continuous cytoplasm, which makes it a syncytium. Most of its volume is taken up by long, parallel, banded threads, the myofibrils (NCERT also calls them myofilaments). Wrapped round them is a network of endoplasmic reticulum, here called the sarcoplasmic reticulum, which stores calcium ions, the switch for contraction. The fibre's plasma membrane has its own name, the sarcolemma, and so does its cytoplasm, the sarcoplasm.

Each myofibril is packed with two kinds of protein filament lying parallel to each other and to its length: narrow thin filaments of actin and wider thick filaments of myosin. Their arrangement creates the stripes. A stretch that holds only thin filaments looks light; it is the I band, short for isotropic, because it barely alters polarised light. A stretch where the thick filaments lie looks dark; it is the A band, for anisotropic. Neighbouring myofibrils keep their bands in line, so the whole fibre looks striped.

The band pattern repeats every few micrometres, and each repeat is a sarcomere, the unit that actually contracts (Fig. 17.4). A sarcomere is bounded at both ends by a dark Z line, which runs through the middle of an I band and anchors the thin filaments. At its centre a fine fibrous M line crosses the A band and holds the thick filaments together. A myofibril is therefore a long chain of sarcomeres joined end to end at their Z lines.

Do the thin filaments from the two ends meet in the middle? Not in a resting sarcomere. They slide in just far enough to overlap the ends of the thick filaments, leaving a central strip of the A band with thick filaments only. This lighter strip is the H zone, and the M line runs through its middle.

17.2.1 Structure of Contractile Proteins

Build a thin filament from the bottom up (Fig. 17.5). Start with small, roughly spherical protein units, G-actin (globular actin). Strung together like beads they make a strand of F-actin (filamentous actin), and two such strands twist round each other like a two-ply rope. Along the grooves of this rope lie two fine threads of tropomyosin, running the whole length. Finally, at regular intervals, a complex protein, troponin, is clipped onto the tropomyosin.

Actin carries active sites where a myosin head could grab it. In resting muscle a subunit of troponin holds the tropomyosin over these sites and keeps them masked, so the thin filament is ready but locked.

Now the thick filament. Picture a few hundred golf clubs bundled together with their shafts side by side and their heads sticking out all along the bundle. Each club is a protein molecule called a meromyosin. Its long shaft, the tail, is the light meromyosin (LMM); its head on a short arm is the heavy meromyosin (HMM). The tails pack together to make the filament, while the heads and arms project from its surface at a regular spacing and angle. Each projecting head and arm is a cross arm.

The head is the working part. It is an ATPase, an enzyme that splits ATP, and it has one binding site for ATP and another, the active site, for actin. A part that can both release energy and grip the thin filament is exactly what a molecular motor needs.

17.2.2 Mechanism of Muscle Contraction

How can a sarcomere get shorter when neither kind of filament changes length? The answer accepted today is the sliding filament theory.

Follow one command from the brain to the filaments (Fig. 17.6). The central nervous system (CNS) sends an impulse down a motor neuron, whose ending sits on the sarcolemma at a neuromuscular junction, also called the motor end plate. A motor neuron branches to reach a number of fibres; the neuron together with all the fibres it supplies is a motor unit, and those fibres always contract together.

At the junction the electrical message is handed over chemically. The nerve ending releases acetylcholine, a neurotransmitter, which triggers an action potential, a travelling electrical signal, in the sarcolemma. As the action potential sweeps through the fibre, the sarcoplasmic reticulum lets calcium ions ($\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$) out into the sarcoplasm.

Calcium is the key to the lock (Fig. 17.7). It binds to a subunit of troponin, which changes shape and lets the tropomyosin roll aside, uncovering actin's active sites. Each myosin head is already primed: it has split an ATP and holds the energy. It now grabs an exposed site, forming a cross bridge between the thick and the thin filament.

Next the head swivels, like a rower's oar, and drags the thin filament towards the centre of the A band. It is pulling on filaments anchored to the Z lines, so on both halves of the sarcomere the Z lines are drawn inwards and the sarcomere shortens: that is contraction. During the stroke the head releases the ADP and inorganic phosphate (${\mathrm{P}}_{\mathrm{i}}$) left from the split ATP and ends in its relaxed position, still attached.

Letting go costs a fresh ATP. Only when a new ATP binds does the cross bridge break. The head then splits that ATP, re-cocks, and grabs a site further along. Like a team hauling a rope hand over hand, thousands of heads bind, pull and release out of step with one another, so the filaments slide smoothly. The cycle continues for as long as calcium stays high.

Now watch what the sliding does to the bands (Fig. 17.8). The thick filaments do not move, so the A band keeps its length. The thin filaments slide deeper into the A band, so each I band shortens and the H zone narrows; when the thin filament tips meet at the M line the H zone disappears. Neither filament has become shorter. Only their overlap has grown.

Relaxation is the switch turned off. Once the nerve impulses stop, the sarcoplasmic reticulum pumps $\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$ back into its cisternae. Troponin, without its calcium, lets tropomyosin cover the active sites again, so no fresh cross bridges can form. The thin filaments slide back out and the Z lines return to where they started.

Muscles differ in how quickly they respond: the small muscles that turn your eyes contract and relax far faster than the big muscles of the calf. When a muscle is worked hard again and again, as in a sprint finish, its fibres cannot get oxygen fast enough and break down glycogen anaerobically. Lactic acid then builds up and the muscle tires, a condition called fatigue.

Muscle fibres also come in two working styles. Fibres built for endurance are crowded with mitochondria and rich in myoglobin, a red pigment that stores oxygen for those mitochondria to use in making ATP. The pigment colours them, so they are red fibres; because they run on oxygen they are also called aerobic muscles. Fibres built for short bursts carry little myoglobin and look pale, so they are white fibres. They have few mitochondria but plenty of sarcoplasmic reticulum, and they get their energy mainly by anaerobic breakdown. Long-distance runners tend to have a larger share of red fibres in their leg muscles, and sprinters a larger share of white.

17.3 Skeletal System

A muscle can only pull, and a pull achieves nothing without something rigid to pull on. The skeleton supplies the rigid parts: levers for muscles to move, a frame that holds the body's shape, and hard cases round the brain, spinal cord, heart and lungs.

Press the tip of your nose, then your forehead. The nose bends slightly because it is cartilage; the forehead does not because it is bone. Both are specialised connective tissues, and the difference lies in their matrix: calcium salts make bone hard, while chondroitin salts leave cartilage firm but pliable. An adult human skeleton has 206 bones plus a few cartilages, in two divisions (Fig. 17.10): the axial skeleton of 80 bones along the body's main axis, and the appendicular skeleton of 126 bones in the limbs and girdles.

Axial skeleton

The axial skeleton is the body's central column and everything fixed to it: the skull, the vertebral column, the sternum and the ribs.

Most of the skull's 22 bones fit together into two sets (Table 17.2): 8 cranial bones form the cranium, the case round the brain, and 14 facial bones build the face, from the jaws and cheeks to the nose and eye sockets. The skull region also holds two surprises. The U-shaped hyoid, at the base of the buccal cavity (the floor of the mouth), is the only bone in the body that does not form a joint with any other bone; muscles and ligaments sling it in place, and it anchors the tongue. Each middle ear holds three ear ossicles, the malleus, incus and stapes; the stapes is the smallest bone in the body.

At its base the skull carries two rounded knobs, the occipital condyles, which rest on the top of the vertebral column. A skull with two condyles is called dicondylic; the human skull is one.

Why is the backbone a chain of 26 pieces rather than one solid rod? A chain can bend while still shielding the spinal cord. The vertebral column runs along the dorsal side, from the base of the skull to the hips, and forms the main frame of the trunk (Fig. 17.11). Through every vertebra passes a hollow, the neural canal, and the canals line up into a tunnel that houses the spinal cord.

Counting from the skull, there are 7 cervical vertebrae in the neck, 12 thoracic in the chest and 5 lumbar in the lower back, followed by one sacral and one coccygeal piece, each made of fused vertebrae. The topmost vertebra, the atlas, articulates with the occipital condyles and carries the skull. A giraffe's long neck has the same seven cervical vertebrae as yours, because almost all mammals have seven; its vertebrae are simply much longer.

The column shields the spinal cord and holds up the head, and both the ribs and the back muscles are fastened to it. In front, the sternum (breastbone) is a flat bone in the ventral midline of the thorax.

Twelve pairs of thin, flat, curved bones, the ribs, wrap round the chest. The back end of a rib has two surfaces for joints with the vertebral column, so a rib is called bicephalic (two-headed). At the front, the ribs differ in how they reach the sternum (Fig. 17.12):

• True ribs, pairs 1–7: attached at the back to thoracic vertebrae and at the front to the sternum, each through its own strip of hyaline cartilage.
• False (vertebrochondral) ribs, pairs 8–10: their cartilage does not reach the sternum but joins the cartilage of the seventh rib.
• Floating ribs, pairs 11–12: attached only at the back, free at the front.

The thoracic vertebrae, the ribs and the sternum together make the rib cage, a springy enclosure round the heart and lungs that also moves when you breathe (Chapter 14).

Appendicular skeleton

The appendicular skeleton is the limbs together with the two girdles that join them to the axial skeleton. Each limb has 30 bones, and the forelimb and hindlimb follow the same plan (Fig. 17.13).

In the forelimb, from shoulder to fingertip, come the humerus of the upper arm, the radius and ulna of the forearm, 8 carpals in the wrist, 5 metacarpals in the palm and 14 phalanges in the digits: two in the thumb and three in each finger.

The hindlimb has the femur in the thigh, the longest bone in the body, the tibia and fibula in the lower leg, 7 tarsals in the ankle, 5 metatarsals in the sole and 14 phalanges in the toes. One more bone makes up the thirty: the cup-shaped patella (kneecap), which covers the knee in front.

The girdles link the limbs to the axial skeleton: the pectoral girdle for the upper limbs and the pelvic girdle for the lower limbs (Fig. 17.14). Each girdle has two halves, one for each side.

Trace one half of the pectoral girdle with your fingers. The bony bar running from the base of your neck out to the shoulder is the clavicle (collar bone), a long, slender bone with two curves. At the shoulder it meets the acromion, a flat, expanded projection that ends the spine, a raised ridge across the back of the scapula (shoulder blade). The scapula itself is a large, flat, triangular bone lying on the back of the thorax over the second to seventh ribs. Just below the acromion it has a shallow cup, the glenoid cavity, into which the humerus fits its rounded head; this ball in a cup is the shoulder joint.

In the pelvic girdle, start at the hip socket. The head of the femur sits in a deep cup, the acetabulum, which lies exactly where three bones, the ilium, the ischium and the pubis, fused into one coxal bone (hip bone). The pelvic girdle consists of two coxal bones, and in front they meet at the pubic symphysis, a joint containing fibrous cartilage.

17.4 Joints

Lift a school bag by bending your elbow. The biceps pulls on the forearm, and the forearm turns about the elbow, which is the fulcrum of this lever. A joint is any point where bones meet, or where a bone meets a cartilage, and it is at joints that muscle force becomes movement. Some joints barely move and others swing freely, depending on how the bones are bound together. On that basis there are three structural types (Fig. 17.15).

Fibrous joints do not move at all. The flat bones of the cranium are locked edge to edge by dense fibrous connective tissue along wavy seams, the sutures. In a newborn baby these seams have not yet closed, which is why a baby's head has soft spots.

In cartilaginous joints the bones are joined by cartilage, which permits a little movement. The bodies of neighbouring vertebrae are joined this way, through pads of cartilage called intervertebral discs: no single joint bends much, but together they let your back curl and twist.

In synovial joints a fluid-filled synovial cavity separates the surfaces of the two bones, so they can move against each other freely. These joints make locomotion and most other movements possible, and their shapes decide which movements each one allows (Fig. 17.16).

17.5 Disorders of Muscular and Skeletal System

Like any machine, the muscle and bone system can fail at different points: where nerve meets muscle, in the muscle fibres, in the bones or in the joints (Fig. 17.17).

Two of these connect directly to the mechanism you studied. In myasthenia gravis the body's own antibodies damage the acetylcholine receptors at the motor end plate, so impulses no longer reliably set off action potentials in the fibres. Tetany is a reminder that calcium matters outside the cell too: when $\mathrm{C}\mathrm{a}{\phantom{X}}^{2+}$ in the blood and tissue fluid falls too low, nerves and muscles become over-excitable and fire on their own.

Osteoporosis is most common in women after menopause, when the level of estrogen, which helps to keep bone strong, falls. Gout follows when uric acid, a nitrogenous waste (Chapter 16), builds up until crystals form in the joints, classically the joint at the base of the big toe.