Locomotion and Movement

Movement means a change in position of a body part, of a single cell, or of the whole organism. All living things move in some way, but not every movement shifts the organism from one place to another. When a movement does carry the organism from one place to another, we call it locomotion. So every locomotion is a movement, but not every movement is locomotion. A cow chewing its cud is moving its jaw; the same cow walking to a field is showing locomotion. The organs used are often the same, which is why the two words get mixed up.

Inside the human body, cells show three broad kinds of movement: amoeboid, ciliary and muscular.

Amoeboid movement. Some specialised cells such as macrophages and leucocytes (white blood cells) crawl the way an Amoeba does. The cell throws out temporary finger-like extensions of cytoplasm called pseudopodia, produced by the streaming of protoplasm from one part of the cell into another. Cytoskeletal elements, especially microfilaments, drive this streaming. This is how a leucocyte squeezes out of a capillary wall and crawls towards a site of infection. The important point is that here the whole cell changes its position.

Ciliary movement. Most of the internal tubular organs of the body are lined by ciliated epithelium. Cilia are tiny hair-like projections on the free surface of these cells, and they beat in a coordinated rhythm, one after another, like a field of grass bending in waves. In the trachea, this beating pushes dust particles and other foreign substances, trapped in mucus, back up and away from the lungs so that they can be coughed out. In the female reproductive tract, the same beating carries the ovum along the oviduct. Notice the difference from amoeboid movement: here the cell itself stays anchored in place, and what actually travels is the material lying outside the cell.

Muscular movement. Moving the limbs, jaws, tongue, eyelids and neck needs muscles. Muscle is a specialised tissue of mesodermal origin, and it makes up roughly 40 to 50 per cent of the body weight of an adult human. Four properties make it special: excitability, the ability to respond to a stimulus; contractility, the ability to shorten and generate force; extensibility, the ability to be stretched; and elasticity, the ability to come back to the original length. Human locomotion, whether walking, running, climbing or swimming, is nothing but muscles pulling on bones across joints. That is why muscles and the skeleton are always studied together in this chapter.

All muscle tissue contracts, but the three types differ in location, appearance and control. Learn each type by three tags: where it sits, striated or not, and voluntary or involuntary.

Skeletal muscle is closely attached to the bones of the skeleton, usually through tough bands of connective tissue called tendons. Under a microscope it shows clear alternating dark and light stripes across each fibre, so it is called striated muscle. It is voluntary, meaning it works under the direct control of your will through the somatic nervous system. Skeletal muscles are responsible for the movements of the limbs, the trunk, the face and the jaws, and therefore for locomotion. They tire quickly compared with the other two types.

Visceral muscle is located in the inner walls of the hollow visceral organs: the alimentary canal, the reproductive tract, the ureters, the blood vessels. It shows no stripes, so it is called non-striated or smooth muscle, and it is involuntary because you cannot decide to contract it. It carries out slow, sustained activities such as the transport of food down the gut and the passage of gametes through the genital tract.

Cardiac muscle is found only in the heart. It is the odd one out because it is striated like skeletal muscle but involuntary like visceral muscle. Its cells are branched and joined end to end by special junctions, so an impulse spreads through the whole sheet and the heart contracts as one unit. It never tires in a normal lifetime.

Skeletal muscle fibres are further sorted into two kinds by the amount of an oxygen-storing pigment called myoglobin present in them. Red fibres contain a high quantity of myoglobin, which gives them their red colour, along with plenty of mitochondria. They can use large amounts of oxygen and are therefore called aerobic muscles; they contract more slowly but keep going for a long time without fatigue, which suits posture and endurance work. White fibres contain much less myoglobin so they look pale, they have fewer mitochondria, and they carry a large amount of sarcoplasmic reticulum. They depend on the anaerobic breakdown of glycogen for energy, so they give quick, powerful bursts but tire fast. Repeated hard use of muscle allows lactic acid to pile up from this anaerobic breakdown of glycogen, and that build-up is what we feel as fatigue.

Take any skeletal muscle, say the biceps, and open it up level by level. The whole muscle is made of many bundles called muscle bundles or fascicles, and these bundles are held together by a common layer of collagenous connective tissue called fascia. Inside one bundle lie many long cylindrical muscle fibres arranged in parallel arrays. A muscle fibre is a single muscle cell, and it is unusual in two ways: it is very long, and it contains many nuclei, so it is described as a syncytium.

Each muscle fibre is lined by a plasma membrane called the sarcolemma, which encloses the cytoplasm of the fibre, called sarcoplasm. The endoplasmic reticulum of the muscle fibre is called the sarcoplasmic reticulum, and its job in this chapter is one thing you must not forget: it is the store house of calcium ions. The characteristic feature of the sarcoplasm is a large number of parallel filaments called myofibrils or myofilaments, running along the length of the fibre.

Now look at one myofibril. Along its length it shows alternate dark and light bands, and because the myofibrils lie side by side in register, these bands line up across the whole fibre and give skeletal muscle its striped look. The striping comes from the arrangement of two proteins, actin and myosin, both arranged as rods parallel to each other and to the long axis of the myofibril.

Here is the layout in words, which is all you need even without a figure. The light band contains only thin actin filaments and is called the I band or isotropic band. The dark band contains the thick myosin filaments and is called the A band or anisotropic band. Actin is therefore the thin filament and myosin the thick filament. In the middle of every I band there runs an elastic fibre called the Z line, and the thin filaments are firmly attached to it, so each Z line has thin filaments radiating out from both its sides. The thick filaments in the A band are held together at their middle by a thin fibrous structure called the M line.

The portion of a myofibril lying between two successive Z lines is called a sarcomere, and this is the functional unit of contraction. In a resting sarcomere the free ends of the thin filaments coming from the two Z lines reach in and partly overlap the free ends of the thick filaments. The central part of the thick filament that is not overlapped by any thin filament is called the H zone. So going across one sarcomere you meet: Z line, then the part of the I band with thin filament only, then the overlap region, then the H zone with thick filament only, then overlap again, then thin filament only, then the next Z line.

Both actin and myosin are polymerised proteins with contractility, which means they can generate a pull. But neither filament is made of just one protein, and the extra proteins are where the real work of this section lies.

The thin filament, called the actin filament, is really a complex of three proteins. Two strands of F actin (filamentous actin) are helically wound around each other, like two strands of a rope. Each F actin strand is itself a polymer of many small round subunits called G actin (globular actin). Lying close to the F actins and running along the whole length of the filament are two filaments of another protein, tropomyosin. Sitting on this tropomyosin at regular intervals is a complex protein called troponin. In the resting state, one subunit of troponin covers, or masks, the active binding sites for myosin that are present on the actin. This masking is the reason a resting muscle does not contract even though actin and myosin are lying right next to each other. Remember the count: G actin and F actin are two forms of the same protein, so the three proteins of the thin filament are actin, tropomyosin and troponin.

The thick filament is the myosin filament. It too is a polymerised protein. Many monomeric units called meromyosins are packed together to make one thick filament. Each meromyosin has two parts. One is a globular head with a short arm, together called heavy meromyosin or HMM. The other is a long tail, called light meromyosin or LMM. The tails lie along the axis of the filament and form its shaft. The head with its short arm projects outwards from the surface of the filament at a regular distance and a regular angle, and this projecting head-plus-arm is called the cross arm.

The globular head is where the chemistry happens. It is an active ATPase enzyme, so it can hydrolyse ATP and capture the energy released. It also carries binding sites for ATP and active sites for actin. Because the heads stick out all around the thick filament, and because the thin filaments from both Z lines reach in on either side, every thick filament is surrounded by thin filaments it can grab. When a head does grab an exposed site on actin, the structure formed is called a cross bridge. Keep the two words apart: the cross arm is the permanent projection of the myosin filament, while the cross bridge is the temporary attachment made when that arm actually binds actin.

The sliding filament theory states that the contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments. Say that sentence carefully. The filaments themselves do not shorten, fold or coil. They simply slide past one another, so the two Z lines are dragged closer together and the sarcomere becomes shorter. Add up thousands of shortened sarcomeres in series and the whole muscle shortens visibly.

Here is the sequence, from the nerve signal to relaxation.

One. The signal for contraction comes from the central nervous system and travels along a motor neuron. The point where the motor neuron meets the sarcolemma of the muscle fibre is called the neuromuscular junction or motor end plate.

Two. When the neural signal reaches the motor end plate, a neurotransmitter, acetylcholine, is released. It generates an action potential in the sarcolemma.

Three. This action potential spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm from the sarcoplasmic reticulum.

Four. The rise in calcium in the sarcoplasm leads to the binding of calcium with a subunit of troponin on the actin filaments. Troponin changes shape, pulls tropomyosin aside, and thereby removes the masking of the active sites for myosin. This is the single job of calcium in this chapter: it unmasks the binding sites. It does not supply energy and it does not bind myosin.

Five. Using the energy from ATP hydrolysis, the myosin head now binds to the exposed active site on actin to form a cross bridge.

Six. The cross bridge swivels and pulls the attached actin filaments towards the centre of the A band. Because the thin filaments are anchored to the Z lines, the Z lines are pulled inwards and the sarcomere shortens. That shortening is the contraction.

Seven. While this happens, the myosin releases ADP and inorganic phosphate and returns to its relaxed position. A new ATP molecule binds to the head and the cross bridge is broken. So one molecule of ATP is used to break the bridge and detach the head, and the ATP is then hydrolysed again by the head so that the cycle of cross bridge formation and breakage repeats and the filaments slide further. This is why a muscle cannot relax without ATP.

Eight. The process continues until the calcium ions are actively pumped back into the cisternae of the sarcoplasmic reticulum. Calcium leaves troponin, tropomyosin slides back over the binding sites, the sites are masked once more, no new cross bridges can form, and the Z lines return to their original positions. That is relaxation.

Finally, keep track of what happens to the bands. During contraction the I bands get reduced in width and the H zone narrows or disappears, because the thin filaments have slid deeper into the A band. The A band keeps its length, because the thick filaments themselves are untouched. If a statement says the A band shortens, it is wrong.

The skeletal system is the framework of bones and a few cartilages that supports the body, protects soft organs, and gives muscles something to pull against. Bone is a hard connective tissue with a matrix rich in calcium salts; cartilage is softer because its matrix is made pliable by chondroitin salts instead, which is why cartilage bends. The adult human skeleton has 206 bones, divided into the axial skeleton and the appendicular skeleton.

The axial skeleton has 80 bones. These lie along the main axis of the body: the skull, the vertebral column, the ribs and the sternum. The skull has 22 bones in two sets, 8 cranial bones that form the box protecting the brain and 14 facial bones. Besides these, each middle ear holds three tiny bones, the malleus, incus and stapes, together called the ear ossicles, giving 6 in all; the stapes is the smallest bone in the body. A single U-shaped bone, the hyoid, lies at the base of the buccal cavity. The skull rests on the vertebral column through two occipital condyles, so the human skull is described as dicondylic.

The vertebral column has 26 bones in an adult and runs from the base of the skull down the middle of the back. Each vertebra has a hollow neural canal through which the spinal cord passes. The count is 7 cervical, 12 thoracic, 5 lumbar, 1 sacral and 1 coccygeal. The sacral bone is a single bone formed by the fusion of five, and the coccygeal by the fusion of four, which is how the number comes down to 26. The first cervical vertebra is the atlas, and it is the one that meets the occipital condyles. Note that seven cervical vertebrae is the rule in almost all mammals, in a giraffe as much as in a human.

There are 12 pairs of ribs. Each rib is a thin flat bone joined at the back to a thoracic vertebra and, in most cases, at the front to the sternum. Each rib has two articulating surfaces at its dorsal end and is therefore called bicephalic. The first seven pairs attach directly to the sternum through hyaline cartilage and are called true or vertebrosternal ribs. The eighth, ninth and tenth pairs do not reach the sternum directly but join the seventh rib through hyaline cartilage, and are called false or vertebrochondral ribs. The last two pairs are not connected in front at all and are called floating ribs. The thoracic vertebrae, ribs and sternum together form the rib cage. So the axial count works out as 22 skull plus 6 ear ossicles plus 1 hyoid plus 26 vertebrae plus 24 ribs plus 1 sternum, which is 80.

The appendicular skeleton has 126 bones: the bones of the limbs and the two girdles that hang them from the axial skeleton. Each forelimb has 30 bones: humerus in the upper arm, radius and ulna in the forearm, 8 carpals in the wrist, 5 metacarpals in the palm and 14 phalanges in the fingers. Each hind limb also has 30 bones: femur, which is the longest bone in the body, then the patella or knee cap covering the knee in front, tibia and fibula in the shank, 7 tarsals in the ankle, 5 metatarsals and 14 phalanges. The pectoral girdle has two halves, each made of a clavicle and a scapula. The scapula is a large flat triangular bone on the back of the thorax; its raised ridge, the spine, ends in a flat process called the acromion with which the clavicle joins, and just below the acromion is a socket called the glenoid cavity that receives the head of the humerus. The pelvic girdle is made of two coxal bones, and each coxal bone is itself formed by the fusion of three bones, the ilium, ischium and pubis. At the point where these three meet is a socket called the acetabulum that receives the head of the femur, and in front the two halves meet at the pubic symphysis, which contains fibrous cartilage. Adding up: 4 pectoral girdle bones plus 60 forelimb bones plus 2 coxal bones plus 60 hind limb bones is 126, and 80 plus 126 is 206.

A muscle can only pull. Without joints, that pull would do nothing but squeeze two bones together. Joints are points of contact between bones, or between bones and cartilages, and the joint acts as the fulcrum about which the bone turns, exactly like the pivot of a see-saw. The muscle supplies the force, the bone acts as the lever and the joint as the fulcrum. Joints are classified into three structural types.

Fibrous joints do not allow any movement. The flat bones of the skull are joined end to end by dense fibrous connective tissue in the form of sutures, and this is what locks them into a rigid cranium that protects the brain.

Cartilaginous joints hold the bones together with cartilage and permit limited movement. The joints between the adjacent vertebrae of the vertebral column are of this kind, which is why you can bend your back a little at each level, and a lot when all the levels add up. The pubic symphysis is another example.

Synovial joints are marked by a fluid-filled synovial cavity between the articulating surfaces of the two bones. The fluid reduces friction and the gap allows considerable movement, so these are the joints that make locomotion possible. Their subtypes are worth learning with one example each. A ball and socket joint, where a rounded head sits in a cup, allows movement in all planes; the joint between the humerus and the pectoral girdle is one. A hinge joint allows movement in one plane only, like a door; the knee joint is the standard example. A pivot joint allows rotation about an axis; the joint between the atlas and the axis, which lets you shake your head, is the example. A gliding joint allows the flat surfaces of two bones to slide over each other, as between the carpals of the wrist. A saddle joint, where each surface is concave one way and convex the other, is found between the carpal and the metacarpal of the thumb and is what lets the human thumb swing across to touch the other fingers.

Now the disorders named in this chapter. State the cause in one line each and stop there.

Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction. The body's own immune system interferes with transmission at the junction, so the signal from the nerve does not reach the muscle properly. The result is fatigue, weakening and paralysis of skeletal muscle.

Muscular dystrophy is a progressive degeneration of skeletal muscle, mostly due to a genetic disorder. The word progressive matters: the muscle is not merely weak, it is being steadily lost.

Tetany is rapid spasms, that is, wild uncontrolled contractions, in the muscle due to a low level of calcium in the body fluid. This connects straight back to the sliding filament theory: calcium handling and muscle behaviour are linked, and here it is a shortage of calcium in body fluid that is at fault.

Arthritis is inflammation of the joints.

Osteoporosis is an age-related disorder with decreased bone mass and a higher chance of fractures. A decreased level of the hormone oestrogen is a common cause, which is why it is seen more often in older women.

Gout is inflammation of the joints due to the accumulation of uric acid crystals. Note the trap: both gout and arthritis involve inflamed joints, but only gout has uric acid crystals named as the cause.