B3.3 Muscle and motility

Movement is one of the functions of life, and every organism has adaptations for it, even if it never leaves the spot where it lives. It helps to separate three kinds of movement.

• Movement inside cells: cytoplasm streams, chromosomes are pulled apart in mitosis and vesicles are carried along the cytoskeleton by motor proteins.
• Internal movements of the body: the heart pumps, peristalsis moves food along the gut and the ribs and diaphragm ventilate the lungs.
• Locomotion: moving the whole organism from place to place.

A species that can move from place to place is motile; one that stays fixed in one position is sessile.

A motile and a sessile example

Plants are also sessile, anchored by their roots, but they still move parts of their bodies. Young sunflower shoots turn to follow the sun across the sky, and the leaflets of Mimosa pudica fold within seconds of being touched.

Skeletal muscle is made of muscle fibres. Each fibre is a single long cell formed by the fusion of many cells, so it is multinucleate. It is surrounded by a plasma membrane called the sarcolemma and is packed with parallel myofibrils, with many mitochondria between them and a network of sarcoplasmic reticulum (SR) wrapped round each one. Switch the 3D model to Muscle to myofibril to see these levels of organization.

The sarcomere

Each myofibril is a chain of sarcomeres, joined end to end at Z-lines (Z-discs). Two kinds of protein filament overlap in a regular pattern:

• Thin filaments of actin, fixed to a Z-line at one end, with the regulatory proteins tropomyosin and troponin.
• Thick filaments of myosin, in the centre, held together at the M-line. Myosin heads project from them towards the actin, except in a bare zone in the middle. In cross-section each myosin filament is surrounded by six actin filaments.

The overlap gives the bands seen in micrographs:

How a sarcomere shortens

When a muscle fibre is stimulated, the myosin heads bind to actin, forming cross-bridges, and pull the actin filaments towards the M-line. The Z-lines are pulled closer together. Neither filament changes length: they slide past each other and their overlap increases. Thousands of sarcomeres in series shorten together, so the whole muscle shortens.

Switching contraction on: calcium

A nerve impulse arriving at a neuromuscular junction triggers an action potential in the sarcolemma. This spreads deep into the fibre along T-tubules, which makes the sarcoplasmic reticulum release calcium ions (Ca²⁺) into the fluid around the myofibrils.

At rest, tropomyosin lies over the myosin-binding sites on actin. Ca²⁺ binds to troponin, which changes shape and pulls tropomyosin aside, so the binding sites are exposed.

The cross-bridge cycle

1. A cocked myosin head, holding ADP and a phosphate (Pi), binds to an exposed site on actin, forming a cross-bridge.
2. Release of Pi triggers the power stroke: the head swivels towards the M-line, sliding the actin a few nanometres. ADP is then released.
3. ATP binds to the head, which detaches from actin.
4. The head hydrolyses ATP to ADP and Pi, and the energy released re-cocks it, ready to bind a site further along.

The heads cycle independently, so at any moment some are attached and pulling while others are detaching, and the filaments slide smoothly, like a team pulling hand over hand on a rope. ATP is needed both to detach the heads and to re-cock them, which is why fibres are packed with mitochondria. After death ATP runs out, the heads stay locked to actin and the muscles become rigid (rigor mortis).

Relaxation

When nerve impulses stop, Ca²⁺ is pumped back into the SR by active transport, which also uses ATP. Tropomyosin covers the binding sites again, no new cross-bridges form and the muscle stops exerting force.

Muscles can only pull

A muscle exerts force only when it contracts, and then it can only pull. It cannot push, and a relaxed muscle cannot lengthen itself. So muscles are arranged in antagonistic pairs that move a part of the body in opposite directions, and each muscle is stretched back to its full length by the contraction of the other.

• The biceps contracts to flex (bend) the elbow; the triceps relaxes and is stretched.
• The triceps contracts to extend (straighten) the elbow; the biceps relaxes and is stretched.

Despite the name, the two muscles work together: their contractions are coordinated so that one relaxes as the other contracts, giving smooth, controlled movement.

Titin

Titin is the largest protein known: a single chain of over 30,000 amino acids. Each titin molecule runs from a Z-line to the M-line, alongside a myosin filament. In the I-band it is folded into a springy section that behaves like an elastic band. Titin has three roles:

• It anchors each myosin filament to the Z-lines, keeping it in the centre of the sarcomere among its six actin filaments.
• It prevents overstretching: it resists more and more strongly as the sarcomere is lengthened, so actin and myosin are not pulled apart.
• It stores potential energy when a relaxed sarcomere is stretched by the antagonist. When the stretching force is removed, titin recoils, helping the sarcomere return to its resting length.

Skeletal muscle contracts only when it is stimulated by a motor neuron. The cell bodies of motor neurons lie in the spinal cord (or brainstem), and each axon runs to one muscle, where it divides into many branches.

• Each branch ends at a neuromuscular junction with one muscle fibre. This is a synapse: the neurotransmitter acetylcholine is released from vesicles, diffuses across the synaptic cleft and binds to receptors in the sarcolemma, causing depolarization and an action potential in the fibre.
• Each muscle fibre receives a junction from only one motor neuron.
• A motor unit is one motor neuron together with all the muscle fibres it stimulates.

Why motor units matter

When a motor neuron fires, all the fibres in its unit contract together. The nervous system controls the force of a whole muscle by changing how many motor units are active (recruitment) and how often they fire.

• Muscles that make fine, precise movements, such as those that move the eyeball or the fingers, have small motor units of a few fibres each.
• Large postural or leg muscles, such as the calf muscles, have large motor units with a thousand or more fibres, producing big forces with relatively few neurons.

Because the fibres of one unit are spread through the muscle, the pull is even across the whole muscle. For how acetylcholine works at synapses, see C2.2 Neural signalling.

A skeleton is a rigid framework that supports and protects the body. For movement it has two roles: it provides anchorage (fixed points for muscles to pull on) and it acts as a set of levers.

Exoskeletons and endoskeletons

Levers

A lever has a pivot (the fulcrum), a force applied to it (the effort) and a force it works against (the load). In a limb, the joint is the fulcrum, the pull of a muscle at its point of attachment is the effort, and the weight of the limb and anything it holds is the load.

Each muscle has a fixed attachment (the origin) and an attachment on the bone it moves (the insertion), on the far side of the joint. The biceps is inserted close to the elbow, so a small shortening of the muscle swings the hand through a large distance, quickly, but the muscle must pull with a much larger force than the load. Arthropod limbs work in the same way: each leg segment is a lever, moved by antagonistic flexor and extensor muscles inside it.

Where two bones meet there is a joint. In a synovial joint the bones can move freely relative to each other. The hip is a ball-and-socket joint: the rounded head of the femur fits into a cup-shaped socket in the pelvis.

The range of motion of a joint is how far it can move in each direction. It depends on the shape of the bones, the ligaments and the muscles around the joint.

• The elbow and knee are hinge joints: they allow flexion (bending) and extension (straightening) in one plane only. (Turning the palm up and down happens at the joints between the radius and ulna, not at the hinge.)
• The hip is a ball-and-socket joint, so it moves in three planes: flexion and extension, abduction (away from the midline) and adduction (towards or across it), and medial and lateral rotation.

How to measure a joint angle

• A goniometer is a protractor with two arms. Place its pivot over the centre of the joint, line the fixed arm up with the stationary bone (such as the upper arm or trunk) and the moving arm with the moving bone, then read the angle at the end of the movement. Step 3 of the antagonistic muscles animation shows this for the elbow.
• Computer analysis of images: photograph or film the movement, mark landmarks on the body (for example the shoulder, elbow and wrist) and measure the angle between the lines with image-analysis software.

Joint angles are usually recorded from the anatomical position, so a straight elbow is 0° of flexion. If the angle between the upper arm and forearm is measured instead, subtract it from 180°: an included angle of 35° is 145° of flexion.

Antagonistic muscles also cause internal movements. Between each pair of ribs are two layers of intercostal muscle, attached to the rib above and the rib below. Their fibres run at different angles, so they move the ribcage in opposite directions.

• External intercostals: fibres run downwards and forwards. When they contract, the ribs are pulled up and out, the volume of the thorax increases, the pressure falls and air flows into the lungs (inspiration).
• Internal intercostals: fibres run downwards and backwards, roughly at right angles to the external fibres. When they contract, the ribs are pulled down and in, decreasing the volume of the thorax and forcing air out (forced expiration).

Whichever layer contracts, it stretches the other, which is relaxed. The titin in the stretched sarcomeres stores potential energy, which is released as elastic recoil when the other layer relaxes, helping the ribs return to their resting position. In quiet breathing, expiration is mainly passive; the internal intercostals are used when breathing out forcefully, for example during exercise. Ventilation is covered in B3.1 Gas exchange.

Locomotion costs energy, so it must bring benefits that outweigh the cost. Four main reasons are listed below, each with an example.

Migration lets an animal use resources in places that are only suitable for part of the year, such as long summer days with plentiful food at high latitudes.

Marine mammals, such as whales, dolphins and seals, evolved from land mammals. Water is much denser and more viscous than air, so moving through it needs different adaptations. The bottlenose dolphin (Tursiops truncatus) shows them clearly.

Streamlining

• The body is fusiform: widest near the front and tapering towards the tail, which minimizes drag.
• The outline is smooth: there are no hind limbs and no external ears, the skin is hairless and a layer of blubber fills out the shape (and adds buoyancy).

Limbs and tail

• The forelimbs are flippers. They still contain the bones of an arm and hand, but these are short, flattened and enclosed in one paddle, used for steering and braking.
• The tail ends in a fluke: two horizontal lobes with no bones inside. Muscles of the back and tail beat the fluke up and down, which provides the thrust. (A fish’s tail fin is vertical and moves from side to side.)
• A dorsal fin on the back stabilizes the body and prevents rolling.

Breathing between dives

• The nostril (in whales, one or two) has moved to the top of the head as the blowhole, so the animal can breathe with only the top of its head out of the water. A muscular plug or flap keeps it closed during a dive.
• The airway from the blowhole to the lungs is separate from the mouth and oesophagus, so the animal can open its mouth underwater to catch prey without water entering the lungs.
• The airways are reinforced with cartilage (and smooth muscle), so they stay open for very rapid exhalation and inhalation at the surface; each breath replaces most of the air in the lungs.