B3.3 Muscle and motility

Every movement you make — from sprinting to blinking to the beating of your heart — depends on proteins sliding past one another. Muscle is a remarkable machine that converts the chemical energy of ATP directly into mechanical force, and it does so with a precision that engineers still envy. For B3.3 the central idea is the sliding filament theory: muscles shorten not because the filaments themselves contract, but because two sets of protein filaments slide over each other. Once you can picture a single sarcomere shortening, almost every detail of this topic — from the role of calcium ions to why muscles must work in antagonistic pairs — follows logically.

The sarcomere: the contractile unit

Skeletal muscle is built from long fibres, each packed with cylindrical myofibrils. Within a myofibril, the repeating functional unit is the sarcomere, defined as the region between two Z discs (also called Z lines). The striped (striated) appearance of skeletal muscle under the microscope comes from the regular arrangement of two types of filament inside each sarcomere.

The thin filaments are made mainly of actin and are anchored to the Z discs. The thick filaments are made of myosin, whose protruding heads form cross-bridges. The pattern of overlap produces the named bands you must be able to label:

• Light I band: contains thin (actin) filaments only.
• Dark A band: the full length of the thick (myosin) filaments, including the region where thick and thin overlap.
• H zone: the central part of the A band with thick filaments only (no overlap).
• Z disc: the boundary of the sarcomere, where adjacent thin filaments are joined.

The sliding filament theory and the cross-bridge cycle

During contraction the actin and myosin filaments do not change length. Instead, the myosin heads pull the thin filaments inwards towards the centre of the sarcomere, so the filaments slide past each other and the whole sarcomere shortens. A crucial exam point follows directly: when a muscle contracts, the I band and H zone both get shorter, while the A band stays the same length (because the thick filaments themselves are unchanged).

The driving mechanism is the cross-bridge cycle, powered by ATP:

• An energised myosin head binds to a site on the actin filament, forming a cross-bridge.
• The head pivots — the power stroke — pulling the thin filament towards the sarcomere centre, and ADP and phosphate are released.
• A new ATP molecule binds to the myosin head, causing it to detach from actin.
• ATP is hydrolysed to ADP + P_i, re-cocking (re-energising) the head so the cycle can repeat.

Many heads cycle out of step, so the filaments are pulled steadily, like many hands hauling on a rope.

Calcium, troponin and the trigger for contraction

At rest the actin binding sites are blocked by a protein called tropomyosin, so cross-bridges cannot form. Contraction is controlled by calcium ions. A nerve impulse arriving at the muscle fibre triggers the release of Ca²⁺ from the sarcoplasmic reticulum (the muscle’s internal calcium store).

The calcium ions bind to troponin, changing its shape. This pulls tropomyosin aside and exposes the binding sites on actin, allowing the myosin heads to attach and the cross-bridge cycle to begin. When stimulation stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum, tropomyosin re-covers the sites, and the muscle relaxes. This is why both contraction (cross-bridge cycling) and relaxation (calcium pumping) require ATP.

A motor unit is a single motor neuron together with all the muscle fibres it stimulates. Recruiting more motor units, or stimulating them more frequently, increases the overall force a muscle produces — this is how the body grades the strength of a contraction.

Antagonistic muscles and movement at joints

Muscles can only actively pull (shorten); they cannot actively push. To move a bone back and forth, muscles must therefore be arranged in antagonistic pairs that pull in opposite directions across a joint. When one muscle contracts, its partner relaxes and is stretched, ready to reverse the movement.

The classic example is the human elbow. The biceps contracts to flex (bend) the arm while the triceps relaxes; to extend (straighten) the arm, the triceps contracts while the biceps relaxes. Such movements depend on the structure of a synovial joint, which the syllabus uses as a model: the bones are capped with cartilage and bathed in synovial fluid to reduce friction, held together by ligaments (bone to bone), while tendons attach the muscles to the bones so that the pulling force is transmitted.