9.3 Skeletal Muscle Contraction and Motor Units

A skeletal muscle contains bundles of muscle fibres. Each fibre is a long multinucleate cell enclosed by a sarcolemma and packed with myofibrils. Myofibrils contain repeating sarcomeres arranged end to end. Mitochondria lie between them and provide ATP aerobically, while the sarcoplasmic reticulum forms an internal membrane network that stores calcium ions close to the contractile machinery.

A sarcomere extends from one Z line to the next. Thin actin filaments are anchored at Z lines and project toward the centre. Thick myosin filaments occupy the centre. Regions containing thick filaments appear dark; regions with thin filaments but no thick filaments appear lighter. Repetition of aligned bands across myofibrils gives skeletal muscle its striated appearance.

During contraction, Z lines approach and the sarcomere shortens. The thin filaments slide farther between the thick filaments, so thin-only regions become narrower. The length of the thick-filament region remains essentially constant because neither actin nor myosin filaments themselves shorten. A micrograph showing a constant dark-band width but reduced light-band width is therefore evidence for sliding rather than filament compression.

An action potential reaches the neuromuscular junction and causes acetylcholine release. Acetylcholine binds receptors on the motor end plate, depolarizing the sarcolemma. The action potential spreads over the surface and descends into the fibre along transverse tubules. T-tubules bring the electrical event close to every myofibril and trigger calcium release from the sarcoplasmic reticulum.

At rest, tropomyosin lies along actin and covers myosin-binding sites; troponin helps position it. Calcium binds troponin and changes the complex's shape, shifting tropomyosin away. Calcium does not supply the energy for contraction and does not form the cross-bridge. It acts as a regulatory signal that makes actin sites accessible.

An energized myosin head carrying ADP and inorganic phosphate binds exposed actin and forms a cross-bridge. Phosphate release strengthens binding and initiates the power stroke; the head pivots and pulls actin toward the centre of the sarcomere, followed by ADP release. Because many heads cycle asynchronously, force is sustained rather than occurring as one synchronized jerk.

A new ATP molecule binds the myosin head and reduces its affinity for actin, causing detachment. ATP hydrolysis then recocks and re-energizes the head. If calcium remains high and ATP remains available, the head binds a new site and repeats the cycle. ATP is thus required both to detach myosin and to prepare it for another stroke. Severe ATP depletion can leave cross-bridges attached, explaining post-mortem rigidity.

Relaxation is active in its energy requirement. Calcium pumps in the sarcoplasmic-reticulum membrane use ATP to lower cytosolic calcium. Troponin loses bound calcium, tropomyosin again blocks actin sites, and cross-bridge formation falls. Elastic components and opposing forces restore length. A motor impulse ceasing is necessary but not sufficient unless calcium is removed and attached heads can detach.

Titin is an exceptionally large elastic protein extending from the Z disc along the thick filament toward the centre. It helps centre and stabilize myosin, resists excessive stretching and behaves as a molecular spring when a relaxed sarcomere is lengthened. Its tension contributes to passive muscle stiffness and recoil. Titin does not replace actin-myosin cycling as the source of active shortening.

When an antagonistic muscle stretches, elastic elements including titin store potential energy. Recoil can assist return toward resting length and helps keep the thick filament positioned for future contractions. The amount of active force depends on initial sarcomere length because actin-myosin overlap must permit many cross-bridges without excessive filament interference. Structure therefore links muscle length to force.

A motor unit consists of one motor neurone and all skeletal muscle fibres it innervates. Each muscle fibre normally receives input from one motor neurone at one neuromuscular junction, but a motor axon branches to many fibres. When that neurone fires above threshold, all fibres in its motor unit are recruited. A whole muscle contains many motor units whose fibres are intermingled.

Fine-control muscles have small motor units, so adding one unit adds only a small force increment. Powerful muscles may have hundreds or more fibres per unit. Increasing force depends partly on recruitment: activating more motor units brings more fibres into contraction. It also depends on firing frequency, because closely spaced stimuli can summate. An individual action potential is all-or-none, while whole-muscle force is graded by population and timing.

Recruitment also supports fatigue resistance. Low-force tasks can rotate activity among smaller, fatigue-resistant units, while demanding tasks recruit additional larger units. Loss of one motor neurone therefore denervates every fibre assigned to its unit rather than removing one compact block of tissue. The nervous system controls force through a distributed map.