Neuromuscular Signalling and Sliding-Filament Contraction

A motor neuron initiates contraction

Voluntary skeletal muscle contraction begins with neural activity in motor pathways. A motor neuron extends an axon to one or more skeletal muscle fibres; the neuron together with its innervated fibres forms a motor unit. At each fibre, the axon terminal faces a specialised region of sarcolemma called the neuromuscular junction, or motor end plate. A narrow synaptic gap separates nerve and muscle membranes, so the electrical signal crosses through a chemical messenger before the fibre produces its own electrical activity.

Arrival of a motor-neuron action potential causes the terminal to release acetylcholine. The transmitter binds receptor channels in the muscle end plate and creates local depolarisation. When this is sufficiently strong, a muscle action potential propagates along the sarcolemma and through transverse tubules. Excitation–contraction coupling then triggers Ca²⁺ release from the sarcoplasmic reticulum into the cytoplasm. Thus the sequence is neural signal, chemical transmission, muscle electrical excitation and internal calcium signalling. All of these stages occur rapidly but are logically distinct.

Calcium removes inhibition of actin sites

In a resting muscle fibre, troponin and tropomyosin reduce access of myosin heads to actin-binding sites. When free Ca²⁺ increases, it binds to a troponin subunit. The protein complex changes conformation, moving tropomyosin and allowing myosin heads to engage actin. Ca²⁺ is a regulatory signal rather than the direct fuel of the power stroke. The mechanical energy comes from the actin–myosin motor system using ATP, and cross-bridge formation can recur for as long as calcium and sufficient ATP remain available.

The power stroke and the ATP cycle

A primed myosin head carrying ADP and inorganic phosphate binds an exposed actin site. Following binding, changes involving phosphate and ADP release accompany a conformational power stroke that pulls the thin filament toward the sarcomere centre. The cross-bridge does not remain attached forever in a normally working fibre: binding of a fresh ATP molecule to the myosin head promotes detachment from actin. ATP hydrolysis then primes or recocks the head for another actin-binding event. Repetition of this sequence causes cumulative filament sliding.

Many cross-bridges cycle asynchronously, so their combined action produces relatively smooth tension. A single muscle action potential is not necessarily equivalent to one myosin stroke: the calcium signal persists long enough for many molecular interactions, depending on its duration. The basic four-stage teaching model represents attachment, power stroke, ATP-mediated detachment and ATP hydrolysis-driven re-priming. Different molecular states can be distinguished in advanced biochemistry, but these four stages account for the main changes needed to understand contraction.

The sliding filament theory explains band changes

Actin filaments are anchored at the Z lines, so when myosin heads pull actin toward the middle of the sarcomere, the Z lines draw closer together. A sarcomere shortens even though the thin and thick protein filaments retain essentially the same lengths. The distance occupied by the thick myosin filament is the A band, and it remains constant. The I bands contain thin filaments without overlap, so their widths become smaller. The H zone consists of central thick-only filaments and also narrows as overlap increases.

This explains why a model in which myosin filaments physically shrink would be wrong. A narrower A band would imply shortened thick filaments rather than sliding. Similarly, Ca²⁺ does not push actin towards the centre by itself; calcium changes access to binding sites, while myosin heads generate force. During a sufficiently strong contraction, the H zone may become very small. The central M line remains a structural landmark organising thick filaments even as Z lines move nearer.

Molecular work becomes limb movement

A whole skeletal muscle contains many myofibrils working in parallel, and each myofibril has sarcomeres in series. Repeated microscopic shortening can therefore create macroscopic movement. Tension generated in the fibre transfers through connective tissues and tendons to bones, which rotate at joints. Coordinated action between muscles and their antagonists may produce motion, control braking, or stabilise posture. The sliding-filament mechanism underlies all of these tasks, even when the final whole muscle length does not decrease visibly.

Quick Quiz

Model Answers

1. Motor axon impulse releases acetylcholine; muscle membrane depolarises and conducts an action potential; SR releases Ca²⁺; calcium binds troponin, moving tropomyosin; primed myosin attaches and generates force.

2. Myosin attaches to accessible actin; a power stroke accompanies product release; new ATP causes detachment; ATP hydrolysis re-primes the head for another attachment.

3. Thin actin slides further between constant-length myosin filaments, reducing thin-only and thick-only regions while the thick-filament span is unchanged.

4. Many sarcomeres shorten; tension is transmitted through fibre and muscle connective tissue into tendons, which pull skeletal levers across joints.

5. Myosin heads require new ATP to release strong actin interactions; absent ATP can leave cross-bridges attached, contributing to rigor.