Mechanism of muscle contraction

The sliding filament theory explains contraction: a muscle fibre shortens because the thin filaments slide over the thick filaments.

The CNS starts contraction by sending a signal along a motor neuron. A motor neuron with the muscle fibres it supplies is a motor unit.

The junction between a motor neuron and the sarcolemma is the neuromuscular junction, or motor end plate. There the signal releases the neurotransmitter acetylcholine, which sets up an action potential in the sarcolemma.

The action potential spreads through the fibre and causes calcium ions to be released into the sarcoplasm.

Ca²⁺ binds a subunit of troponin, which uncovers the active sites for myosin on actin.

Using energy from ATP hydrolysis, the myosin head binds the exposed sites to form a cross bridge, and pulls the attached actin filaments towards the centre of the A band.

The Z lines attached to those actins are pulled inwards too, so the sarcomere shortens. During contraction the I bands shorten while the A bands keep their length.

The myosin head releases ADP and Pi and returns to its relaxed state; a fresh ATP binds and the cross bridge breaks. The ATP is hydrolysed again and the cycle of making and breaking cross bridges repeats, sliding the filaments further.

This goes on until Ca²⁺ is pumped back into the sarcoplasmic cisternae, the actin sites are covered again, and the Z lines return to their original place: relaxation.