A skeletal muscle cell, or muscle fibre, contains many nuclei and is several centrimetres long. The cytoplasm contains blocks of parallel structures called myofibrils, which are bundles of thin actin and thick myosin filaments. One such block is called a sarcomere. The cell surface membrane, or sarcolemma, has inturnings called T-tubules, which surround the myofibrils. The endoplasmic reticulum is specialised as a sarcoplasmic reticulum into which calcium ions are pumped, using ATP, when the muscle is relaxed.
When a nerve impulse arrives at a neuromuscular junction, the following events lead to muscle contraction:
- The sarcolemma of the muscle fibre is depolarised, and an action potential spreads cross it, including the membranes of the T-tubules.
- This, in turn, depolarises the membrane of the sarcoplasmic reticulum, which becomes permeable to its enclosed calcium ions.
- Calcium ions flood into the cytoplasm and bind to a protein associated with the actin filaments.
- This makes it possible for myosin to bind to actin, and for each sarcomere to shorten.
Role of ATP in muscle contraction