Triggering contraction

Nerve impulses trigger muscle contraction by causing depolarisation of the plasmamembrane (sarcolemma). Key to this are Transverse tubules (T-tubules) which are invaginations of the plasmamembrane which lie adjacent to the outer face of the sarcoplasmic reticulum (SR) around each myofibril. An action potential causes opening of a voltage gated Ca2+ channel which release a small burst of Ca2+ into the cytosol. This Ca2+ binds to a channel in the SR which triggers massive and explosive release of Ca2+ into the cytosol. The increase in calcium concentration causes myosin to bind to actin.