During the muscles contraction, the thin filaments slide over the thick filaments. A signal sent by the central nervous system via motor neuron initiates muscle contraction. The neuromuscular junction is the junction between a motor neuron and sarcolemma. Acetylcholine is released when a neural signal reaches this junction and action potential is generated in the sarcolemma. When this spreads through the muscle fibre, calcium ion is released in the sarcoplasm. Calcium then binds to troponin on actin filaments and exposes the active sites for myosin. Myosin binds to the exposed active site on actin using energy from the hydrolysis of ATP. This pulls the actin towards the centre. The Z lines attached to these are also pulled, and contraction occurs. Myosin is in a relaxed state.
Consequently, the hydrolysis of ATP at the myosin head continues and this leads to further sliding. This is repeated till calcium ions are pumped back to the sarcolemma and results in covering of the actin sites again. The Z lines move back to their original positions. This causes relaxation. Muscle fatigue occurs due to repeated activation of the muscles leading to accumulation of lactic acid.