Explain the process of muscle contraction in a myofibril following nervous stimulation.

Muscle contraction can be explained using the 'Sliding Filament Hypothesis'. Following nervous stimulation, a nerve impulse will be transmitted to a neuromuscular junction. This will cause a release of the neurotransmitter acetylcholine, which will cause the motor end plate to become depolarised. As a result, Ca²⁺ions are released from the sarcoplasmic reticulum of the muscle fibre, which binds to troponin, and this displaces tropomyosin. This allows for the mysoin heads to bind to the actin filaments, forming cross-bridges. Once the mysoin head is attached it 'rocks back' pulling the actin filament over the adjecant myosin filaments, effectively shortening the muscle fibre. Through the hydrolysis of an ATP molecule, the myosin head is released from the actin filament, and returns to its orginial position. It is now available to reattach again. The process can then be repeated, provided there are sufficient ATP and Ca²⁺ stores.