Muscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron. A motor neuron along with the muscle fibers connected to it constitute a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fibers is called the neuromuscular junction or motor end plate.

1. A neural signal reaching this junction releases a neurotransmitter (Acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibers and causes the release of calcium ions into the sarcoplasm. 

2. Increase in Ca²⁺ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. 

3. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin laments towards the centre of ‘A’ band. 

4. The 7’ line attached to these actions are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. 

5. It is clear from the above steps, that during shortening of the muscle, i.e., contraction, the T bands get reduced, whereas the A’ bands retain the length. 

6. The myosin, releasing the ADP and PI goes back to its relaxed state. A new ATP binds and the cross-bridge is broken. The ATP is again hydroysed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding. 

7. The process continues till the Ca²⁺ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin laments. This causes the return of 7 lines back to their original position, i.e., relaxation.