How muscle contract

1. Skeleton muscle contraction is initiated by arrival of a nerve impulse at the nerve muscle junction.
2. Arrival of the nerve impulse cause influx of calcium ions into the pre-synaptic knob leading to release of acetylcholine from pre-synaptic knob into the synaptic vesicle.
3. acetylcholine depolarizes the muscle fiber
4. the impulse generated thereof is transmitted via T-tubules through sarcoplasm.
5. Ca²⁺ are released from sarcoplasmic reticulum to the muscle fibre.
6. Calcium ions initiates contraction of muscles by binding troponin
7. Tropomyosin shift from actin binding site
8. myosin heads get instantly attached to the binding sites on actin filament and contraction of muscles occurs.

Relaxation phase

1. After action potential has cholinesterase is released and breaks down acetylcholine.
2. Sarcolemma & T-tubules repolarized
3. the calcium gates close, and calcium pumps located on the sarcoplasmic reticulum remove calcium from the cytoplasm.
4. As the calcium gets pumped back into the sarcoplasmic reticulum, calcium ions come off the troponin.
5. The troponin returns to its normal shape and allows tropomyosin to cover the actin-myosin binding sites on the actin filament.
6. Because no binding sites are available now, no cross bridges can form, Passive sliding of filaments occur and the muscle relaxes.