Describe the mechanism of muscle contraction

Muscles are the structures which in coordination with bones bring about movement in an organ. Muscle contraction is clearly explained by sliding-filament theory. Muscles are made up of 2 types of proteins namely Actin and Myosin filaments.

•  Muscle starts to contract when it receives information from motor neurons.
•  Neuromuscular junctions are formed by the junctions between motor neuron and sarcolemma of individual muscle fibre.
•  As the neural signal reaches neuro-muscular junction, acetyl choline is released at this place.
•  Acetyl choline causes an action potential in the sarcolemma and releases calcium ions in the sarcoplasm.
•  Calcium ions bind to the troponin of the actin and bring a change in its structure. This causes a movement in tropomyosin.
•  This structural change exposes myosin binding sites on actin filaments. Myosin binds to actin forming a cross bridge.
•  These cross bridges pull the attached actin to the centre of the A band. Z-line is also pulled inwards. This causes shortening of sarcomere which is termed as muscle contraction.
•  During contraction, I bands are reduced, A bands retain their length. Actin and myosin filaments slide over one another during contraction.
•  Myosin releases ADP and pi and returns to its original state. New ATP binds to myosin head and the cross bridge is broken.
•  Myosin head hydrolyses ATP to form a cross bridge.
•  These events are repeated till calcium ions return into sarcoplasmic reticulum. Calcium ions are released till acetyl choline is present. Acetyl choline is released till the muscle fibre receives signal from motor neurons.