Question

How does a muscle shorten during its contraction and return to its original form during relaxation?

Answer 1

The steps followed during relaxation are:

1. At the instant when the neural signal reaches the neuromuscular junction and acetylcholine the neurotransmitter acetylcholine is released which creates an action potential in the sarcolemma.
2. Due to this action potential, calcium ions are released which spread through muscle fibres.
3. Calcium ion bind to the subunit of troponin on the actin filament due to the increase in the concentration of the calcium ions which further remove the active sites in the myosin.
4. The myosin binds with the actin and forms a cross bridge.
5. 5. This causes the attached actin filaments to move towards the centre of A-band. The Z-line also moves inwards, and a contraction of the sarcomere occurs in the muscles. During muscle contraction, the I-band gets reduced while the A-band retains its original length.
6. Breaking of the cross-bridge occurs when the ADP and Pi releasing myosin go back to their relaxed state and another new ATP binds which breaks the cross-bridge. Again the formation and breakage of the cross-bridge occur when ATP is hydrolysed again.
7. The process remains continues as the concentration of calcium ions causes the formation of the action potential.