question_answer 3)

Describe the important steps in muscle contraction. 

Answer:

A neural signal reaching the neuromuscular junction releases a neurotransmitter (acetyl choline) which generates an action potential in the sarcolemma. (ii)       This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. (iii)      Increase in  level leads to the binding of calcium with a sub-unit of troponin on actins filaments and thereby remove the masking of active sites for myosin. (Iv)     Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed activesites on actins to form a cross bridge. (v)      This pulls the attached actins filaments towards the centre of A-band. (vi)     The Z-line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. (vii)    During shortening of the muscle, i.e., contraction, the I-bands get reduced, whereas, the A-bands retain the length. (viii)   The myosin, releasing the ADP and P1 goes back to its relaxed state. A new ATP binds and the cross-bridge is broken. (ix)     The ATP is again hydrolysed by the myosin head and the Cycle of cross bridge formation and breakage is repeated causing further sliding. (x)      The process continues till the  ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of Z-lines back to their original position, i.e., relaxation.