Question

Discuss the role of Ca²⁺ ions in muscle contraction. Draw neat sketches to illustrate your answer.

Answer 1

Muscle contraction is initiated by a neural signal, which after reaching the neuromuscular junction or motor end plate releases a neurotransmitter, as a result, an action potential in the sarcolemma is generated. Action potential spreads through muscle fibre and causes the release of calcium ions into the sarcoplasm. An increase in Ca²⁺ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby removes the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active site on actin to form a cross-bridge. This pulls the attached actin filaments towards the centre of ‘A’ band. The ‘Z’ line attached to these actions is also pulled inwards thereby causing the shortening of the sarcomere, i.e., contraction.

A new ATP binds to the myosin head and the cross-bridge is broken. The ATP is again hydrolysed by the myosin head and the cycle of cross-bridge formation and breakage is repeated causing further sliding. The process continues till the Ca⁺⁺ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments and the breakage of all cross-bridges. This cause the return of ‘Z’ lines along with filaments back to their original position, i.e., relaxation.