Question

Detail how calcium ions work to allow for muscle contraction. Be sure to describe the roles of sarcoplasmic reticulum, tropomyosin, troponin, ATP, actin and myosin.

Answer 1

Role of calcium ion in muscle contraction:

Muscle contraction is initiated by a signal sent by the central nervous system through a motor neuron. Junction between motor neurone and the sarcolemma of muscle fibre is called neuromuscular Junction. A neural signal reaching neuromuscular Junction releases a neurotransmitter that is acetal choline which generates and action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ion into the sarcoplasmic. Increasing calcium Ion concentration in the sarcoplasm leads to binding of calcium with the subunit of troponin on actin filaments and thereby remove the masking off active sites for myosin. Utilising the energy from ATP hydrolysis the myosin head now binds to active site on actin to form a cross bridge. This pulls the attached actin towards the centre of A band . The Z line attached to these actins are also pulled towards thereby causing a shortening of sarcomere ie, contraction. During shortening of muscle ie, contaction, the I band get reduced, whereas the A band retain the length.

The myosin releasing the ADP andPi goes back to its relaxed state. A new ATP binds 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 Ca2+ ions are pumped back to sarcoplasmic cisternae resulting in the masking of actin filaments. This causes return of Z line back to their original poistion ie relaxation.

Hence ca2+ ion play vital role in muscle contraction.