Question

Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction.
A. Sodium
B. Potassium
C. Calcium
D. Magnesium

Answer 1

The skeletal muscle contraction is best explained by a theory named Sliding Filament Theory. This theory explains muscle contraction i.e. muscle contracts when two types of proteins slide one over each other. It is sliding of thin filaments over the thick filaments. During muscle contraction, a molecule of ATP is used and various ions perform their function in muscle contraction.

Complete step-by-step answer:
There are two contractile proteins present in muscle thin filament known as actin filament and the thick filament is known as myosin filament having differences in their structure. The initiation of muscle contraction is with the signal sent by the Central nervous system. A motor neuron along with the muscle fibres constitute a motor unit. The junction between a motor neuron and sarcolemma is called the neuromuscular junction. A signal reaching its junction release acetylcholine which generates an action potential in the sarcolemma. It leads to spread of signal through muscle fibre and calcium ions are released into the sarcoplasm. Calcium ions increase and it binds to troponin subunit on actin filaments. By binding, they remove the masking site for myosin filaments and thereby binding to this utilise energy from ATP hydrolysis. Therefore now myosin head binds to the exposed active sites on actin to form a cross-bridge between them.
By this, they pull the actin filament towards the centre of ‘A’ band. The ‘Z’ line attached to these actin filaments pull inwards causing contraction of muscles I.e. shortening of the sarcomere. During muscle contraction the length of the ‘I’ band is reduced but the length of the ‘A’ band remains the same. It is repeated again and again causing muscle contractions. This continues till calcium ions are pumped back into sarcoplasmic cisternae causing masking of actin filaments again to stop the attachment of myosin head to the active site.

This is how muscle contraction occurs. So, the correct answer to this question is the option” C” calcium as it binds to troponin subunit of actin filament and therefore causes unmasking of active sites for myosin filaments.

Note: Muscles contract with the use of an energy molecule I.e. hydrolysis of an ATP molecule. Muscles subsequently contract with the use of ATP molecules and binding of actin and myosin filaments. The skeletal muscles are known to be the contracting muscles of the body thereby exerting locomotion and movement of the body. As stated sliding filament theory is the best explanation of muscle contraction.