Describe the important steps in muscle contraction.

Asked by Abhisek | 11 months ago |  72

1 Answer

Solution :-

During skeletal muscle contraction, the thick filament slides past the thin filament (actin) via repeated binding, releasing myosin along the filament. This entire process takes place in sequential order.

(i) Step 1: Muscle contraction is initiated by a signal that is sent by a CNS (Central nervous system) through a motor neuron. A neuromuscular junction or motorend

plate is a junction between a motor neuron and the sarcolemma of the muscle fibre. When a signal reaches the neuromuscular junctions, Acetylcholine (a neurotransmitter) is released which results in the generation of an action potential in the sarcolemma.

(ii) Step 2: This spreads through the muscle fibres, resulting in the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm.

(iii) Step 3: The increased calcium ions \( (Ca ^{2+})\) in the sarcoplasm leads to the activation of actin sites. Calcium ions \( (Ca ^{2+})\) bind with the subunit of troponin on actin filaments and thus remove the masking of active sites for myosin. Hence, active sites on actin are exposed and this allows myosin heads to attach to this site.

(iv) Step 4: The myosin head now attaches to the exposed site of actin to form a cross-bridge by utilizing energy from ATP hydrolysis. The actin filaments are pulled towards the centre of the A-band. The Z line attached to the actin filaments is also pulled, causing shortening of the sarcomere, i.e., the contraction of the muscle occurs. It is clear from the above steps, that during contraction, the length of the A-band or anisotropic band remains constant as its original length whereas the I-band or isotropic band gets reduced.

(v) Step 5: After muscle contraction, the myosin head goes back to its relaxed state, releasing ADP and inorganic phosphate (Pi). A new ATP molecule binds and

detaches myosin, thus the cross-bridges are broken.

(vi) Step 6: This process of formation and breaking down is repeated causing further sliding. This process continues until the calcium ions are pumped back to the

sarcoplasmic cisternae. Hence, the calcium ions concentration decreases. This results in masking the actin filaments and leading to muscle relaxation.

Answered by Pragya Singh | 11 months ago

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