Describe the important steps in muscle contraction. - Vidyadip

Question

Describe the important steps in muscle contraction.

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Answer

The major phases of muscle contraction are as follows:
1. Excitation: This is the initial phase of muscle contraction. Due to neural stimulation, acetylcholine (a neurotransmitter) is released from nerve endings at the neuromuscular junction. This acetylcholine increases the permeability of the sarcolemma to Na⁺ ions, leading to the generation of an inward positive charge on the inner surface of the sarcolemma. This charge spreads across the entire sarcolemma, triggering the excitation of the muscle fibre.
2. Excitation-Contraction Coupling: During this phase, the excitation activates the muscle fibre. The excitation spreads rapidly through the muscle fibre and triggers the release of Ca⁺⁺ ions from the sarcoplasmic reticulum. These ions bind to troponin-C, causing a conformational change in the troponin-tropomyosin complex. As a result, active sites on the actin filament are exposed, allowing myosin heads to bind to actin, initiating muscle contraction.
3. Contraction: Binding of the myosin heads to the active sites on the actin filament leads to the power stroke. ATP attached to the myosin head is broken down into ADP and Pi by the ATPase enzyme on the myosin head, providing energy for the power stroke. This results in the sliding of actin filaments toward the centre of the sarcomere, causing muscle contraction.
4. Relaxation: When neural stimulation ceases, Ca⁺⁺ ions are actively transported back into the sarcoplasmic reticulum. This removal of Ca⁺⁺ ions from troponin-C causes the troponin-tropomyosin complex to return to its original position, covering the active sites on actin. As a result, the muscle relaxes and returns to its resting state.

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