Question:
Can you explain the process of muscle contraction.
Answer:
Muscle contraction essentially depends on the interaction between actin light chains and myosin heavy chains in muscle fibres. We will first consider light chains; these are made up of actin filaments, around which are wound tropomyosin molecules. Attached to these tropomyosin molecules are molecules called troponin. When the muscle is not contracting, tropomyosin will sit in the myosin binding sites, this prevents actin and myosin interacting with one another which is needed for contraction. Myosin thick filaments contain globular, ‘golf club shaped’ heads. These interact with the binding sites on actin molecules during muscle contraction. When an action potential arrives down transverse tubules in muscle fibres, it results in calcium ion release from intracellular stores. The calcium ions bind to troponin (troponin C) and cause it to change shape (conformational change). This pulls tropomyosin away from the myosin binding site on the actin filaments. This allows ‘cross bridge cycling’ – which goes as follows. Myosin heads bind the newly uncovered sites on actin molecules. Myosin then heads flex in unison, pulling the actin filaments over them ‘the power stroke’. The back of the myosin head then hydrolyses ATP into ADP and phosphate, allowing detachment. The myosin heads are now free to attach to another binding site and pull the actin thin filaments even further over them, repeating the cycle.
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