When a skeletal muscle fibre contracts, a complex process occurs within the sarcomere, the basic unit of muscle contraction. The sarcomere is made up of a series of overlapping thick and thin filaments, which slide past each other during muscle contraction, resulting in a shortening of the sarcomere and the muscle as a whole.
The thick filaments are made up of myosin, a motor protein that has the ability to bind to actin, a thin filament protein, and generate the force necessary for muscle contraction. The thin filaments are composed primarily of actin, but also contain other proteins such as tropomyosin and troponin, which regulate the interaction between myosin and actin.
When a muscle fibre receives a signal to contract, calcium ions are released from the sarcoplasmic reticulum, a specialized network of tubules within the muscle cell. These calcium ions bind to troponin, causing a conformational change that exposes the binding sites on the actin filaments.
The myosin heads then bind to these exposed sites on actin, forming cross-bridges. The energy released during this binding process causes the myosin heads to change shape, pulling the actin filaments towards the centre of the sarcomere. This shortens the sarcomere and causes the muscle to contract.
As the myosin heads continue to bind and release from the actin filaments, the thin filaments are pulled towards the centre of the sarcomere, resulting in a complete contraction. The force generated by this process is transmitted to the muscle as a whole, allowing it to produce movement.
In summary, when a skeletal muscle fibre contracts, calcium ions are released from the sarcoplasmic reticulum, which bind to troponin and expose the binding sites on the actin filaments. Myosin heads then bind to these sites, forming cross-bridges, and generate the force necessary to pull the actin filaments towards the centre of the sarcomere, resulting in muscle contraction.
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