How do muscles contract and relax?
Muscles contract and relax through a complex process involving the interaction of several proteins, primarily actin and myosin. Here's a simplified overview of the process:
1. Stimulation: The process begins when a nerve impulse reaches the muscle, triggering the release of calcium ions (Ca2+) from specialized structures called sarcoplasmic reticulum.
2. Calcium Binding: The calcium ions bind to specific receptors on the surface of the myofilaments, which are thin filaments made of actin and other proteins.
3. Myosin Head Activation: The binding of calcium ions causes a conformational change in the myosin head, which contains a globular domain known as the myosin head group. This change exposes the binding sites on the myosin head, allowing it to interact with the actin filaments.
4. Cross-Bridge Formation: The myosin head binds to a specific site on the actin filament, forming a cross-bridge between the thick (myosin) and thin (actin) filaments. ATP, the energy molecule of the cell, binds to the myosin head.
5. Power Stroke: Hydrolysis of ATP provides energy for the "power stroke," where the myosin head undergoes a conformational change, pulling the actin filament towards the center of the sarcomere (the repeating unit of the muscle). This sliding movement of the actin filaments is what causes muscle contraction.
6. Detachment of Myosin: After the power stroke, the myosin head detaches from the actin filament and returns to its original conformation.
7. Relaxation: Relaxation occurs when the calcium ions are pumped back into the sarcoplasmic reticulum, lowering the calcium concentration in the muscle fibers. This causes the myosin heads to detach from the actin filaments, allowing them to slide back to their initial positions. The muscle fiber relaxes, and the cycle can repeat upon receiving a new nerve impulse.
This sequence of events, involving cross-bridge formation, power strokes, and detachment, continues until the muscle contraction stops. The frequency of these cycles determines the strength and duration of the muscle contraction.