Physiology of muscle contraction
Muscle contraction is a complex physiological process that involves the interaction between muscle fibers, myofibrils, and motor neurons. The contraction of skeletal muscle can be divided into several stages:
- Excitation: The process of muscle contraction begins with an action potential, or electrical signal, that is generated by a motor neuron in the spinal cord. This signal travels down the motor neuron and reaches the neuromuscular junction, where it triggers the release of acetylcholine, a neurotransmitter.
- Excitation-Contraction Coupling: The release of acetylcholine causes depolarization of the muscle fiber membrane, which triggers the release of calcium ions from the sarcoplasmic reticulum, a specialized organelle in the muscle fiber. The calcium ions bind to the protein complex called troponin, which causes tropomyosin to move away from the actin-binding site on myosin, allowing myosin to bind to actin.
- Contraction: Once myosin binds to actin, it undergoes a conformational change, which generates force and pulls the thin filaments toward the center of the sarcomere. This process is known as the power stroke. ATP hydrolysis is required to release myosin from actin, so that it can bind again and repeat the power stroke.
- Relaxation: When the action potential ceases and calcium is pumped back into the sarcoplasmic reticulum, the tropomyosin-troponin complex returns to its original position, blocking the actin-binding site on myosin. This prevents further power strokes, and the muscle relaxes.
Overall, muscle contraction is a highly regulated process that involves the precise interaction between muscle fibers, myofibrils, and motor neurons. The process of excitation, excitation-contraction coupling, and contraction ultimately leads to the generation of force and movement, while relaxation requires the cessation of the electrical signal and the sequestration of calcium ions.