Muscle contraction is a complex process that involves the interaction of myofilaments (actin and myosin) within muscle fibers, leading to muscle shortening and force generation.
The process can be broken down into four key steps: neuromuscular transmission, excitation-contraction coupling, cross-bridge cycling, and muscle relaxation.
1. Neuromuscular transmission:
Action Potential Initiation: Muscle contraction begins with an action potential (nerve impulse) generated in a motor neuron.
Acetylcholine Release: The action potential travels down the motor neuron’s axon to the neuromuscular junction (NMJ), where it triggers the release of acetylcholine (ACh).
Muscle Action Potential: ACh diffuses across the synaptic cleft and binds to receptors on the muscle fiber's sarcolemma (plasma membrane), generating an action potential in the muscle fiber.
2. Excitation-contraction coupling:
Propagation of Action Potential: The action potential spreads along the sarcolemma and down the T-tubules, invaginations of the sarcolemma that reach deep into the muscle fiber.
Calcium Release: This electrical signal triggers the release of calcium ions from the sarcoplasmic reticulum (SR) into the sarcoplasm (the cytoplasm of the muscle fiber).
Calcium Binding: Calcium ions bind to troponin on the thin filaments (actin), causing a conformational change that moves tropomyosin and exposes the myosin-binding sites on actin.
3. Cross-bridge cycling:
Power Stroke: Myosin heads, energized by the hydrolysis of ATP, pivot and pull the actin filaments toward the center of the sarcomere, shortening the muscle.
Release and Reset: Myosin heads release ADP, detach from actin when a new ATP binds, and return to their original position after ATP is hydrolyzed, ready to form another cross-bridge.
Continuation: This cycle repeats as long as calcium ions remain bound to troponin and ATP is available.
4. Muscle relaxation:
Cessation of Neural Signal: Muscle relaxation occurs when the motor neuron stops releasing ACh, halting the action potential in the muscle fiber.
Calcium Reuptake: Calcium ions are pumped back into the SR, reducing calcium levels in the sarcoplasm.
Blocking of Myosin-Binding Sites: As calcium dissociates from troponin, tropomyosin moves back to block the myosin-binding sites on actin, preventing further cross-bridge formation.
Relaxation: The muscle fiber returns to its resting state.