Relaxation of Skeletal Muscles

Muscle relaxation begins when the motor neuron terminal stops releasing acetylcholine vesicles. The enzyme acetylcholinesterase breaks down remaining acetylcholine in the synaptic cleft into choline and acetic acid. This cessation of neurotransmitter activity prevents new action potentials, allowing the muscle fiber to return to its resting state and dissipate contraction-associated tension.

When action potentials cease, the sarcoplasmic reticulum closes calcium-release channels and actively pumps accumulated calcium ions back using calcium ATPase pumps. As calcium concentration decreases, troponin undergoes a conformational change that causes tropomyosin to block myosin binding sites on actin filaments. This prevents cross-bridge cycling and allows the muscle fiber to relax passively.

ATP binding to myosin heads deactivates cross-bridges between myosin and actin filaments. After deactivation, ATP-bound myosin heads reorient away from actin filaments, preventing sarcomere contraction. This ATP-dependent process is essential for releasing the myosin-actin interaction and allowing the muscle to relax completely.

Acetylcholinesterase rapidly degrades acetylcholine in the synaptic cleft, preventing continuous receptor activation. Swift removal and inactivation of the neurotransmitter stops excessive firing of action potentials in the postsynaptic membrane. This ensures that a single action potential has only brief impact on the muscle fiber, allowing proper relaxation between contractions.

When acetylcholine is degraded and removed from receptors, the muscle membrane repolarizes, restoring its resting potential. Repolarization closes voltage-gated calcium channels and prevents further calcium entry into the sarcoplasm. This electrical change is coupled with calcium reuptake, initiating the reversal of the contraction process and enabling muscle relaxation.

As calcium concentration in the sarcoplasm decreases, troponin releases calcium ions and reverts to its original conformation. This conformational change causes tropomyosin to move back and cover the myosin-binding sites on actin filaments. Blocking these binding sites prevents myosin heads from attaching to actin, stopping the contraction cycle and allowing the sarcomere to relax.

When action potentials fire rapidly at the neuromuscular junction, acetylcholine is released continuously into the synaptic cleft. This triggers repeated action potentials in the muscle membrane and maintains high calcium levels in the cytosol. Under these conditions, the contraction cycle repeats without muscle fibers undergoing relaxation, resulting in sustained muscle contraction or tetanus.