Final answer:
The relaxation of skeletal muscle fiber starts with the motor neuron stopping ACh release, leading to muscle repolarization and Ca++ reuptake into the SR. This causes the actin-binding sites to be reshielded, stopping cross-bridge formation, and resulting in muscle relaxation.
Step-by-step explanation:
Relaxation Process of Skeletal Muscle Fibers
The relaxation of a skeletal muscle fiber follows the contraction phase and it involves several steps. The process begins when the motor neuron ceases the release of the neurotransmitter Acetylcholine (ACh) into the synapse at the neuromuscular junction (NMJ). When the signal stops, the muscle fiber repolarizes, causing the sarcoplasmic reticulum (SR), which stores calcium ions (Ca++), to close its gates, preventing further release.
Subsequently, ATP-driven pumps move Ca++ out of the sarcoplasm and back into the SR. This action allows for the reshielding of actin-binding sites by the regulatory protein tropomyosin, preventing the formation of cross-bridges between actin and myosin, which is necessary for muscle contraction. As the cross-bridge formation stops, muscle tension is lost, leading to muscle relaxation.
A relaxation phase follows the twitch contraction when tension decreases. This is facilitated by the reuptake of calcium ions into the SR, the reshielding of actin, and the muscle fiber returning to a state of rest. Muscle fatigue can also contribute to the cessation of contraction when ATP sources are depleted.