What sequence describes the phases of muscle contraction and relaxation?

The sequence of muscle contraction and relaxation is accurately described by the phases of excitation, coupling, contraction, and relaxation.

In the excitation phase, a motor neuron releases acetylcholine at the neuromuscular junction, leading to depolarization of the muscle fiber's sarcolemma and subsequent generation of an action potential. This action potential then triggers the coupling phase, where calcium ions are released from the sarcoplasmic reticulum into the cytosol of the muscle cell.

Next, during the contraction phase, the calcium binds to troponin, which leads to a series of interactions within the actin and myosin filaments, ultimately resulting in muscle shortening and force production. Finally, during the relaxation phase, calcium ions are actively transported back into the sarcoplasmic reticulum, leading to the detachment of myosin heads from actin, thus allowing the muscle to return to its resting state.

Understanding this sequence is crucial for comprehending how muscles contract and relax, as well as the biochemical and physiological processes involved in muscle movement.