ANATOMY

Detailed explanation-1: -Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. Recall that this excitation actually triggers the release of calcium ions (Ca++) from its storage in the cell’s SR.

Detailed explanation-2: -As ACh binds at the motor end plate, this depolarization is called an end-plate potential. The depolarization then spreads along the sarcolemma and down the T tubules, creating an action potential.

Detailed explanation-3: -When a muscle action potential propagates along the sarcolemma and into the T tubules, it causes Ca2+ release channels in the SR membrane to open. As these channels open, Ca2+ flows out of the SR into the sarcoplasm around the thick and thin filaments.

Detailed explanation-4: -The membrane depolarization at the synaptic cleft triggers nearby voltage-gated sodium channels to open. Sodium ions enter the muscle fiber further depolarizing the membrane, and an action potential rapidly spreads (or “fires”) along the entire membrane to initiate excitation-contraction coupling.