Membrane potential electromechanical coupling

Neuromuscular transmission (B) of motor nerve impulses to the striated muscle fiber takes place at the motor end plate. The nerve impulse liberates acetylcholine (ACh) from the axon terminal. ACh binds to nicotinic cholinoceptors at the motor end plate. This causes depolarization of the postsynaptic membrane, which in turn elicits a propagated action potential (AP) in the surrounding sarcolemma. The AP triggers a release of Ca2+ from its storage organelles, the sarcoplasmic reticulum (SR), within the muscle fiber the rise in Ca2+ concentration induces a contraction (electromechanical coupling). Meanwhile, ACh is hydrolyzed by acetylcholinesterase (p. 104) excitation of the end plate subsides. If no AP follows, Ca2+ is taken up again by the SR and the myofilaments relax. 

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