The steps of neuromuscular transmission are as follows:
(1) An action potential is conducted down the motor axon to the prejunctional axon terminal.
(2) Depolarization of the terminal buttons opens up voltage-gated Ca2+ channels in its membranes. Ca2+ moves into the terminals along an electrochemical gradient.
(3) Elevated Ca2+ concentration in the terminal button causes exocytosis of the ACh-containing synaptic vesicles in the terminal button into the myoneural cleft.
(4) The acetylcholine released from the axon terminal then diffuses across the myoneural cleft and binds to a specific acetylcholine receptors on the motor end- plate.
(5) The binding of ACh with the ACh receptor (ACh-R) increases the conductance of the postjunctional membrane to Na+ and K+, resulting in a transient depolarization of the postjunctional membrane. This depolarization is called the endplate potential (EPP).
(6) The EPP is transient because acetylcholine is quickly hydrolyzed by the enzyme acetylcholinesterase (ACh-E) into choline and acetate. AChE is present in high concentration in the junctional cleft.
(7) The postjunctional membrane cannot generate action potentials.
However, the EPP depolarizes the adjacent muscle membrane by electro tonic conduction. When the depolarization exceeds the threshold, an action potential is triggered in the adjacent muscle membrane.