Physiology

The neuromuscular junction lies between motor nerves and striated muscle. The transmitter acetylcholine is synthesized in the cytoplasm of the prejunctional nerve ending, stored in vesicles, and released in response to a nerve impulse. The receptors for acetylcholine, which are integral with their ion channels, lie on the postjunctional membrane. The simultaneous binding of two acetylcholine molecules is required for a channel to open, whereas occupation of either or both sites by an antagonist molecule will inactivate the channel. The ion channels are selective for small cations, and it is the sudden transmembrane flux of sodium ions through a large number of simultaneously opened channels which is responsible for the generation of an endplate potential and consequently the onward propagation of the impulse. Receptors also exist on prejunctional nerve endings, and these are involved in modulation of impulse transmission. Stimulation of the prejunctional nicotinic receptors by acetylcholine increases transmitter mobilization at higher frequencies.

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