Physiology of the Neuromuscular Junction
The process of neuromuscular transmission is conceptually simple, the main basic events having been established and widely accepted for more than 40 years. The neurotransmitter acetylcholine is synthesized within the nerve endings and stored in vesicles. A small, ineffective amount is continuously released spontaneously, mainly from the axoplasm but also from the vesicles. Vesicular release (but not axoplasmic release) is abruptly and greatly accelerated, through a Ca2+-dependent mechanism, by the arrival of a nerve impulse at the nerve endings. The released acetylcholine diffuses across the narrow junctional cleft and combines fleetingly with the nicotinic acetylcholine receptors on the postjunctional membrane of the motor endplate. The consequence is a localized depolarization, the endplate potential, which initiates a propagating action potential that passes around the muscle fibe membrane to trigger the contractile mechanism. Although these basic tenets of the transmission process have been accepted for many years, important advances in detailed knowledge continue to be made through the accelerating developments of new skills and techniques, especially in the fields of molecular biology and electrophysiology. The nicotinic acetylcholine receptors of the neuromuscular junction were the first to be isolated, biochemically and biophysically characterized, reconstituted into artificial bilayers, and cloned and sequenced.
KeywordsSynaptic Vesicle Neuromuscular Junction Nicotinic Receptor Nerve Impulse Acetylcholine Release
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