7 - Neuromuscular Transmission

Summary

Numerous terminal branches of motor nerves each end on individual component muscle fibres making up their motor unit. Each ramification terminates in a neuromuscular junction comprising pre- and postsynaptic membranes separated by the synaptic cleft. The neurotransmitter acetylcholine is released in response to depolarisation from presynaptic vesicles as discrete quanta. Each such event elicits a postsynaptic miniature endplate potential. This probabilistic release process follows a Poisson distribution whose probability increases with presynaptic depolarisation. The released acetylcholine diffuses across the synaptic gap to access postsynaptic ACh receptors, proteins each comprising five transmembrane subunits surrounding a central pore. Two of these (α) subunits each include an ACh binding site. ACh binding to both sites causes pore opening, permitting Na+ and K+ permeation, generating endplate currents demonstrable under voltage clamp. The resulting endplate potentials trigger a propagated action potential in the innervated muscle fibre when the resulting depolarisation attains the Na+ channel threshold.