Quantal transmitter release onto different patterns of receptor distributions at somatic neuromuscular junctions.

The effect of different distributions of acetylcholine receptors at the active zone of somatic motor-nerve terminals on the amplitude-frequency histogram of spontaneous excitatory currents due to the secretion of a quantum transmitter has been analysed by numerically solving the diffusion and binding equations. The time course and amplitude of the spontaneous currents of different arrangements of receptors has been determined. Organization of receptors into stripes of different patterns beneath the active zone, which is known to occur, does not affect the amplitude or temporal characteristics of the spontaneous currents. Increases in the amount of transmitter in a quantum released onto the stripes does not introduce any subunit structure into the spontaneous currents. It is concluded that if subunits are detectable in measured amplitude-frequency histograms of spontaneous potentials they arise because of presynaptic rather than postsynaptic factors.