Spontaneous calcium transients in automatic boutons and varicosities

Spontaneous multiquantal events are recorded at many different boutons and varicosities for which there is evidence that the receptor patch at these individual synapses is saturated by the transmitter unit. In order to reconcile these observations, a model is considered in which calcium release from a ryanodine channel within a nerve terminal can reach adjacent active zones in single synapses in sufficient concentration to occasionally trigger exocytosis from adjacent zones synchronously, giving rise to multiquantal spontaneous events. It is shown that the spatial and temporal distribution of calcium concentration at the active zone after a spontaneous opening of a ryanodine channel can predict the amplitude and time course of observed calcium-activated potassium channel currents. Similar calcium transients are sufficient to give rise to multiquantal events. Such events suggest a multi hypothesis for secretion.