QUANTAL ANALYSIS OF SYNAPTIC POTENTIALS IN NEURONS OF THE CENTRAL NERVOUS-SYSTEM

There is considerable evidence supporting the all-or-none hypothesis for the quantal synaptic potential at synapses in the CNS; a constant amplitude synaptic potential is generated at a single synaptic site each time transmitter is released at that site. The evidence is indirect: the definitive experiment of recording postsynaptically, or focally, from a single release site has not yet been achieved. It is not known if the release of more than one vesicle of transmitter from the same site can be triggered by a single impulse. The evidence for lack of significant variability in the quantal amplitude at release sites arising from the same axon and located at the same electronic location is again indirect but compelling, especially for synapses on DSCT neurons. Quantal size and variability have been measured directly for IPSPs in M cells, and the results confirm earlier assumptions that the variability is small compared with the recording noise. The best explanation for the lack of variability of quantal size is that a quantum of transmitter opens all, or almost all, of the subsynaptic channels. Variations in quantal size for synapses located at different electrotonic locations on dendrites but arising from the same axon might be expected and would be unresolvable using present methods of recording and analysis. The morphology of the connections made by 1a axons with motoneurons indicates that many dendritic connections involve only a small number of contacts at a localized region of the dendrites, and, under these circumstances, any variation in quantal size will not be caused by electrotonic factors. The invariance of quantal size measured at the soma suggests that the quantal EPSP generated at dendritic synapses results from a larger quantal current than that which occurs at somatic synapses.