Relation between structural and release parameters at the frog sensory‐motor synapse.

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Abstract
The sensory-motor synaptic connections in the frog lumbar cord were used to examine the relationship between the statistical characteristics of the unitary excitatory post-synaptic potential (e.p.s.p.) and the number and organization of synaptic contacts. The primary afferent fiber used in evoking the e.p.s.p., and a motoneuron in which it was recorded, were both labeled with horseradish peroxidase (HRP). A significant correlation was found between the number of contacting boutons and the amplitude of the chemical component of the unitary e.p.s.p. generated at the same connections. The amplitude fluctuation patterns of the single-fiber e.p.s.p. could be fitted by both Possion and binomial distribution. The number of presumed Poisson release sites as estimated from the ratio Vmax/v (where Vmax is the maximal amplitude of the chemical component of e.p.s.p. and v is quantal size) is always less than or equal to the total number of boutons observed histologically. In 3 connections there was a close correspondence between the number of binomial release units, n and the number of contact regions formed by the tight clusters of contacting boutons. The unit potential amplitude estimated from the Poission distribution was 2 to 3 times smaller than the quantal size calculated from binomial distribution. A similar numerical relationship was found between the number of contacting boutons and the number of contact regions. It is suggested that at a single bouton, transmission results in release of a single quantum of transmitter, whereas the binomal quantum probably reflects the multi-quantal release occurring simultaneously at boutons comprising a contact region. A significant correlation wsa found between the mean quantum content estimated either from Poisson or binomial distribution and the number of contacting boutons and contact regions respectively, indicating the dependence of quantal release on the magnitude of synaptic surface. No correlation was found between the motoneuronal soma diameter and the quantal size, although the former is significantly correlated with the number of contacting boutons.