Non‐quantal fluctuations and transmission failures in charge transfer at Ia synapses on spinal motoneurones.

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Abstract
The origins of fluctuations in charge transfer during the generation of Ia EPSP [excitatory postsynaptic potential] [in the cat] were investigated. The discrete components which make up the fluctuating EPSP were separated. Some EPSP fluctuated between 2 different amplitudes and time courses. These fluctuations were analyzed to show that charge transmission always occurred at 1 synaptic location, but not always at a 2nd synaptic location. The failures in transmission were studied by stimulating the afferent fiber at different frequencies. Although different probabilities of failure were obtained at different frequencies, there was no systematic change in probability with increasing frequency. Single afferents were tetanized and histograms of charge transfer computed during post-tetanic potentiation (PTP). Only half of the units studied showed any PTP. In those that did, evidence was found for a decrease in the probability of failure during potentiation. The results could not be used to distinguish between failure of the impulse to always propagate into the terminals, and failure of the terminals to release transmitter following adequate depolarization. The fluctuations in transmission at a single synapse can be described by a binomial process with n = 1 and p .ltoreq. 1. Junctional mechanisms consistent with this description were discussed. Alternative mechanisms which associate failures with failure of impulse transmission at afferent fiber branch points were also suggested.