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

1. The origins of fluctuations in charge transfer during the generation of Ia e.p.s.p.s have been investigated. The discrete components which make up the fluctuating e.p.s.p. have been separated. 2. Some e.p.s.p.s fluctuate between two different amplitudes and time courses. These fluctuations have been analysed to show that charge transmission always occurs at one synaptic location, but not always at a second synaptic location. 3. The failures in transmission were study by stimulating the afferent fibre at different frequencies. Although different probabilities of failure were obtained at different frequencies, there was no systematic change in probability with increasing frequency. 4. Single afferents were tetanized and histograms of charge transfer computed during post-tetanic potentiation (p.t.p.). Only half of the units studied showed any p.t.p. In those that did, evidence was found for a decrease in the probability of failure during potentiation. 5. 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. 6. The fluctuations in transmission at a single synapse can be described by a binomial process with n = 1 and p less than or equal to 1. Junctional mechanisms consistent with this description are discussed. Alternative mechanisms which associate failures with failure of impulse transmission at afferent fibre branch points are also suggested.