Statistical fluctuations in charge transfer at Ia synapses on spinal motoneurones.

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Abstract
Net inward charge was calculated for unitary Ia EPSP [excitatory postsynaptic potential] evoked in [cat] spinal motoneurons. Fluctuations in this charge were used to describe the fluctuations in transmission at the Ia synapse. The statistical details of variation in transmission at Ia synapses were largely obscured by noise, arising from spontaneous activity and recording apparatus. A computational procedure was described, which utilized the measured statistical properties of the recording noise, and the measured charge histogram of the evoked EPSP (with noise) to calculate the variations in charge due to the EPSP alone. The fluctuation in charge transmission at Ia synapses was, in general, non-quantal. Nor were they described by binomial or Poisson statistics. The basic mode of transmission at a single terminal is probably all-or-none. The fluctuations result from the combined effect of failures at several terminals arising from a single afferent fiber.