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

1. Net inward charge is calculated for unitary Ia e.p.s.p.s. evoked in spinal motoneurones. Fluctuations in this charge are used to describe the fluctuations in transmission at the Ia synapse. 2. The statistical details of variation in transmission at Ia synapses are largely obscured by noise, arising from spontaneous activity and recording apparatus. 3. A computational procedure is described, which utilized the measured statistical properties of the recording noise, and the measured charge histogram of the evoked e.p.s.p. (with noise) to calculate the variations in charge due to the e.p.s.p. alone. 4. The fluctuation in charge transmission at Ia synapses are, in general, non-quantal. Nor are they described by binomial or Poisson statistics. 5. It is suggested that the basic mode of transmission at a single terminal is all-or-none. The fluctuations result from the combined effect of failures at several terminals arising from a single afferent fibre. This hypothesis is supported by further analysis of these results in a subsequent paper (Edwards, Redman & Walmysley, 1976a).