Fluctuations in the peak amplitudes of composite excitatory postsynaptic potentials (EPSPs) in cat spinal motoneurons were analyzed during posttetanic potentiation (PTP). Each of a series of identical tetanic stimulus trains delivered to a muscle nerve was followed by 45 test stimuli applied at 2-s intervals. The mean peak amplitude and mean peak variance were calculated for EPSPs evoked by all those stimuli following a tetanus with the same time interval. It was assumed that the variance arises primarily from the probabilistic all-or-none behavior of single synaptic boutons and background noise due to spontaneous synaptic activity and thermal noise in the recording system. The variance was corrected for the contribution from additive Gaussian background noise. 2. If it is assumed that individual synaptic boutons behave independently, corrected mean peak variance and mean peak amplitude are related by a parabolic function. The expected parabolic relationship was seen in 9 of 31 cases studied, and the parameters of the best parabolic fit to the data allowed estimation of some synaptic properties. From these parameters, the mean amplitude of the unit EPSP (v) was estimated to be 102.1 +/- 57.4 (SD) microV. An average of 3.7 boutons comprised each Ia-motoneuron contact system. 3. On average, only 27% of all synaptic boutons given off by the stimulated Ia fibers to one motoneuron were active and releasing transmitter during unpotentiated reflex transmission. The remaining 73% of the synapse population was intermittently silent. The population of boutons which took part in synaptic transmission could be divided into two subpopulations, one with a release probability P = 1 and a second with a mean release probability P = 0.13 +/- 0.086. 4. We conclude that synaptic boutons connecting Ia afferents to motoneurons exist in two populations, one having a high and one a low probability of transmitter release. Transmitter release is quantal, resulting in a unit EPSP of approximately 100 microV measured at the motoneuron soma.
