Amplitude fluctuations in synaptic potentials evoked in cat spinal motoneurones at identified group Ia synapses.

Abstract

Excitatory post-synaptic potentials (EPSP) were evoked in spinal motoneurons (of anesthetized cats) by impulses in single group Ia axons. The morphological details of the Ia axons-motoneuron connection involved in generating each EPSP were subsequently rconstructed. The fluctuation pattern of the peak amplitude of each EPSP was determined using a deconvolution method, taking into account the recording noise. Four EPSP were analyzed. One did not fluctuate in amplitude; the others fluctuated betwen discrete amplitudes which were separated by quantal increments. The number of increments which must be added to produce the largest peak amplitude of each EPSP was always less than, or equal to, the number of synaptic boutons in the connection at which the EPSP was generated. The results are consistent with the all-or-none transmission manner hypothesis at each synaptic bouton. Different boutons in the termination of a Ia axon with a motoneuron have different probabilities of releasing transmitter, and this probability is sometimes zero at low stimulation rates. The idea that the discrete amplitudes of an evoked EPSP result from intermittent transmission, in an all-or-none manner, at some or all of the boutons in the termination is supported.