1. Monosynaptic i.p.s.p.s were produced in spinal motoneurones of the cat by stimulation of a pool of interneurones following chronic degeneration of descending tracts and primary afferent fibres in the lumbosacral cord.2. Monosynaptic i.p.s.p.s so evoked by supramaximal stimuli often showed a fluctuation in amplitude with occasional failures of response.3. When two successive stimuli were applied at a short interval, the mean amplitude of the second i.p.s.p.s was greater than that of the first. This facilitation was associated with a decrease in the number of failures, a decrease in the coefficient of variation of the amplitude distribution and an increase in the probability of occurrences of large i.p.s.p.s.4. A statistical analysis of the i.p.s.p. amplitude fluctuation showed that the monosynaptic i.p.s.p. is composed of discrete unit potentials evoked with a certain probability in a manner described by a binomial law.5. The application of strychnine decreased the mean amplitude of i.p.s.p.s with little change in the coefficient of variation of the i.p.s.p. amplitude distribution.6. It is concluded that the release of inhibitory transmitter occurs in quantal steps and that strychnine blocks primarily the post-synaptic receptors for the inhibitory transmitter.
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