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One goal of the neurophysiological approach to the study of nervous systems is to analyze neuronal circuitry in terms of the synaptic actions of one cell on another, particularly in instances in which both cells are functionally identifiable and components of a circuit whose overall structural and functional properties can be analyzed with experimental techniques. The present project contributed to this type of effort by providing an analysis of the recurrent Renshaw circuit, a prominent pathway in the mammalian spinal cord which includes recurrent motoneuronal collaterals, Renshaw cells and other interneurons, which, in turn, project to motoneurons. The project describes the use of a relatively new data processing technique, spike-triggered averaging, to study the effects of the single impulses of single motor axons on the postsynaptic activity of single motoneurons which were responsive to the test impulses by way of components of the recurrent Renshaw circuit. The experimental paradigm involved intracellular recording from single motoneurons in low-spinal cats, either anesthetized with chloralose-urethane or unanesthetized after their ischemic decapitation. The synaptic noise recorded in each motoneuron served as the input to a signal averager which was triggered by brief electrical shocks used to activate single antidromic impulses in single motor axons, either by way of an intra-axonally positioned microelectrode in the muscle nerve or by microstimulation of the muscle supplied by the axon. The resultant average revealed the motoneuron's response to each single antidromic impulse; a recurrent inhibitory postsynaptic potential, recorded for the first-ever time in this project and termed a single-axon RIPSP. The experimental results described in the report include: first, the measurement, incidence and characterization of single-axon RIPSPs; and second, their use to test a hypothesis concerned with the distribution of Renshaw-cell effects within the spinal cord. The single-axon RIPSP measurement was shown to be the clearest example yet provided in the neurophysiological literature that spike-triggered averaging can be used to detect synaptic activity crossing two or more synapses within the central nervous system. Furthermore, the hypothesis was confirmed that Renshaw-cell effects within a single spinal motor nucleus are distributed according to the principle of topographic specificity.

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