CENTRAL PATHWAY FOR DIRECT INHIBITORY ACTION OF IMPULSES IN LARGEST AFFERENT NERVE FIBRES TO MUSCLE

Abstract

After any lonitudinal intramedullary conduction in the inhibitory pathway is measured and allowed for, the central latency for the inhibitory post-synaptic potential generated by direct inhibitory action on motoneurones is about 0.8 msec longer than for the monosynaptic excitatory post-synaptic potential. None of this differences can be attributed to a longer duration for the true synaptic delay; hence there is likely to be a synaptic relay on the direct inhibitory pathway. It is shown that the primary afferent impulses from annulospiral endings excite synaptically special neurones of the intermediate nucleus of Cajal and that the properties of this synaptic relay conform with the known properties of the direct inhibitory pathway. Systematic exploration of the potential fields in the spinal cord has shown that impulses appear in the inhibitory presynaptic terminals soon after the intermediate neurons discharge impulses, but their axonal pathway has remained indeterminate. Further evidence relating to the pathway is provided by the brief positive wave that is produced on the dorso-lateral regions of the spinal cord as current flows into the activated presynaptic terminals in the motoneuronal nucleus. Other inhibitory pathways also operate on motoneurones by relaying through short-axon neurones. The relationship of these findings to the hypothesis of chemical transmission at central synapses is discussed.