INTRACELLULAR POTENTIALS RECORDED FROM MOTONEURONS FOLLOWING PRECENTRAL GYRUS STIMULATION IN PRIMATE

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Abstract
Descending volleys from the primary motor cortex have been shown to initiate both excitatory and inhibitory processes within segmental motoneurons of the monkeys lumbosacral spinal cord. Excitatory post-synaptic potentials are initiated in some motoneurons as early as 3.5 msec. after application of an electrical stimulus to the precentral gyrus. This brief latency must be presumed to be the result of direct projections from cerebral cortex to spinal motoneurons since the earliest potentials to be recorded from descending tracts at the same spinal level occur only 0.3 msec. prior to the time of EPSP initiation in some motoneurons. The majority of motoneurons, however, are excited after a delay which would allow ample time for interneuronal relay. More than one-half of the motoneurons which are depolarized by descending pyramidal volleys can be caused to initiate spike discharge by single volleys originating from the precentral gyrus. In addition to excitatory influences, in some motoneurons precentral stimulation results in the development of inhibitory post-synaptic potentials which are capable of preventing spike discharge resulting from dorsal root stimulation. Evidence also has been presented to suggest that some motoneurons receive both excitatory and inhibitory inputs from the pathways mediating precentral gyrus effects.

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