Excitability changes along pyramidal tract axons after sensory stimulation

Recordings were made extracellularly of the activity of single pyramidal tract (PT) neurons in the forepaw focus of the cat's postcruciate cerebral cortex. Activity was evoked antidromically by near-threshold stimulation of the contralateral dorsolateral funiculus of the C6 spinal cord (corticospinal tract), the ipsilateral medullary pyramid, the ipsilateral cerebral peduncle, and the ipsilateral internal capsule. Orthodromic activity was initiated by shocks to each of the four paws. The excitability of PT axons at the spinal cord, medullary pyramid, cerebral peduncle, and internal capsule was tested by measuring the threshold to antidromic activation in conditioning-testing interaction. The PT axons with small contralateral receptive fields (small-field cells) showed no change in excitability at any of the four test sites after stimulation of any of the four paws. In contrast, most PT axons with large bilateral receptive fields (wide-field cells) showed increases in excitability after stimulation of the contralateral forepaw at one or more of the test sites, in many cases, at all four sites. Although not tested in all neurons, increases in excitability of wide-field axons were also seen after stimulation of the ipsilateral forepaw and the contralateral and ipsilateral hind paws. The increased excitability was detectable 20 to 25 ms after the contralateral forepaw stimulus, was maximum at 25 to 30 ms, and then returned to baseline by 75 to 100 ms. Maximum changes in excitability varied from 5% to more than 60% in different axons. The time course of the change in excitability was nearly the same at each site, but the magnitude of the change was most often greater at the pyramid and cerebral peduncle than at the internal capsule. In some cells, a period of decreased excitability, in most cases apparent inexcitability, followed the period of increased excitability. This could also be observed at all four test sites. The period of increased excitability most likely resulted from depolarization of terminals of branches of the PT axon near the testing site, whereas the period of decreased excitability may have resulted from strong depolarization or hyperpolarization of such terminals or from hyperpolarization of one or more nodes of Ranvier, the site situated nearer to the soma than the test site.