Pyramidal tract responses during seizures

Abstract

1. 1. Responses to stimulation of the sensorimotor cortex were recorded from the ipsilateral medullary pyramid during seizures in locally anesthetized, paralyzed cats. Seizures were induced either by i.v. administration of Metrazol or by high frequency stimulation of the ipsilateral sensorimotor cortex. 2. 2. Changes in PT responses that were observed include: an increased fluctuation in amplitude of both D and I components during seizures as compared with the resting period and the addition of a large late I component not obtainable by strong, brief stimulation of normal cortex. In addition, paroxysmal activity in the ECoG usually accompanied that in the PT. 3. 3. Before the amplitude of the PT response can be used as a measure of the number of responding PT neurons (and hence as a measure of PT excitability) any change in size of the action potentials of PT axons during a seizure must be determined. An average decrement of 5–10% was indirectly estimated from the decrement in population responses to direct stimulation of PT axons at several intensities. 4. 4. The PT response to a test stimulus given more than 250 msec after the onset of an antecedent paroxysmal PT discharge usually had the largest late I wave, some enhancement of the early I waves, but a slightly reduced D wave. Such reduction is mainly attributable to reduced size of the individual axonal potentials and is only occasionally attributable to reduced excitability of PT neurons. By contrast, a test stimulus given during the latter part of a paroxysmal discharge, or soon thereafter, elicited an enhanced D wave and reduced I waves. 5. 5. The interneuronal circuits subserving the late I wave are located in the homolateral sensorimotor cortex because of the following: 1. (a) After unilateral topical application of Metrazol to the sensorimotor cortex, the late I wave response was initially confined to the same side. 2. (b) The late I wave was not elicited by subcortical stimulation of white matter during a seizure. 3. (c) Late I waves could still be elicited by stimulation of the sensorimotor cortex after extensive acute ipsilateral thalamic lesions (including the ventral thalamic group), or after an extensive lesion of the ipsilateral head of the caudate nucleus, or after contralateral hemispherectomy. 4. 6. All I wave responses were markedly depressed during the post-ictal period. 5. 7. The characteristic changes in PT responses during seizures did not depend in contamitant changes in arterial pressure. 6. 8.The findings suggest that at certain times during a seizure, PT neurons have their excitability at, and occasionally slighly below, control levels until powerfully driven by the cortical interneuronal circuits subserving either the late I waves or paroxysmal discharges.