Prolonged changes in excitability of pyramidal tract neurones in the cat: a post‐synaptic mechanism.

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Abstract
Prolonged changes in the excitability of cortical neurons can be produced by altering their firing rates for brief periods. In the anesthetized cat, increased firing of pyramidal tract cells induced by trains of antidromic conditioning shocks led to increases in cell excitability, as measured by the size of the mass response at the medullary pyramid to test shocks applied to the cortical surface. Post-synaptic mechanisms could be responsible in 2 ways. In the 1st experiment, MgCl2 solution (1 mol/l.) was applied to the cortical surface to block synaptic activity throughout the cortical depth. Following antidromic conditioning trains, cell excitability was increased; the size of the mass response was up to 30% larger than the control values. This persisted undiminished for up to 3 h. In the 2nd experiment, synaptic activity was not blocked, but the effects of antidromic plus synaptic activation of pyramidal tract cells were compared to the effects of synaptic activation alone. Antidromic plus synaptic activation was obtained by applying conditioning trains to the pyramidal tract at the medulla ipsilateral to the cortical test shock; prolonged increases in the ipsilateral response to the test shock were produced. Synaptic activation alone was obtained by the same conditioning trains, but in those cells whose axons projected into the contralateral pyramidal tract; prolonged increases in the contralateral response to the cortical test shock were never seen. In many instances prolonged decreases in excitability were found. Prolonged increases in excitability of pyramidal tract cells can occur in the absence of any synaptic input, demonstrating that the underlying mechanism is post-synaptic; this does not preclude the action of synaptic mechanisms when synaptic transmission is not blocked.