ACTIVATION OF INTERNUNCIAL NEURONS THROUGH COLLATERALS OF PYRAMIDAL FIBERS AT CORTICAL LEVEL

Abstract

The large surface-negative wave occurring in the later phase of the cortical response to antidromic stimulation of the medullary pyramid was identified as related to the activity of collaterals of pyramidal fibers at cortical level. This wave can be abolished in about 1.5 minutes either by N2 anoxia or by superficial application of KCN. In both cases it recovers earlier than the dendritic component of the response. It can be enhanced by the action of strychnine, and can be recruited by repeated shocks delivered during a period of 5-8 msec. after first stimulus. To low frequency tetanic stimulation this wave appears only following the first shock. Properties of this wave suggests that it is produced by the activity of cortical internuncial neurons involving synapses. The experimental circumstance suggests the collaterals of the large pyramidal neurons as the only possible origin for synaptic excitation in the elaboration of this wave. During the period of collateral activity, unit discharges presumably produced by internuncial neurons can be recorded from the cortex with a microelectrode. Such discharges are proved to be capable of reexciting the pyramidal fibers which have originally carried antidromic impulses. A returning volley of impulses resulting from collateral activity in the cortex is detectable at the site of stimulation in the medullary pyramid.