Cerebral response to medullary pyramid stimulation in the rabbit

The response evoked in the cerebral cortex of the rabbit by stimulation of the medullary pyramid was studied electrophysiologically. The response was everywhere surface-positive and began 2-3 ms after the stimulus. It exceeded 1 mV in amplitude and generally lasted 2-4 ms. Maps of the response showed it to extend throughout the anterior one-third of the cortex (in the 'motor' cortex) and into adjacent tissue, including the limbic cortex. Although it depended on antidromic conduction in pyramidal tract fibers for its production, it varied in amplitude, configuration and latency at different sites and at the same sites on repeated trials. It was found to reverse polarity deep in the cortex, to become a large, negative wave in layer V and to maintain that polarity into the underlying white matter. The response disappeared during a spreading depression, though it recovered more rapidly than the primary response evoked by skin stimulation. Maps of the primary response showed a strong overlap with the 'antidromic' response, though the response was largest slightly caudal to the maximal focus of the 'antidromic' response and extended more caudally through transitional and into limbic cortex. The 'antidromic' response showed two major foci on the cortex, with an additional minor focus located more laterally. It is argued that the 'antidromic' response in the rabbit is the same as that found in the opossum, woodchuck, rat and slow loris, and is markedly different from that found in the cat and macaque monkey. It is postulated that this response reflects synaptic action in pyramidal tract axon collaterals, probably onto cell bodies in layer V, rather than being a purely antidromic event.