Antidromic response to medullary pyramid stimulation in rats and its relation to that in cats

The response evoked in the cerebral cortex of laboratory rats after stimulation of the medullary pyramid is surface-positive. It begins 0.9-1.6 ms after the stimulus, attains peak amplitude (up to 2 mV) in 0.8-1.2 ms and lasts 2-4 ms. It occurs throughout the anterior two-thirds of the dorsal cortex and is largest lateral to bregma, with a secondary maximum in the somatosensory area II. Although it depends on antidromic conduction in pyramidal tract fibers for its production, it varies in amplitude, configuration and latency at different recording sites and at the same sites on repeated trials. It reverses polarity deep in the cortex to become a large, negative wave deep in layer V, and maintains that polarity into the white matter. Current source density analysis reveals a strong sink in layer V, with a strong source just superficial to that sink and a weaker source in layer VI. The antidromic response disappears during spreading depression, but recovers more rapidly than the primary response evoked by skin stimulation. It decreases progressively in amplitude with continuous 200-Hz iterative stimulation, and recovers slowly at the end of stimulation. The primary response evoked by contralateral forepaw and hindpaw stimulation is highly localized, being entirely within the antidromic response distribution. The antidromic response in laboratory rats consists of a small, surface-positive component analogous to the pure antidromic response of cats, and of a large, surface-positive response analogous to that found in woodchucks, rabbits, opossums and slow lorises. It is argued that this latter response results from synaptic action in pyramidal tract axon collaterals, probably onto cells in layer V, rather than being a purely antidromic event.