ACTION OF STRYCHNINE ON RECRUITING RESPONSES OF DENDRITES OF CAT CORTEX

Abstract

Strychnine was applied topically to cat cortex in solutions so dilute as to reveal minimal changes in recruiting responses to thalamic stimulation and in responses to stimulation at the cortical surface, in visual and association areas. Records were taken from surface to white and of cortical fractions. The first effect on either response is to increase the amplitude and duration of the dendritic wave superficially in cortex. At higher concentrations the amplitude is decreased, but responses to direct stimulation are affected only at higher concentrations than affect indirectly stimulated responses. Finally all responses are reversed in sign as recorded transcortically, the positivity being accounted for by deeper regions not affected by strychnine, when concentrations of 0.1% or higher depressed dendritic terminals near the surface. Spontaneous strychnine spikes first appearing had the same form as stimulated responses. At concentrations which reduced negative response, some higher responses were presumably due to firing of elements not directly stimulated, at short latencies after the stimulated responses or summated with these. Sharply diphasic waves surface plus-minus appearing at higher concentrations are taken to indicate conduction downward along dendrites. From previous work on the effect of strychnine on nerve fibers and from current work it is inferred that above a minimal concentration the effect of strychnine is to depress amplitude of response of both pre- and postsynptic elements but to render postsynaptic regions more excitable than normal, an effect similar to that of KC1 and cathodal polarization on peripheral nerve. Strong stimuli, or spontaneously excited responses may break through the depression to fire dendrites still depressed to weaker stimuli.