Activity of neurons of somatosensory cortex during local strychnine poisoning

Abstract

The types of re sponse of neurons in the somatosensory cortex to peripheral stimulation and the character of their reaction to direct electrical stimulation has already been examined (2, 3). A common component of all the individual responses of these neurons to these forms of stimulation was a temporary quiescence, probably due to inhibition. Since observations (4, 8, 9) have shown that strychnine abolishes the phenomena of inhibition, it was interesting to study the changes in the character of the responses to peripheral stimulation during local strychnine poisoning. It might also be expected that the application of strychnine, by abolishing inhibitory influences, would in turn facilitate excitation of the neurons of the surface layers of the cortex. The activity evoked in this manner would readily be distinguished by its selectivity and its greater potency from the activity evoked by direct electrical stimulation of the cortex. It was assumed that by performing experiments along these lines a more meaningful analysis could be made of interneuronal relationships in the primary somatosensory cortex. METHOD Cats were anesthetized with Nembutal (50 mg/kg, intraperitoneall y). Preparatory operations included tracheotemy, the application of stimulating electrodes to forelimb nerves, drainage of the 4th ventricle, and exposure of the sigmoid gyri of both hemispheres. Neurons in circumscribed areas of the posterior sigmoid gyri corresponding to the site of primary projection of forelimb nerves were investigated. Strychnine poisoning was produced by application of filter paper soaked in a 1% solution of strychnine nitrate to the area of the cortex to be investigated. The potentials of the neurons were detected by glass microelectrodes filled with 3 M KC1 with a resistance of 5-15 M~. The surface corticogram was recorded with a silver ball electrode. Potentials from the microelectrode were passed through a cathode follower (while those from the macroelectrode were passed directly)to a UBP 1-02 amplifier and, with a "Meopta" tape winder, were photographed from the screen of a D-581 twin-beam oscilloscope. The grid current of the cathode follower was less than 10-11 A. The nerves were stimulated with 0.5-msec rectangular pulses, applied through radio-frequency output from EST-7 generators.