Refractory phase in cerebral mechanisms

1. 1. Stimulation of the sciatic nerve in cats under deep barbiturate narcosis has previously been shown to evoke a primary electric response in the sensory leg area of the cortex and a secondary discharge, more widespread in the cortex, with latencies from 30 to 80 msec. When a series of stimuli is applied at more than about 5 per sec., each stimulus evokes a primary response, but only the first evokes the secondary discharge. A rest of at least 0.2 sec. is needed before another stimulus can evoke the discharge. 2. 2. This behavior resembles the Wedensky inhibition in peripheral nerve, in which when a series of impulses so frequent that each one after the first falls in the relative refractory phase and is therfire subnormal, a partial block in the nerve will stop all but the first (full-sized) impulse. The experiiments described herein were designed to determine the effect of varying the depth of narcosis on the properties of the primary response, on the time of recovery of responsiveness in the sensory discharge (S. D.) mechanism, and on the critical stimulus frequency requisite to maintain blockingof the S.D. It was hoped that such information might aid in estimating whether the block could be interpreted as essentially a Wedensky effect.v 3. 3. The quantitative estimation of the depth of narcosis became a major issue. For this purpose we recorded respiration, blood pressure and spontaneous electric waves in the cortex, graded them on an empirical percentage scale, and designated their average the “functional mean” — a convenient measure of the depression of the cerebrum and the vital centers. We studied the effects of injection of dial in urethane and of nembutal on the functional mean and the rate of recovery as the effect of the drug wore off, and by trial and error arrived at formulae that gave fairly good correlation of estimated narcotic concentration with observed depression of functional mean (fig. 2). 4. 4. Analysis of blood samples by spectrophotometry when nembutal alone was used, showed that the actual concentration of the drug in the blood was nearly what we had estimated for the active tissues, and further gave evidence that the concentration of the narcotic was not in itself enough too account for the depression of activity after narcosis had been in progress more than 4 or 5 hours (fig. 3). The observations of Beecher and Moyer suggest that increased CO 2 in the blood may be the explanation. 5. 5. Increasing the depth of narcosis greatl