1. The tail-flick reflex elicited by noxious beat in lightly anesthetized rats is known to be prevented by trains of low-amplitude current pulse passed through a monopolar microelectrode in the rostromedial medulla (RMM). The effect of the distance from such an electrode on the threshold of cell bodies was described in the preceding paper (11). This paper estimates the density of cell bodies in the RMM and, subsequently, estimates the number of cell bodies excited by the aforementioned pulses, a figure whose upper bound is between 30 and 75. 2. The mean chronaxy for suppression of tail flick was found to be 162 μs. Correspondingly, for activation of spikes in somata of the RMM, it was found to be 170 μs. The axons belonging to these somata, located in the spinal lateral columns, had mean chronaxies of 360 μs. These comparisons favor the idea that cell bodies in RMM, not axons, mediate the suppression of tail flick. Other evidence for this conclusion is given in the text. 3. Resting activity in the RMM was found to average 6.33 Hz. Thus if the inhibitory process depends only on the instantaneous sum of activity in the many thousands of RMM neurons, all nocifensive reflexes should be continuously suppressed. But since this is not so, the relative timing of spikes in the population may also be critical. The synchronizing effect of electrical stimulation then explains the low number of cells needed to prevent the reflex.
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