ELECTRICAL STUDIES IN MAMMALIAN REFLEXES

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Abstract
An afferent nerve in the hind limb of the decerebrate cat was stimulated with 2 successive maximal stimuli at various intervals, and the resulting action currents in the motor nerve involved in the flexion reflex were recorded. Individual decerebrate preparations often differed in their behavior with respect to 2 stimuli, but each preparation was usually consistent in the character of response throughout the experiment. In the majority of experiments the 2d response was smaller than the 1st at all stimulus intervals from 1.5 [sigma] to 50 [sigma], the longest interval tested. In some experiments there was a maximum in the size of the second response when the stimulus interval was between 20 and 30 [sigma]. In a minority of experiments the second response was larger than the first at all intervals up to about 40 [sigma]. In these cases the first response was abnormally small. In one experiment the condition changed spontaneously, the 1st response increasing in size till it was larger than the 2d. A similar change was caused, in two cases, by low spinal transection. This suggests that the first response was subject to inhibition by impulses from higher centers, and that central summation released the motor neurones from this inhibition and enabled more of them to respond to the second stimulus. The earliest modifi- cation of the response to a single stimulus by a second stimulus tends to show that the motor neurones can discharge a second volley of impulses about 3 < after the first. In the terminal fall of blood pressure following extreme hemorrhage, when the response to a single stimulus had decreased considerably, the response to 2 stimuli at a brief interval sometimes remained more nearly at its original value. This type of summation may depend on the supernormal phase of recovery conditioned on acidity in the nerve center. The small size of an early 2nd response in the majority of experiments suggests a refractory phase in the center, exceeding that of the peripheral fiber. If each response consists of a brief tetanus, the reduced size of response may be due to reduced duration and frequency of discharge in each motor neurone. The evidence does not settle the question whether central function is of the all-or-none type, like that of peripheral nerve, or of a different character subject to gradation, as has been suggested by Sherrington.

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