A STUDY IN REFLEXES: IDENTIFICATION OF THE CUTANEOUS AFFERENT FIBERS WHICH EVOKE IPSILATERAL EXTENSOR AND FLEXOR REFLEXES

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Abstract
Exp. showed that qualitatively different reflex responses are evoked by relatively discrete groups of afferent fibers as characterized by diam., irritability and conduction rate. Different cutaneous nerves of the leg of Rana catesbiana differ in the reflexes that they evoke, and also in their fiber composition. The nerves containing large fibers (17.5-24/[mu]. max. diam.) elicit chiefly a low threshold ipsilateral extensor reflex, whereas the nerve whose largest fiber is 11-13/[mu] elicits the flexor reflex with ease and vigor, but the ipsilateral extensor reflex not at all or very slightly. The relation of reflex threshold to frequency of stimulation is studied. At the optimally effective rates the extensor reflex threshold is the same as the motor nerve threshold, but the flexor reflex threshold is 1.6-1.7 times as high. The fibers which evoke the extensor reflex conduct at rates ca. 40-14 m.p.s. while the fibers which evoke the flexor reflex conduct at about 27-6 or less. While exciting a distinctive group of afferents a change in frequency is without effect on the quality of response. Both the central and afferent peripheral processes are found to be faster for ipsilateral extension than for flexion. Myographic recording, the cathode ray oscillograph, and histologic methods were employed.

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