Abstract
Changes in excitability of spinal motorneurons produced by a volley of impulses in cutaneous nerves of the contralateral hindleg were studied in cats acutely made spinal or decerebrate. Motorneuron excitability was determined by measuring changes in electrical activity recorded from ventral roots or muscle nerves. Depending upon the size of fibers fired, at least two different effects were produced by synchronous impulses in crossed cutaneous nerves. When the cutaneous nerve activity was confined to the low threshold, rapidly conducting fibers, 14–6 µ in diameter, crossed knee and ankle flexor motorneurons were facilitated after a central latency of some 3 msec., and on occasion discharged. In several instances crossed extensor motorneurons were depressed during the course of flexor facilitation. When the cutaneous nerve activity involved the smaller (6–2 µ) myelinated fibers as well, prolonged inhibition of the crossed flexor motorneurons followed the initial facilitation and the crossed extensor motorneurons were facilitated, after a delay of 6–30 msec., for periods up to 300 msec.
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