TOPOGRAPHICAL REPRESENTATION OF MUSCLES IN MOTOR CORTEX OF MONKEYS

Abstract

The topographical representation in the motor cortex of individual muscles acting over the ankle joint has been studied in 9 Macaque monkeys. Simultaneous responses of 8 muscles were recorded individually by isometric myographs during the stimulation mm. by mm. of the motor cortex with a Goodwin thyratron stimulator. The records were analyzed in terms of "solitary response", tension-ratio and latency. Evidences were presented that individual muscles each have a focal point in the cortex from which a solitary response of that particular muscle is elicitable or the response is stronger and more prompt relative to other muscles. Among the muscles investigated, the anatomical extensors were found more responsive than the flexors and the distal muscles better cortically represented than the proximal ones. The inhibition of flexor responses by simultaneous activation of the cortical extensor points has been demonstrated and considered to be one factor in the relative unresponsiveness of the flexor muscles. A hypothetical organization of the cortical motor neurons into a field composed of a focus and a fringe, the latter exhibiting overlap, is suggested to account for the type of muscle representation observed.