INFLUENCE OF DEAFFERENTATION ON STIMULATION OF MOTOR CORTEX

Abstract

The considerable increase in electromyographic response to stimulation of the motor cortex under the influence of proprioceptive reflexes prompted an investigation of the question of the relative contribution to muscle response of proprioceptively induced reflexes and of the efferent impulses originating on the motor cortex. Consequently the E.M.G. and tension response to cortical stimulation was studied before and after sectioning of the posterior roots of the hind limb in anesthetized cats. Slight variations in intensity and frequency of motor cortex stimulation result in graded responses of skeletal muscles as indicated by the magnitude of EMG and developed tension in normal cats as well as after deafferentation, showing that the motor cortex is itself capable of eliciting graded responses independent of proprioceptive recruitment. However, after deafferentation even maximal cortical stimulatoin never induced the degree of tension nor the amplitude of EMG seen in control conditions. An initial stretch of muscle causes an increased EMG and tension developed in response to motor cortical stimulation compared to that developed with the muscle slack under control conditions; after deafferentation the EMG response is seen to be independent of initial length of the muscle whereas tension remains directly related to initial length. Deafferentation markedly diminishes a co-contraction seen in gastrocnemius in most cases, suggesting a largely proprioceptive origin for co-contraction.