A PROPRIOSPINAL-LIKE CONTRIBUTION TO ELECTROMYOGRAPHIC RESPONSES EVOKED IN WRIST EXTENSOR MUSCLES BY TRANSCRANIAL STIMULATION OF THE MOTOR CORTEX IN MAN

We tested the hypothesis that some of the electromyographic (EMG) resp onses elicited in preactivated forearm muscles by transcranial stimula tion of the human motor cortex are produced by activity in a disynapti c corticospinal linkage involving propriospinal-like interneurones wit h cell bodies in the spinal C3-4 segments. The experimental design inc orporated a previous observation that stimulation of afferents in the superficial radial nerve inhibits propriospinal-like neurones projecti ng to the extensor carpi radialis (ECR) muscle. Surface EMG responses were recorded from the active ECR muscle after transcranial electrical or magnetic stimulation over the motor cortex. In random trials, sing le conditioning stimuli at twice perceptual threshold were given to th e superficial radial nerve at the wrist at different times before a co rtical shock. When the cortex was stimulated electrically, the conditi oning stimulus suppressed the EMG responses when the interval between the shocks was 11 ms or more. This was about 3.5 ms longer than the mi nimum time calculated for a possible direct cutaneous effect on spinal motoneurones. The time course of suppression began earlier and was mo re complex during magnetic stimulation of the cortex. It is argued tha t this difference is due to the repetitive I waves generated by the ma gnetic shock, Whether electrical or magnetic stimulation was used, the first 1-3 ms of the EMG response was relatively unaffected by superfi cial radial nerve stimulation at any interstimulus interval, whereas c lear suppression was seen in the later portion of the response. In con trast, if the EMG response in ECR was suppressed by a conditioning sti mulus to the median nerve at the elbow, then all portions of the EMG r esponse were inhibited including the first 1-3 ms. The median nerve ef fect is thought to be due to direct reciprocal inhibition of the exten sor motoneurones. Thus sparing of the initial part of the cortically e voked response with superficial radial stimulation suggests that the l atter type of inhibition occurs at a premotoneuronal level. The timing of the effect is compatible with the explanation that corticospinal e xcitation is produced in ECR motoneurones through both monosynaptic an d disynaptic (including propriospinal premotoneuronal) pathways, with superficial radial nerve inhibition being exerted at the propriospinal level.