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) responses elicited in preactivated forearm muscles by transcranial stimulation of the human motor cortex are produced by activity in a disynaptic corticospinal linkage involving propriospinal-like interneurones with cell bodies in the spinal C3-4 segments. The experimental design incorporated a previous observation that stimulation of afferents in the superficial radial nerve inhibits propriospinal-like neurones projecting 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, single conditioning stimuli at twice perceptual threshold were given to the superficial radial nerve at the wrist at different times before a cortical shock. When the cortex was stimulated electrically, the conditioning 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 minimum time calculated for a possible direct cutaneous effect on spinal motoneurones. The time course of suppression began earlier and was more complex during magnetic stimulation of the cortex. It is argued that this difference is due to the repetitive I waves generated by the magnetic shock. Whether electrical or magnetic stimulation was used, the first 1-3 ms of the EMG response was relatively unaffected by superficial radial nerve stimulation at any interstimulus interval, whereas clear suppression was seen in the later portion of the response. In contrast, if the EMG response in ECR was suppressed by a conditioning stimulus to the median nerve at the elbow, then all portions of the EMG response were inhibited including the first 1-3 ms. The median nerve effect is thought to be due to direct reciprocal inhibition of the extensor motoneurones. Thus sparing of the initial part of the cortically evoked response with superficial radial stimulation suggests that the latter type of inhibition occurs at a premotoneuronal level. The timing of the effect is compatible with the explanation that corticospinal excitation is produced in ECR motoneurones through both monosynaptic and disynaptic (including propriospinal premotoneuronal) pathways, with superficial radial nerve inhibition being exerted at the propriospinal level.