NONMONOSYNAPTIC TRANSMISSION OF THE CORTICAL COMMAND FOR VOLUNTARY MOVEMENT IN MAN

1. The possibility was investigated that, in man, some of the descendi ng command for tonic voluntary wrist extension is transmitted to exten sor motoneurones over a non-monosynaptic pathway. 2. Stimulation of th e cutaneous superficial radial nerve at 3 times perceptual threshold d epressed the electromyogram (EMG) of extensor carpi radialis (ECR) and the discharge of single ECR motor units, both with a mean central del ay of 4.2 ms. Such stimuli depressed the response to transcranial magn etic stimulation of the motor cortex, but had little effect on the H r eflex. 3. The possibility that the relative sparing of the H reflex wa s due to an alteration in transmission of the afferent volley for the H reflex was excluded. 4. The central latency of the cutaneous-induced depression of the discharge of single motor units in biceps brachii ( C5-C6) was shorter by about 1 ms than that of the more caudal wrist an d finger extensor motor units. This suggests that the locus for the cu taneous-induced effects was spinal but above the cervical enlargement. 5. The pattern of EMG depression (evoked by superficial radial but no t palmar stimuli, in wrist extensors but not wrist flexors) is that pr eviously described for the presumed propriospinal system of human subj ects. 6. It is concluded that a significant component of the voluntary command for tonic wrist extension reaches the relevant motoneurone po ol via a non-monosynaptic pathway. It is suggested that the interposed neurones could be C3-C4 propriospinal neurones.