RECURRENT INHIBITION BETWEEN MOTOR NUCLEI INNERVATING OPPOSING WRIST MUSCLES IN THE HUMAN UPPER-LIMB

1. Effects of conditioning motor volleys of increasing amplitude on an tagonistic motor nuclei were tested at the wrist and elbow level in ma n. 2. The modifications of biceps, triceps, flexor carpi radialis (FCR ) and extensor carpi radialis (ECR) motoneurone excitability were test ed both in post-stimulus time histogram (PSTH) and rectified averaged EMG experiments. 3. Conditioning motor volleys were evoked in biceps, triceps, FCR and ECR muscles by electrical stimuli applied to the corr esponding nerves. The intensity of the conditioning stimuli was system atically varied from an intensity just below motor threshold to one gi ving rise to a motor response equal to half of the maximal direct moto r response (M(max)). 4. The effect of the injection of a cholinergist agonist, L-acetylcarnitine (L-AC), on antagonistic conditioning motor volleys was tested for each motor nucleus. 5. Results obtained at the wrist and elbow level were strikingly different. Antagonistic motor vo lleys resulted in an early and long-lasting inhibition enhanced by L-A c injection in wrist motor nuclei, while no modification was observed in elbow motor nuclei. 6. The characteristics of the early and long-la sting inhibition evoked in wrist motor nuclei by antagonistic motor vo lleys suggest that they are due to the activation of Renshaw cells by the conditioning motor volleys. 7. It is therefore concluded that thes e experiments provide further evidence that the interconnections betwe en motoneurones, interneurones mediating reciprocal inhibition and Ren shaw cells at the wrist level are different from those described elsew here. Indeed, it has already been shown that the interneurones mediati ng reciprocal inhibition between FCR and ECR are not inhibited by Rens haw cells, and the results presented here suggest that FCR-coupled Ren shaw cells inhibit ECR motoneurones and vice versa.