1. Effects of conditioning motor volleys of increasing amplitude on antagonistic motor nuclei were tested at the wrist and elbow level in man. 2. The modifications of biceps, triceps, flexor carpi radialis (FCR) and extensor carpi radialis (ECR) motoneurone excitability were tested 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 corresponding nerves. The intensity of the conditioning stimuli was systematically varied from an intensity just below motor threshold to one giving rise to a motor response equal to half of the maximal direct motor response (Mmax). 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 volleys resulted in an early and long-lasting inhibition enhanced by L-Ac injection in wrist motor nuclei, while no modification was observed in elbow motor nuclei. 6. The characteristics of the early and long-lasting inhibition evoked in wrist motor nuclei by antagonistic motor volleys suggest that they are due to the activation of Renshaw cells by the conditioning motor volleys. 7. It is therefore concluded that these experiments provide further evidence that the interconnections between motoneurones, interneurones mediating reciprocal inhibition and Renshaw cells at the wrist level are different from those described elsewhere. Indeed, it has already been shown that the interneurones mediating reciprocal inhibition between FCR and ECR are not inhibited by Renshaw cells, and the results presented here suggest that FCR-coupled Renshaw cells inhibit ECR motoneurones and vice versa.
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