Temporal Changes in the Excitability of the Reciprocal Inhibitory Interneurones Preceding Cocontraction of the Antagonistic Muscle in the Human Wrist

The supraspinal control of reciprocal inhibitory interneurones preceding cocontraction of antagonistic muscles in the human wrist was investigated in 5 subjects. The method combined a reaction-time task paradigm and a technique for conditioning the H-reflex. Unconditioned H-reflexes were elicited from flexor carpi radialis (FCR) and extensor carpi radialis (ECR) muscles by using electrical stimulation to median and radial nerves. The conditioned H-reflex of the FCR muscle was obtained by simultaneous electrical stimulation to the median and the radial nerves. While subjects performed the bilateral isometric cocontraction task, excitabilities of the alpha motoneurone pools and that of the reciprocal inhibitory interneurones were assessed by the unconditioned and conditioned H-reflex, respectively. Alpha mononeurone pools of FCR and ECR muscles were facilitated starting from approximately 50 msec. preceding EMG activities. Reciprocal inhibitory interneurones which suppressed FCR alpha motoneurones were disinhibited at almost same time. The present results suggested that supraspinal control of the reciprocal inhibitory interneurones preceding antagonist cocontraction is different from that of preceding agonist contraction.