Inhibition versus facilitation of the reflex responsiveness of identified wrist extensor motor units by antagonist flexor afferent inputs in humans

The question of whether Ia reciprocal inhibition might depend on the motor task and on the type of motor unit activated was investigated in the human extensor carpi radialis muscles. Ia reciprocal inhibition induced by stimul ating the median nerve (conditioning stimulation) was estimated by measurin g the changes in the firing probability of 37 extensor motor units in respo nse to the radial nerve stimulation (100 test stimuli) delivered 1 ms after the conditioning stimulation, Six subjects were asked to perform a task co nsisting of either selectively contracting their wrist extensor muscles or co-activating their wrist and finger antagonist muscles by clenching their hand around a manipulandum. In the control recordings (test stimulation alo ne), the mean response probability of the 37 motor units was found to be gr eater during hand clenching. The motor units were identified on the basis o f their force thresholds, their macro-potentials, and their twitch contract ion times. The data obtained in the control recordings were consistent with the size principle. In the recordings where the responses were conditioned by applying median nerve stimulation, the response probability of the moto r units with low force thresholds, small macro-potential areas, and long tw itch contraction times tended to decrease, in line with the presence of Ia reciprocal inhibition, whereas the response probability of the motor units with higher force thresholds, larger macro-potential areas, and shorter twi tch contraction times tended to increase. The median nerve stimulation may therefore have altered the efficiency with which the extensor Ia inputs rec ruited the homonymous motoneurones in the pool. The flexor group I afferent s activated while the median nerve was stimulated had inhibitory effects on the slow contracting motor units, and facilitatory effects mainly on the f ast contracting motor units, Both of these effects were stronger during han d clenching, in which the numerous cutaneous receptors of the palm and fing ertips are liable to be activated. Besides their own effects on the excitab ility of the various types of motor units, cutaneous inputs are known to po tentiate the Ib interneurones. In addition, the effects of the conditioning stimulation were superimposed on the tonic activity of the Ia and Ib affer ents from the flexor wrist and finger muscles. This may explain why both th e inhibitory and facilitatory effects of the median nerve stimulation were enhanced during hand clenching.