Distribution of presynaptic inhibition on type-identified motoneurones in the extensor carpi radialis pool in man

1. The question was addressed as to whether the magnitude of Ia presynaptic inhibition might depend on the type of motor unit activated during volunta ry contraction in the wrist extensor muscles. For this purpose, we investig ated the effects of applying electrical stimulation to the median nerve on the responses of 25 identified motor units to radial nerve stimulation deli vered 20 ms after a conditioning stimulation. 2. The reflex responses of the motor units yielded peaks in the post-stimul us time histograms with latencies compatible with monosynaptic activation. Although median nerve stimulation did not affect the motoneurone net excita tory drive assessed from the mean duration of the inter-spike interval, it led to a decrease in the contents of the first two 0.25 ms bins of the peak . This decrease may be consistent with the Ia presynaptic inhibition known to occur under these stimulation conditions. 3. In the trials in which the median nerve was being stimulated, the findin g that the response probability of the motor units, even in their monosynap tic components, tended to increase as their force threshold and their macro -potential area increased and as their twitch contraction time decreased su ggests that the median nerve stimulation may have altered the efficiency wi th which the Ia inputs recruited the motoneurones in the pool. 4. These effects were consistently observed in seven pairs of motor units e ach consisting of one slow and one fast contracting motor unit which were s imultaneously tested, which suggests that the magnitude of the Ia presynapt ic inhibition may depend on the type of motor unit tested rather than on th e motoneurone pool excitatory drive. 5. The present data suggest for the first time that in humans, the Ia presy naptic inhibition may show an upward gradient working from fast to slow con tracting motor units which is able to compensate for the downward gradient in monosynaptic reflex excitation from 'slow' to 'fast' motor units. From a functional point of view, a weaker Ia presynaptic inhibition acting on the fast contracting motor units may contribute to improving the proprioceptiv e assistance to the wrist myotatic unit when the contraction force has to b e increased.