Effects produced in human arm and forearm motoneurones after electrical stimulation of ulnar and median nerves at wrist level

Effects of electrical stimulation of ulnar and median nerves at wrist level were investigated in post-stimulus time histograms (PSTHs) of single motor units from both flexors and extensors in human arm and forearm. Stimulation of ulnar nerve produced late (mean extra time-after monosynaptic group Ia excitation-10.7 +/- 0.1 ms) high-threshold (> 1.2 x motor threshold, MT) excitation, which was not reproduced by purely cutaneous stimulation, in all the investigated motor nuclei except in Extensor Carpi Radialis. Stimulation of median nerve, and of the skin of fingers II and III (at palmar side level), produced short latency inhibition (mean extra time 3.8 +/- 0.3 ms), which was most often truncated or followed by late excitation (mean extra time 11.8 +/- 0.3 ms); both effects were of low threshold (0.8 x MT). Short latency inhibition was very strong, and late excitation was rare and weak in almost all the investigated motor units except in those supplying flexors in forearm, in which the main effect was the late facilitation (stronger than in other motoneurones). Since extra time was not more than 13 ms, it is suggested that the late effects may be mediated through spinal pathways, at least during their 3-5 first ms. Based on the electrophysiological results and on the anatomical characteristics of ulnar and median nerves, it is assumed that ulnar-induced late high-threshold peak in PSTHs might reflect group II excitation in spinal motoneurones, and median-induced modifications in motor unit discharge, mainly cutaneous control of motoneurone discharge. Since the central delay of median-induced inhibition is longer the more caudal the motoneurone, inhibitory propriospinal-like interneurones are supposed to mediate cutaneous inhibitory control from hand upon muscles in arm and forearm. Potential roles of proprioceptive and cutaneous control from hand to more proximal musculature, provided by ulnar and median nerve, respectively, during precise hand movements are discussed.