Pattern of cutaneous inhibition of the propriospinal-like excitation to human upper limb motoneurones.

1. The pattern of cutaneous inhibition of non-monosynaptic excitation (or 'propriospinal-like' excitation) to various species of motoneurones-wrist flexors and extensors, biceps and deltoid - was investigated in man. 2. Changes in the firing probability of individual voluntarily activated motor units were studied following various stimuli. The effects of weak cutaneous stimuli applied to the skin of either the palmar or the dorsal side of the fingers on propriospinal-like excitation evoked by stimulation of different mixed nerves (median, radial, musculo-cutaneous) were investigated. 3. Cutaneous stimuli were found to produce an inhibition with a very specific pattern, which depended on the afferent input evoking the excitation. Thus, the median nerve-induced excitation of wrist and elbow flexors was depressed by cutaneous afferents from the palmar side but not from the dorsal side of the fingers; the radial nerve-induced excitation of wrist extensors was only depressed from the dorsal side: the musculo-cutaneous nerve-induced excitation of motor units of various wrist, elbow and shoulder muscles was depressed from both sides. 4. It is argued that this pattern implies that the excitation is mediated to the motoneurones through different subsets of neurones, organized according to the afferent input. 5. The functional significance of the pattern of cutaneous inhibition is discussed and it is argued that it could contribute to an appropriately timed termination of a movement mediated through the propriospinal system.