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Department of Rééducation, Hôpital de la Salpêtrière, Paris, France.

1. The possibility that activation of low-threshold afferents in the median and ulnar nerves in man evokes an interneuronally mediated excitation of wrist flexor motoneurones was investigated. Two independent techniques were used: (i) the indirect technique of spatial and temporal facilitation of the flexor carpi radialis H reflex; (ii) the post-stimulus time histogram (PSTH) method for measurement of the firing probability of voluntarily activated motor units following different stimuli. 2. Conditioning volleys were evoked by weak electrical stimuli (to the median nerve, the ulnar nerve, the skin and cutaneous nerve branches) and by a tap to the tendon of the flexor carpi radialis muscle. 3. In the H reflex experiments the comparison was drawn between the effects of two conditioning stimuli applied separately or together. In some experimental conditions the facilitation of the reflex evoked when combining two conditioning stimuli was larger than the algebraic sum of the effects from separate stimuli. The central latency of this additional facilitation, which is denoted 'extra facilitation', was 3 ms. It is argued that the extra facilitation reflects summation at a premotoneuronal level. 4. PSTHs of voluntarily activated flexor carpi radialis and flexor carpi ulnaris motor units were computed following stimulation of the median or the ulnar nerve. In 60% of the recordings the afferent volley evoked a peak of increased firing probability with a latency which was 3-6 ms longer than the monosynaptic Ia latency. 5. Both the extra facilitation of the reflex and the late peak in the PSTH were evoked from very low-threshold afferents. A contribution from Ia afferents was demonstrated in reflex experiments. Group I afferents alone are responsible for the onset of the non-monosynaptic excitation but it is probable that cutaneous afferents give a later contribution. 6. On the basis of the characteristics of the non-monosynaptic excitation studied with the two methods (same very low-threshold, comparable central latencies), it is argued that both the extra facilitation of the H reflex on combined stimulation and the late peak in the PSTHs from individual motor units are mediated through the same pre-motoneuronal pathway. 7. The possibility that the non-monosynaptic excitation may be mediated through propriospinal neurones is discussed.

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