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Département de Rééducation, Hôpital de la Salpêtrière, Paris, France.

1. The pattern of distribution of non-monosynaptic (propriospinal-like) excitation to various motor nuclei (deltoid, extensors and flexors of the elbow, the wrist and the fingers) was investigated. 2. Changes in the firing probability of individual voluntarily activated motor units were studied following conditioning stimuli. Conditioning volleys were evoked by weak electrical stimuli applied to various mixed nerves (circumflex, musculocutaneous, median, radial, ulnar) and to the skin. 3. In all investigated nuclei stimulation of the 'homonymous' nerve evoked a peak of increased firing probability with a latency which was 2-7 ms longer than the monosynaptic Ia latency. The average central delay of the late excitation, measured from monosynaptic latency, seems to depend only on the segmental level of the motor nucleus: the more caudal the nucleus the longer the latency. This strongly suggest a transmission through neurones located above the cervical enlargement, as are C3--C4 propriospinal neurones in the cat. 4. Both group I muscle and cutaneous afferents were shown to contribute to propriospinal-like excitation. It is argued that a spatial facilitation of the effects evoked by these two inputs might explain why the threshold of late excitation is always below that of the monosynaptic Ia excitation in motoneurones. 5. The pattern of distribution of propriospinal-like excitation was diffuse: stimulation of each mixed nerve was able to evoke excitation in all investigated motor nuclei. Similarly, stimulation of a given skin field could produce excitation of biceps and wrist flexor and extensor units. 6. Each motor nucleus therefore receives excitation from a multimodal and wide range peripheral input. However, it is argued that what appears as a diffuse pattern might simply reflect connections which are not used in each movement but appropriately selected by higher centres.

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