Peripheral and descending control of neurones mediating non-monosynaptic Ia excitation to motoneurones: a presumed propriospinal system in man.

Evidence for a non-monosynaptic Ia excitation of wrist flexor and quadriceps motoneurones (MNs) in man has been provided while using two independent methods: construction of post-stimulus time histograms (PSTHs) of voluntarily activated motor units and the spatial facilitation of the H-reflex. This non-monosynaptic Ia excitation has a long central latency (3-6 ms) and a very low threshold. Neurones mediating this effect seem to receive a strong descending excitation at the onset of voluntary movement and it is argued that they might mediate part of the descending command to MNs. In contrast, increasing the afferent input produces an inhibition of the transmission in this pathway. Several characteristics of the non-monosynaptic excitation and its depression bear resemblance to those of the C3-C4 propriospinal system in the cat, which is used as a model for the discussion of the present results.