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Does a C3-C4 propriospinal system transmit corticospinal excitation in the primate? An investigation in the macaque monkey

M. A. Maier, M. Illert *, P. A. Kirkwood, J. Nielsen * and R. N. Lemon

1. Synaptic responses to electrical stimulation of the contralateral pyramidal tract were measured in intracellular recordings from 206 upper limb motoneurones in ten chloralose-anaesthetized macaque monkeys. The objective was to search for evidence of a disynaptic excitatory pathway via C3-C4 propriospinal interneurones similar to that in the cat.

2. In monkeys with intact spinal cords, only a small proportion of motoneurones (19 %) responded with late EPSPs to repetitive stimulation of the pyramid; only 3 % had segmental latencies that were appropriate for a disynaptic pathway.

3. From previous studies in the cat, it was expected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upper limb muscles, whilst leaving intact excitation transmitted via a C3-C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was made to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the pyramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, even after the lesion the proportion of motoneurones with such late EPSPs was still low (18 %); 14 % of motoneurones had EPSPs within the disynaptic range.

4. In addition, some EPSPs with relatively long segmental latencies (> 1·1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable latencies and did not facilitate with repetitive shocks, it is likely that they represent monosynaptic EPSPs evoked by slowly conducting corticospinal fibres which survived the lesions.

5. In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral reticular nucleus (LRN) were also tested. Most motoneurones showed EPSPs with short latencies (1·2-2·5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of collaterals of C3-C4 propriospinal neurones to the LRN, as demonstrated in the cat.

6. These short-latency EPSPs evoked from the LRN were just as common before (77 %) as after (75 %) the C5 lesion. They had small amplitudes both before (mean ± s.d. 1·1 ± 0·59 mV) and after (1·2 ± 0·72 mV) the lesion. Unlike the situation in the cat, only a small proportion (16 %) of motoneurones activated from the LRN showed late EPSPs after repetitive stimulation of the pyramid.

7. The results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3-C4 propriospinal neurones in the monkey. The general absence of responses mediated by such a system in the macaque, under experimental conditions similar to those in which they are seen in the cat, show that extrapolation of results from the cat to the primate should be made with considerable caution.

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