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1 Department of Developmental Physiology, National Institute for Physiological Sciences, Okazaki, Aichi, Japan; Core Research for Evolutionary Science and Technology (CREST), Japan Science and Technology Agency (JST), Kawaguchi, Japan
2 Department of Physiology, Kyorin University Medical School, Mitaka, Tokyo, Japan
3 Department of Developmental Physiology, National Institute for Physiological Sciences, Okazaki, Aichi, Japan
4 Department of Integrative Medical Biology, Section of Physiology, Umea University, Umea, Umea, Sweden
* To whom correspondence should be addressed. E-mail: tisa{at}nips.ac.jp.
Candidate propriospinal neurones (PNs) that mediate disynaptic pyramidal excitation to forelimb motoneurones were studied in the C3-C4 segments in anaesthetized macaque monkeys (n = 10). A total of 177 neurones were recorded (145 extracellularly, 48 intracellularly and 16 both) in laminae VI-VII. Among these, 86 neurones (73 extracellularly, 14 intracellularly and 1 both) were antidromically activated from the forelimb motor nucleus or from the ventrolateral funiculus just lateral to the motor nucleus in the C6/C7 segment and thus are identified as PNs. Among the 73 extracellularly recorded PNs, 60 cells were fired by a train of 4 stimuli to the contralateral pyramid with segmental latencies of 0.8 - 2.2 ms, with most of them (n = 52) in a monosynaptic range (< 1.4 ms including one synaptic delay and time to firing). The firing probability was only 21% from the third pyramidal volley, but increased to 83% after intravenous injection of strychnine. In most of the intracellularly recorded PNs, stimulation of the contralateral pyramid evoked monosynaptic EPSPs (12/14) and disynaptic IPSPs (14/14) which were found to be glycinergic. In contrast, cells that did not project to the C6-Th1 segments where forelimb motoneurons are located were classified as segmental interneurones. These were fired from the third pyramidal volley with a probability of 71% before injection of strychnine. It is proposed that some of these interneurones mediate feed-forward inhibition to the PNs. These results suggest that the C3-C4 PNs receive feed-forward inhibition from the pyramid in addition to monosynaptic excitation and that this inhibition is stronger in the macaque monkey than in the cat. Another difference with the cat was that only 26 of the 86 PNs (30 %, as compared to 84% in the cat) with projection to the forelimb motor nuclei send ascending collaterals terminating in the lateral reticular nucleus (LRN) on the ipsilateral side of the medulla. Thus, we identified C3-C4 PNs that could mediate disynaptic pyramidal excitation to forelimb motoneurones in the macaque monkey. The present findings explain why it was difficult in previous studies of the macaque monkey to evoke disynaptic pyramidal excitation via C3-C4 PNs in forelimb motoneurones and why - as compared with the cat- the monosynaptic EPSPs evoked from the LRN via C3-C4 PNs were smaller in amplitude.
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