Disynaptic Pyramidal Excitation in Forelimb Motoneurones mediated via C3-C4 Propriospinal neurones in the Macaca Fuscata. B. Alstermark1, T. Isa2, Y. Ohki3 and Y. Saito2. 2Department of Integrative Physiology, National Institute for Physiological Sciences, Myodaiji, Okazaki 444-8585, Japan, 1Department of Integrative Medical Biology, Umeå University, S-901 87 Umeå, Sweden, 3 Department of Physiology, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo 181-8611, Japan.
APStracts 6:0482N, 1999.
In contrast to findings in the cat, it has recently been shown that disynaptic pyramidal EPSPs are only rarely observed in forelimb motoneurones of the macaque monkey in the intact spinal cord or after a corticospinal transection in C5. This finding has been taken to indicate that the disynaptic pyramidal excitatory pathway via C3-C4 propriospinal neurones (PNs) is weakened through phylogeny when the monosynaptic cortico-motoneuronal connection has been strengthened. We have now re-investigated this issue with special focus on the possibility that the inhibitory control of the C3-C4 PNs may be stronger in the macaque monkey as compared to the cat. The effect in forelimb motoneurones of electrical stimulation in the contralateral pyramid was investigated in anaesthetised macaque monkeys (Macaca Fuscata). We confirmed the low frequency of disynaptic pyramidal EPSPs in forelimb motoneurones. However, after intravenous injection of strychnine, disynaptic EPSPs could be evoked in 39 of 41 forelimb motoneurones recorded after lesion of the corticospinal fibres in C5. After a corresponding lesion in C2, disynaptic pyramidal EPSPs were observed in 2 out of 25 motoneurones. In contrast to previous reports, we conclude that C3-C4 PNs can mediate disynaptic pyramidal excitation in high frequency of occurrence to forelimb motoneurones in the C6-8 segments and that this transmission is under a stronger inhibitory control as compared to the cat. Thus, the hypothesis that the disynaptic excitatory cortico-motoneuronal pathway via the C3-C4 PNs is weakened in parallel with the strengthened monosynaptic connection, through phylogeny, is not supported by the present findings.
Received 13 July 1999; accepted in final form 7 September 1999.
APS Manuscript Number J577-9.
Article publication pending Journal of Neurophysiology.
ISSN 1080-4757 Copyright 1999 The American Physiological Society.
Published in APStracts on 21 December 1999