Disynaptic pyramidal excitation in forelimb motoneurons mediated via C(3)-C(4) propriospinal neurons in the Macaca fuscata.

In contrast to findings in the cat, it recently has been shown that disynaptic pyramidal EPSPs only rarely are observed in forelimb motoneurons of the macaque monkey in the intact spinal cord or after a corticospinal transection in C(5). This finding has been taken to indicate that the disynaptic pyramidal excitatory pathway via C(3)-C(4) propriospinal neurons (PNs) is weakened through phylogeny when the monosynaptic cortico-motoneuronal connection has been strengthened. We reinvestigate this issue with special focus on the possibility that the inhibitory control of the C(3)-C(4) PNs may be stronger in the macaque monkey than in the cat. The effect in forelimb motoneurons of electrical stimulation in the contralateral pyramid was investigated in anesthetized macaque monkeys (Macaca fuscata). We confirmed the low frequency of disynaptic pyramidal EPSPs in forelimb motoneurons. However, after intravenous injection of strychnine, disynaptic EPSPs could be evoked in 39 of 41 forelimb motoneurons recorded after lesion of the corticospinal fibers in C5. After a corresponding lesion in C(2), disynaptic pyramidal EPSPs were observed in 2 of 25 motoneurons. In contrast to previous reports, we conclude that C(3)-C(4) PNs can mediate disynaptic pyramidal excitation in high frequency of occurrence to forelimb motoneurons in the C(6)-C(8) segments and that this transmission is under a stronger inhibitory control than in the cat. Thus, the hypothesis that the disynaptic excitatory cortico-motoneuronal pathway via the C(3)-C(4) PNs is weakened in parallel with the strengthened monosynaptic connection through phylogeny is not supported by the present findings.