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 disynap tic pyramidal EPSPs only rarely are observed in forelimb motoneurons of the macaque monkey in the intact spinal cord or after a corticospinal transect ion in C-5. This finding has been taken to indicate that the disynaptic pyr amidal excitatory pathway via C-3-C-4 propriospinal neurons (PNs) is weaken ed through phylogeny when the monosynaptic cortico-motoneuronal connection has been strengthened. We reinvestigate this issue with special focus on th e possibility that the inhibitory control of the C-3-C-4 PNs may be stronge r 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 freque ncy of disynaptic pyramidal EPSPs in forelimb motoneurons. However, after i ntravenous injection of strychnine, disynaptic EPSPs could be evoked in 39 of 41 forelimb motoneurons recorded after lesion of the corticospinal fiber s in C-5. After a corresponding lesion in C-2, disynaptic pyramidal EPSPs w ere observed in 2 of 25 motoneurons. In contrast to previous reports, we co nclude 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 an d that this transmission is under a stronger inhibitory control chan in the cat. Thus, the hypothesis that the disynaptic excitatory cortico-motoneuro nal pathway via the C-3-C-4 PNs is weakened in parallel with the strengthen ed monosynaptic connection through phylogeny is not supported by the presen t findings.