Origin of corticospinal neurones evoking monosynaptic excitation in C3--C4 propriospinal neurones in the cat.

Intracellular recording was made from propriospinal neurones (PNs) in the C3-C4 spinal cord segments in the cat (alpha-chloralose anaesthesia). The effect of electrical stimulation of corticospinal neurones (CSNs) in the cortex was investigated. Short C3-C4 PNs were identified by antidromic activation of their axons in the ventral horn in C6/C7 and in the lateral reticular nucleus. Long PNs were antidromically identified from Th12-13. In short PNs, monosynaptic excitory postsynoptic potentials (EPSPs) were elicited from the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6. Two subtypes of short PNs were identified. PNs of type I received monosynaptic EPSPs from the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma, which is from the same region as disynaptic cortical EPSPs were evoked in forelimb motoneurones. PNs of type II received monosynaptic EPSPs from regions slightly more rostrally in the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6, which is outside the region from which disynaptic EPSPs could be evoked in forelimb motoneurones. Long PNs received monosynaptic EPSPs, like the short PNs, by stimulation in the rostral part of the lateral sigmoid gyrus, the lateral part of the anterior sigmoid gyrus in area 4 gamma and in the adjacent area 6. In contrast, the long PNs also received monosynaptic EPSPs from area 3b near the border of area 1. The present results show segregation of the cortical control to functionally different premotoneuronal systems and suggest that this control could in part be separated for subtypes of short C3-C4 PNs.