Origin of corticospinal neurones evoking disynaptic excitation in forelimb motoneurones mediated via C3-C4 propriospinal neurones in the cat

Intracellular recording was made from forelimb motoneurones in the cat (alpha-chloralose anaesthesia) during electrical stimulation of corticospinal neurones (CSNs) and their afferents in the contralateral cortex. Axons of the CSNs were stimulated in the contralateral pyramid. The corticospinal tract was transected at the C5/C6 segmental border in order to restrict transmission through the C3-C4 propriospinal neurones (C3-C4 PNs). Di- and trisynaptic cortical EPSPs could be evoked after transection of the corticospinal fibres in C5/C6 but not after a corresponding transection in C2/C3. Pyramidal stimulation elicited disynaptic EPSPs that were abolished after a C2/C3 transection. Disynaptic pyramidal EPSPs, mediated via C3-C4 propriospinal neurones could be facilitated by a single cortical stimulation. It is concluded that di- and trisynaptic cortical EPSPs and disynaptic pyramidal EPSPs are mediated via the same C3-C4 PNs. Cortical surface stimulation showed that di- and trisynaptic cortical EPSPs could be evoked from distinct spots in the lateral part of the anterior sigmoid gyrus (Sig. a) and/or in the rostral part of the lateral sigmoid gyrus (Sig. l). No cortical EPSPs or facilitation of pyramidal disynaptic EPSPs was evoked from the posterior part of the Sig. l, posterior sigmoid gyrus, coronal gyrus, lateral gyrus, suprasylvian gyrus and ectosylvian gyrus. It is concluded that the CSNs, which issue the command for visually guided target reaching with the forelimb via the C3-C4 PNs, originate in the lateral part of the Sig. a and in the rostral part of the Sig. l. A dual representation of the forelimb in the primary motor cortex of the cat has previously been proposed. The present results show that with respect to one identified interneuronal system like the C3-C4 propriospinal system, the CSNs may have their origin restricted to one region of the primary motor cortex.