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

Intracellular recording was made from forelimb motoneurones in the cat (alp ha-chloralose anaesthesia) during electrical stimulation of corticospinal n eurones (CSNs) and their afferents in the contralateral cortex. Axons of th e CSNs were stimulated in the contralateral pyramid. The corticospinal trac t was transected at the C5/C6 segmental border in order to restrict transmi ssion through the C3-C4 propriospinal neurones (C3-C4 PNs). Di- and trisyna ptic cortical EPSPs could be evoked after transection of the corticospinal fibres in C5/C6 but not after a corresponding transection in C2/C3. Pyramid al stimulation elicited disynaptic EPSPs that were abolished after a C2/C3 transection. Disynaptic pyramidal EPSPs, mediated via C3-C4 propriospinal n eurones could be facilitated by a single cortical stimulation. It is conclu ded that di- and trisynaptic cortical EPSPs and disynaptic pyramidal EPSPs are mediated via the dame C3-C4 PNs. Cortical surface stimulation showed th at di- and trisynaptic cortical EPSPs could be evoked from distinct spots i n the lateral part of the anterior sigmoid gyrus (Sig. a) and/or in the ros tral part of the lateral sigmoid gyrus (Sig. l). No cortical EPSPs or facil itation of pyramidal disynaptic EPSPs was evoked from the posterior part of the Sig. l, posterior sigmoid gyrus, coronal gyrus, lateral gyrus, suprasy lvian gyrus and ectosylvian gyrus. It is concluded that the CSNs, which iss ue the command for visually guided target reaching with the forelimb via th e 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 m otor cortex of the cat has previously been proposed. The present results sh ow that with respect to one identified interneuronal system like the C3-C4 propriospinal system, the CSNs may have their origin restricted to one regi on of the primary motor cortex. (C) 2000 Elsevier Science Ireland Ltd and t he Japanese Neuroscience Society. All rights reserved.