Branching and termination of C3-C4 propriospinal neurones in the cervical spinal cord of the cat.

Antidromic stimulation and intra-axonal injections of horseradish peroxidase have been used to investigate axonal branching and termination of single C3-C4 propriospinal neurones (PNs) that project to the forelimb segments (C6-Th1). Branching at several spinal cord levels and terminations were found in laminae VI-VIII and IX. With respect to terminations in laminae VII and IX, 3 patterns were observed: (i) termination only in lamina IX, (ii) only in lamina VII in the region of Ia inhibitory interneurones and (iii) in both laminae VII and IX. These findings are consistent with previous results showing monosynaptic projections of C3-C4 PNs to forelimb motoneurones and Ia inhibitory interneurones. Terminations in laminae VI, VIII and other parts of lamina VII suggest that C3-C4 PNs also project to other neurones in the forelimb segments.