Projection from excitatory C3-C4 propriospinal neurones to lamina VII and VIII neurones in the C6-Th1 segments of the cat.

Intracellular recording and injection of horseradish peroxidase (HRP) were made in neurones located medially in lamina VII and in lamina VIII of the forelimb segments (C6-Th1). The cells received disynaptic excitation from the contralateral pyramid after corticospinal transection in C5/C6 and monosynaptic excitation from the ipsilateral lateral reticular nucleus. The pyramidal excitation was facilitated by a conditioning volley evoked from the contralateral nucleus ruber, which suggests convergence of cortico- and rubrospinal fibres on the intercalated neurones. It is proposed that laminae VII and VIII neurones receive a collateral input from the same excitatory C3-C4 propriospinal neurones which project to motoneurones and/or Ia inhibitory interneurones. Reconstruction of HRP-stained lamina VII and VIII neurones revealed ipsi- and contralateral ascending and/or descending axonal projections and termination in laminae VII and VIII in the forelimb segments.