Pyramidal tract and corticospinal neurons with branching axons to the dorsal column nuclei of the cat.

Extracellular single activity was recorded from pericruciate neurons in anaesthetized, paralysed, artificially ventilated cats. A total of 309 neurons were identified antidromically by stimulation of the dorsal column nuclei (229 from the nuneate nucleus and 80 from the gracile nucleus). The study addressed the question whether pericruciate-dorsal column nuclei neurons (corticonuclear cells) sent collaterals to the ipsilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. Also, the ipsilateral pyramidal tract was stimulated at mid-olivary level, as was the crossed corticospinal tract at C2, Th2 and L2 levels in order to know whether the corticonuclear cells sent their axons to the spinal cord and if so to which level. It was found that more than 95% of the corticonuclear fibres coursed through the pyramidal tract. A significant (28.4%; 88/309) proportion of the the corticonuclear neurons sent collaterals to the red nucleus and/or to the nucleus reticularis gigantocellularis. About 68% (209/309) of the corticonuclear cells did not send their axons to the spinal cord and the remainder were corticospinal neurons. Most of the corticospinal fibres terminated at the cervical level (72/100) and the remaining ended at thoracic (18/100) and lumbar (10/100) segments of the cord. While 63.4% (123/194) of the corticonuclear fibres coursing through the pyramidal tract and ending at supraspinal levels were slow conducting, the great majority of the corticospinal neurons were fast conducting (91/100). The non-corticospinal neurons were significantly slower conducting than the corticospinal cells. The corticogracile neurons were slower conducting than the corticocuneate cells. Of the 88 corticonuclear neurons that sent at least a branch to the sites tested, 50% branched into the red nucleus, 35.2% into the nucleus reticularis gigantocellularis and 14.7% into both nuclei, without significant difference between non-corticospinal and corticospinal cells. Most of the main axons of the corticonuclear cells ended at bulbar and cervical levels (281/309 or 90.9%). The data indicate that pericruciate-dorsal column nuclei neurons form a particular substrate within pyramidal tract cells. They can serve precise functions in motor coordination associated with the selection of their own sensory input. The results are discussed from this point of view.