PYRAMIDAL TRACT AND CORTICOSPINAL NEURONS WITH BRANCHING AXONS TO THEDORSAL COLUMN NUCLEI OF THE CAT

Extracellular single activity was recorded from pericruciate neurons i n anaesthetized, paralysed, artificially ventilated cats. A total of 3 09 neurons were identified antidromically by stimulation of the dorsal column nuclei (229 from the nuneate nucleus and 80 from the gracile n ucleus). The study addressed the question whether pericruciate-dorsal column nuclei neurons (corticonuclear cells) sent collaterals to the i psilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. Also, the ipsilateral pyramidal tract was stimulat ed at mid-olivary level, as was the crossed corticospinal tract at C2, Th2 and L2 levels in order to know whether the corticonuclear cells s ent their axons to the spinal cord and if so to which level. It was fo und that more than 95% of the corticonuclear fibres coursed through th e 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 t he remainder were corticospinal neurons. Most of the corticospinal fib res terminated at the cervical level (72/100) and the remaining ended at thoracic (18/100) and lumbar (10/100) segments of the cord. While 6 3.4% (123/194) of the corticonuclear fibres coursing through the pyram idal tract and ending at supraspinal levels were slow conducting, the great majority of the corticospinal neurons were fast conducting (91/1 00). The noncorticospinal neurons were significantly slower conducting than the corticospinal cells. The corticogracile neurons were slower conducting than the corticocuneate cells. Of the 88 corticonuclear neu rons that sent at least a branch to the sites tested, 50% branched int o the red nucleus, 35.2% into the nucleus reticularis gigantocellulari s and 14.7% into both nuclei, without significant difference between n on-corticospinal and corticospinal cells. Most of the main axons of th e corticonuclear cells ended at bulbar and cervical levels (281/309 or 90.9%). The data indicate that pericruciate-dorsal column nuclei neur ons form a particular substrate within pyramidal tract cells. They can serve precise functions in motor coordination associated with the sel ection of their own sensory input. The results are discussed from this point of view.