Extracellular single activity was recorded from pericruciate neurons in anaesthetized, paralysed, artificially ventilated cats. A total of 455 neurons were classified antidromically according to their sites of termination along the corticospinal tract and whether they sent collateral branches to the ipsilateral red nucleus and/or to the contralateral nucleus reticularis gigantocellularis. It was found that the majority of the branching fibres that reached the most caudal segments of the cord were fast conducting, while the slower branching axons tended to terminate at more rostral levels of the corticospinal tract. Most of the branching fibres terminated at bulbar and cervical levels (153/182: 84%), and the remaining ended at thoracic (21/182: 11.5%) and at lumbar (8/182: 4.4%) segments of the cord. The non-corticospinal, pyramidal tract fibres branched more (56%) than the corticospinal fibres (26.6%). Within the corticospinal neurons, the degree of branching decreased with distance along the spinal cord. While 57.5% of the pericruciate fibres that projected only as far as the pyramidal tract were slow conducting, the majority of the corticospinal neurons were fast conducting (74.6%). Both pyramidal tract and corticospinal neurons that sent branches to one or to the two sites tested were significantly faster conducting than the neurons which did not branch. A total of 101 corticorubral and corticobulbar neurons which did not respond to pyramidal tract stimulation was also recorded. The data can be of significance in the understanding of co-ordination of different muscles in order to couple movement and posture into a common act. The results are discussed from this point of view. © 1994.
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