Relations between spinocervical and post-synaptic dorsal column neurones in the cat.

1. In chloralose-anaesthetized cats single-unit micro-electrode recordings were made at the lumbosacral level either from axons in the dorsolateral funiculus and dorsal columns, identified as belonging to the spinocervical tract (s.c.t.) or post-synaptic dorsal column (p.s.d.c.) pathway respectively, or from neurones in the dorsal horn similarly identified. 2. Attempts were made to show that s.c.t. and p.s.d.c. neurones had axons that bifurcated, so that they sent branches into both the ipsilateral dorsolateral funiculus and the dorsal columns. That is, that some, or all, of the presumed s.c.t. or p.s.d.c. axons were common to both populations. In addition, the effects of stimuli applied to the ipsilateral dorsolateral funiculus at C3 and C1 on the resting discharges of p.s.d.c. neurones were examined in order to determine the effectiveness of the link between the s.c.t. and the p.s.d.c. pathway. 3. Thirty-three s.c.t. units (twenty-six axonal recordings and seven soma-dendritic recordings) and thirty p.s.d.c. units (twenty-four axonal and six soma-dendritic recordings) were examined for bifurcating axons by electrically stimulating the dorsolateral funiculus at C3 and the dorsal columns at C4. None of the p.s.d.c. units could be antidromically activated from the ipsilateral dorsolateral funiculus with stimulus strengths up to 40 V or seventy times threshold for antidromic activation from the dorsal columns. Similarly, twenty s.c.t. units could not be activated antidromically from the dorsal columns at stimulus strengths up to 30 V or thirty times threshold for their antidromic excitation from the dorsolateral funiculus. Thirteen s.c.t. units were antidromically activated from the cervical dorsal columns, eight at seventeen or more times threshold for their activation from the dorsolateral funiculus and five at between two and nine times threshold. All s.c.t. units that were activated antidromically from both the cervical dorsal columns and the dorsolateral funiculus showed similar latencies for the two responses. 4. Twenty-five p.s.d.c. units were examined for the effects of ipsilateral dorsolateral funiculus stimulation on their resting activity. In thirteen, clear evidence of facilitatory effects from C3 were observed, whereas similar results were seen in only six of these units when C1 was stimulated and the effects were less. The facilitation had a latency of 3-16 ms and lasted for 6-22 ms. In all but one of the twenty-five units, stimulation at both C1 and C3 produced profound inhibition of the resting discharge that began at between 8 and 26 ms and lasted for up to 300 ms.(ABSTRACT TRUNCATED AT 400 WORDS)