Segmental and supraspinal input to cells of origin of non‐primary fibres in the feline dorsal columns.

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Abstract
The synaptic input to ascending tract cells with axons in the dorsal columns was investigated using intracellular recording. EPSP (excitatory post synaptic potentials) evoked by stimulation of the lateral funiculus were analysed to test for the possibility of collateral connexions between spino-cervical tract cells and dorsal column cells. Groups of fibers (3) contributed to such EPSP: fibers which terminated or originated between spinal segments C[cervical]3-4 and C1, or Th[thoracic]9 and C3-4 and cortico-spinal tract fibers. The latencies and thresholds of EPSP evoked by stimulation of the 1st group of fibers were compatible with their origin via axon collaterals of spino-cervical tract cells. The occurrence of these EPSP in dorsal column cells which were disynaptically excited from cutaneous afferents further corroborated this possibility. EPSP of specifically cervical origin were also found in some other neurons in the dorsal horn, probably segmental interneurons, but were absent in spino cervical tract cells. Convergence of group I muscle afferents (possibly both group Ia and group Ib) and cutaneous afferents was found in about 50% of the dorsal column cells. The shortest latency EPSP from cutaneous and group I afferents were evoked with segmental delays indicating monosynaptic and disynaptic coupling. IPSP were evoked from cutaneous and group I muscle afferents in either the same or different nerves as those from which the EPSP were elicited. Excitatory potentials were dominating.