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Experiments were carried out to find if there were post-synaptic effects produced by impulses in the long ranging primary afferents, which had been shown by Wall & Werman (1976) to extend from upper lumbar dorsal roots to the sacral segments. Dorsal rootlets were stimulated in decerebrate low-spinal adult cats.

1. The dorsal root potential and ventral root reflex were recorded on S1 root filaments, in response to stimulation of dorsal rootlets extending from L1 to S1. With increasing distance between stimulating and recording segments, these potentials became smaller and more delayed. In two animals, there was no response at S1 to stimulation of L1 and L2 dorsal roots.

2. In all animals, stimulation of L3 or L4 dorsal roots produced cell responses in dorsal horn segments L7 or S1. The density of such cells was variable, from animal to animal. Responding cells were mainly concentrated laterally in the dorsal horn.

3. The latency and response variability of L7-S1, dorsal horn cells to L3-L4 stimulation was consistent with at least some of them being fired monosynaptically.

4. Cells that respond to stimulation of one distant rootlet respond to many closer rootlets as well.

5. The receptive fields of L7-S1 dorsal horn cells, responsive to stimulation of L3-L4 rootlets, were typical of those generally found in the L7-S1 segments, and were at some distance from the L4 dermatome. Only twenty cells had receptive fields which extended into the dermatome of the rootlets stimulated.

6. It was established that some L4 cells respond to S1 dorsal root stimulation, just as the main study had shown that S1 responds to L4.

7. It is concluded that substantial numbers of dorsal horn cells, including cells with many types of cutaneous receptive field, respond to two classes of synaptic in-put: one effective in firing the cell upon natural cutaneous stimulation, and one relatively ineffective, capable of driving the cell only when stimulated electrically and thus carrying a synchronous volley from a number of highly convergent axons. The contribution of this secondary afferent channel to normal and pathological cord physiology has now to be determined.

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