Observations on neuronal pathways subserving primary afferent depolarization.

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Abstract
Latencies of presynaptic depolarization of group I muscle afferents and cutaneous afferents were investigated in cats to define the minimal number of interneurons interposed in the pathways of the presynaptic depolarization. Intra-axonal records from individual afferents, records of dorsal root potentials and measurements of changes in excitability of individual afferents were used for this purpose. Some of the animals were treated with 4-aminopyridine to facilitate transmission via neuronal pathways. The minimal segmental latencies were measured relative to afferent volleys evoked by single electrical stimuli applied to peripheral nerves and to the 2nd of a pair of stimuli. Due to subthreshold facilitatory effects of the 1st stimulus, the responses to the 2nd stimulus appeared at a reduced latency. The minimal latencies of primary afferent depolarization found in this way were 1.7-2.0 ms. They were similar for group I and cutaneous afferents and in both cases compatible with a trisynaptic pathway (2 interposed interneurons). Primary afferent depolarization was also evoked by local intraspinal stimuli applied in the dorsal horn, in the intermediate zone and in the ventral horn to define the location of the last-order interneurons mediating depolarization group I and cutaneous afferents. Lowest threshold responses appearing with the shortest latencies (.apprx. 0.8 ms) and attributed to activation of the last-order interneurons were found when stimulating fairly restricted areas: within laminae V-VI for group I afferents and within laminae III-IV for cutaneous afferents. Both areas corresponded to the regions where the largest monosynaptic field potentials were evoked by fibers receiving the presynaptic depolarization. Both the 1st- and last-order interneurons of the pathways may be located within these areas.