1. The origin of the dorsal root potential observed following a volley in cutaneous unmyelinated or C fibres (C-DRP) has been investigated by several assays.2. Measurements of the spatial decrement of the C-DRP along the dorsal roots indicate that mainly the thick myelinated primary afferent fibres contribute to this C-DRP.3. The spatial configuration of the intraspinal potential field during the course of the C-DRP is similar to that of the field correlated to the A-DRP, i.e. the DRP evoked by a volley in the A fibres of the same cutaneous nerve. It is concluded therefrom that both A- and C-DRPs signal presynaptic depolarizations of afferent fibres in the same regions of the dorsal horn.4. Intrafibre recordings reveal that C input produces primary afferent depolarization (C-PAD) mainly in cutaneous and articular afferents, but not in muscle Group Ia afferents.5. These findings and direct measurements of the interactions between A- and C-DRPs indicate that the pathways involved in A- and C-PAD exhibit convergence.6. C-DRPs were also recorded from contralateral rootlets.7. Some functional aspects of the PAD produced by an afferent volley in C fibres are discussed.
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