Presynaptic depolarization of myelinated afferent fibres evoked by stimulation of cutaneous C fibres

Abstract

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.