Studies on the presence and functional properties of afferent C-fibers in the cat's dental pulp.

The present investigation describes how intradental afferent C-fibers can be identified and characterized in the anaesthetized cat. Functional single fibers innervating the lower canine tooth were recorded from filaments split from the inferior alveolar nerve. Fibers responding to monopolar electrical stimulation of the tooth were classified as slowly and fast conducting according to their conduction velocity (c.v.). The axonal c.v. was determined for 31 slowly conducting fibers by electrical stimulation of the nerve. Sixty-eight percent of these proved to be C-fibers. The slowly conducting fibers had higher activation thresholds, smaller amplitudes and longer durations of the action potentials than the fast conducting fibers. Many of the fast but none of the slowly conducting fibers were repetitively activated by a single stimulus pulse. The highest frequency at which the fibers responded to every stimulus pulse was much lower for the slowly than for the fast conducting fibers. Most of the slowly conducting intradental fibers tested were activated by both hot and cold stimulation of the tooth, by pressure applied to the pulp chamber and by the application of KCl or bradykinin to the pulp. Eight C-fibers responded to mechanical stimulation of the pulp. For some fibers the response magnitude correlated with the stimulus intensity. Many slowly conducting intradental fibers developed an ongoing discharge after heat and pressure stimulation. The results indicate that there exists in the dental pulp of the cat afferent C-fibers that resemble polymodal C-nociceptors. They might be involved in the generation of the dull aching pain present in inflammatory conditions of the pulp. The activation of intradental C-fibers in pulpitis might result either from the elevated pressure and/or by sensitization of the C-fiber endings by inflammatory substances released.