Reflex responses in the digastric and tongue muscles to stimulation of intradental nerves in the cat.

Electrical stimulation of tooth pulp nerves induces the digastric jaw-opening reflex in the cat, apparently due to activation of intradental A-fibres; C-fibres do not seem to be involved. In fact, reflex responses to activation of pulpal C-fibres have not been studied. In the present experiments on anesthetized cats we recorded EMG reflex responses of the digastric and tongue muscles to stimulation of the intact tooth crown, exposed dentine, and the pulp. We used stimuli that selectively activate either A- or C-fibres of the pulp. Slow heating of the tooth and application of capsaicin into the pulp, both procedures known to excite only C-fibres in the pulp, evoked licking movements of the tongue and prolonged EMG-responses in the tongue and, less consistently, digastric muscles. Similar muscle responses were elicited by high intensity electrical current pulses applied to the tooth. At low current intensities only short-duration digastric activation (jaw-opening reflex) was induced. Similar digastric jaw-opening was also evoked by drilling and air-drying of dentine, both stimuli able to activate only intradental A-fibres. These results indicate that activation of both A- and C-type pulp nerve fibres can induce reflectory muscle activation, and further, they support the concept of afferent intradental C-fibre innervation.