Somatosensory afferents mediating the bilateral reflex vasodilatation in cat palate induced by noxious tooth‐pulp stimulation

Noxious stimulation of the tooth pulp is known to evoke autonomic and other visceral reflexes, e.g. blood pressure and cardiac changes (1-3). We have found recently that electrical stimulation of the tooth pulp caused reflex blood flow increases at various sites in the cat's ipsilateral oral mucosa (upper and lower gingivae, lower lip, buccal mucosa and tongue and, most notably, bilaterally in the palatal mucosa) (4-6). We also reported that the bilateral reflex vasodilator response in the palatal mucosa was mediated via parasympathetic vasodilator fibres that emerge from the brain stem with the facial nerves on each side and reach the blood vessels via the pterygopalatine ganglion (6). Although the efferent fibres mediating this reflex vasodilatation in the cat palatal mucosa were thus identified, we are unclear as to the nature of the receptors and afferent fibres from the tooth pulp involved in this response. The tooth pulp is known to be innervated by large numbers of myelinated and unmyelinated axons (7, 8) with conduction velocities in the A- and C-fibre range (9). Further, it has been shown that there are two pharmacological types of nerve fibres in the tooth pulp, i.e. capsaicin-sensitive and -insensitive nociceptive nerve fibres. In this experiment, we (1) applied capsaicin to the exposed pulp to determine whether the reflex vasodilatation in the palate involves capsaicin-sensitive nociceptive nerve fibres as the afferent arm of the reflex and (2) electrically stimulated one upper canine tooth before and after application of capsaicin to the superior alveolar nerve on the side ipsilateral to the stimulus to determine whether capsaicin-insensitive nerve fibres participate in the reflex vasodilatation. In addition, the question of whether A- and C-fibres represent capsaicin-insensitive and -sensitive nociceptive nerve fibres, respectively, is also discussed.