Depolarization of the tooth pulp afferent terminals in the brain stem of the cat.

1. Tooth pulp afferent fibres belonging exclusively to the Adelta group were stimulated bi-polarly with electrical pulses applied to the dentine and the central effects of the stimulation were examined in the nucleus of the spinal trigeminal complex of anaesthetized cats.2. Field potentials evoked by single pulses to the upper or lower canine tooth pulp were explored over the nucleus of the spinal trigeminal tract in the region 5 mm caudally to the obex up to 8.5 mm rostrally to the obex. They were found to be restricted to a region of 5.5-8.5 mm rostrally to the obex, 4-5 mm laterally from the mid line, with a maximum amplitude at a depth of 5 +/- 0.5 mm.3. Antidromic action potentials were recorded from the tooth pulp afferents. The stimulating micro-electrode was inserted in the region of the medulla from which distinct field potentials could be recorded by orthodromic stimulation.4. Excitability of the central terminals of the tooth pulp primary afferents was increased when the test stimulus was preceded either by a conditioning volley in low threshold afferent fibres of the infraorbital nerve or in Adelta fibres of another tooth.5. Single pulses applied to the afferent fibres of the tooth pulp produced changes in the excitability of central terminals of the fast conducting afferent fibres of the infraorbital nerve.6. The greatly increased excitability of central terminals of tooth pulp primary afferent fibres was accompanied by a small decrease in their synaptic efficiency as estimated from the changes of the post-synaptic component of the field potential evoked by their stimulation.7. It is concluded that an analogous presynaptic control mechanism is present at the central terminals of the tooth pulp primary afferent fibres as is known for the fast conducting cutaneous system. But a difference in the specifity of the action from this afferent system is suggested. While activity from Aalpha fibres is very effective in evoking primary afferent depolarization at both fast conducting trigeminal fibres and slow conducting Adelta system, the Adelta fibre activity is effective only at the latter, and not at the former.