Analysis of the Central Pathway of the Trigeminal-Salivary Reflex by the Spinal Trigeminal Tractotomy

The aim of this study was to clarify the role of spinal trigeminal subnucleus interporalis (Sp 5 I) and subnucleus caudalis (Sp 5 C) on the trigeminal-salivary reflex. Extracellular recordings were made from a total of 119 superior salivatory nucleus (SSN) neurons innervating the submandibular and sublingual glands in urethane-chloralose anaesthetized cats. Responses to stimulation of the inferior alveolar (IA) nerve were investigated in trigeminal tractotomized animals at the level of the obex (Sp 5 C animals) and at the level of 2.5-3mm rostral from the obex (Sp 5 I animals) as well as in non-tractotomized (control) animals. In control, Sp 5 C and Sp 5 I animals, 50% (23/46), 46% (15/33) and 10% (4/40) of the identified SSN neurons orthodromically responded to stimulation of the IA nerve, respectively.In Sp 5 C animals, no significant reduction of the response rate was found in comparison with control animals, and there was no significant difference in the mean onset latency between control and Sp 5 C animals. However, only the responses activated by the medium-threshold afferent fibres or by high-threshold afferent fibres could be observed.In Sp 5 I animals, a marked reduction of the response rate was noted in comparison with control animals, and there was no significant difference in the mean onset latency between control and Sp 5 I animals. However, only the responses excited by the medium-threshold afferent fibres or by high-threshold afferent fibres could be observed.As the relay nucleus in the trigeminal-salivary reflex, Sp 5 I has a more important role and Sp 5 C has a lesser role.