INTERNEURONS OF THE SUPRATRIGEMINAL AREA MEDIATING REFLEX INHIBITION OF TRIGEMINAL AND FACIAL MOTOR-NEURONS IN THE RAT

Whether sensory information from the inferior alveolar nerve is mediat ed by different types of interneurones in the supratrigeminal area (Su 5) and whether different types of these interneurones have different i nhibitory actions on jaw-closing motor neurones of the trigeminal moto r nucleus was investigated. The intracellular responses of periodontal afferents in the mesencephalic trigeminal nucleus, Su5 interneurones and jaw-closing motor neurones were studied in response to graded, sin gle-shock stimulation of the ipsilateral inferior alveolar nerve. It w as found that the inhibitory action of afferent inflow from the inferi or alveolar nerve to jaw-closing motor neurones is possibly mediated b y two types of Su5 interneurones (T-I and T-II). These Su5 neurones we re discriminated on the basis of their firing characteristics. The fin dings also indicated that: (1) T-I neurones are responsible for short- latency, low-threshold inhibitory postsynaptic potentials (IPSPs) obse rved in the trigeminal motor nucleus neurones; (2) T-II interneurones mainly contribute to the amplitude of these IPSPs at higher stimulus s trengths; (3) the late part of plateau IPSPs in the jaw-closing motor neurones is induced by a characteristic firing of T-II neurones. It wa s also shown that afferent inflow from the inferior alveolar nerve, pr obably mediated by collaterals of T-I and T-II interneurones, also evo kes IPSPs in neurones of the intermediate subnucleus of the facial mot or nucleus. The characteristics of these IPSPs resemble those of the I PSPs recorded in the jaw-closing motor neurones.