Differences in the excitability of two populations of trigeminal primary afferent central terminals

The excitability of lingual and inferior dental nerve terminals in the trigeminal main sensory nucleus was increased following a conditioning stimulus delivered to the same nerves of the contralateral side. The lingual nerve primary afferent depolarization (PAD) began at a shorter CS-TS interval (CS, conditioning stimulus; TS, test stimulus), and reached a maximum level of depolarization sooner, than did the PAD evoked in the inferior dental nerve. This difference in the time course of the PAD was not dependent on the site of the CS; i.e., the same time course was observed for the lingual nerve terminals whether the CS was delivered to either the contralateral lingual, or inferior dental nerve. The effect of the contralateral CS was also tested on the ipsilateral lingual-digastric and inferior dental-digastric reflexes in order to determine if the PAD observed in the ipsilateral nerve terminals would be reflected in similar changes in reflexes mediated by those nerves. It was found that both digastric reflexes were inhibited by the CS. The time course of the inhibition showed similar characteristics to that of the previously discussed PAD; i.e., the onset of the lingual-digastric reflex inhibition began a shorter CS-TS interval and reached maximum effectiveness sooner than did the inhibition of the inferior dental-digastric reflex. The possible significance of the results in relation to the role of presynaptic inhibitory mechanisms in the reflex control of jaw movement is discussed.