Response characteristics of primary periodontal mechanoreceptive neurons in the trigeminal mesencephalic nucleus to trapezoidal mechanical stimulation of a single tooth in the rat.

The response characteristics of primary periodontal mechanoreceptive neurons in the trigeminal mesencephalic nucleus (MeNV) were studied by changing the rate and magnitude of trapezoidal pressure applied to the upper incisor in very lightly anesthetized rats. Using a metal microelectrode in the MeNV either for recording or for stimulation, the projection site of primary afferents in the anterior superior alveolar nerve innervating the upper incisor was determined. Its stereotaxic coordinates were 1.0--2.0 mm posterior to the interaural plane and 1.2--1.3 mm lateral to the mid-sagittal plane, corresponding to the caudal part of the MeNV. From this site of 19 animals, single unitary activity from 41 primary periodontal mechanoreceptive neurons was recorded, which were identified by: (1) the constant, brief latency of 1.0--4.0 ms (2.0 +/- 0.6 ms, mean +/- S.D.) from the onset of single tooth tapping; (2) the wave forms of their unitary spike responses; and (3) the ability to follow faithfully trains of repetitive stimuli applied to the anterior superior alveolar nerve at rates of more than 100 Hz for 2 s. The responses of the identified primary mechanoreceptive neurons in the MeNV were tested for repetitive tapping and trapezoidal pressure to the ipsilateral upper incisor. The highest frequency of one-to-one following was observed in repetitive tapping at 100 Hz. All of the 27 tested neurons exhibited such very rapid adaptation as to show only on-off responses to trapezoidal pressures, and did not respond at all unless the pressures were applied more rapidly than 0.6 Newtons/s (N/s). In most of these neurons, the number of evoked spikes was greater in on-responses than in off-responses, and the number and frequency of spikes were increased with an increase in the rate of pressure application. But in several neurons only one spike was triggered even when the pressures were applied at the rate of more than 83.1 N/s.