歯髄の電気刺激に応答するネコ三叉神経中脳路核一次ニューロンの性質

Abstract

The existence of primary afferent neurons innervating the tooth pulp (TP) has been demonstrated by the HRP tracing method in the mesencephalic trigeminal nucleus (MTN) of the cat, dog and monkey by Amano et al. (1987) and Yoshino et al. (1989), (1993). The intention of the present study was to confirm electrophysiologically that the neurons in the feline MTN that respond to electrical stimulation of the TP are cell bodies of primary afferent fibers and to examine their functional properties. Adult cats of either sex were anesthetized with urethane-chloralose and mounted on a stereotaxic apparatus. For electrical stimulation and whole nerve recordings, the right inferior alveolar nerve (IAN) was exposed and a pair of Ag-AgCl wire electrodes were placed in contact with the intact nerve. For bipolar EMG recording, a pair of copper wire electrodes were sutured into the ipsilateral anterior digastric muscle (DIG). The right maxillary and mandibular canine teeth were used for electrical stimulation of the TP. To activate as many pulpal afferent fibers as possible with constant low current stimulation, two Ag-AgCl wire electrodes were implanted into deep dentin cavities prepared in the canine root portions corresponding to the apical one- and two-fourths. Single-unit recordings were made with glass-coated elgiloy microelectrodes in the region of P2.5 and L2.5, corresponding to the caudal part of the MTN. During recordings in the MTN, unitary spindle and periodontal activities were detected with occasional manual force to the mandible or to the canine teeth. The only unitary activity related to the electrical TP stimulation was investigated. The recording sites were histologically verified with the aid of electrode tip marking. A total of 25 single neurons were recorded from the MTN of 20 cats. These neurons followed faithfully double pulse stimuli separated by 2 msec (i.e., 500 Hz) delivered to the canine TP, responded with a consistent short latency ranging 1.2-2.0 msec (1.6±0.2 msec, mean±SD) and were histologically confirmed to locate in the MTN. Their conduction velocities were 38.0-63.3 m/sec (48.9±6.4 m/sec, mean±SD) and agreed well with the aβ range of 35-70 m/sec. Prior to paralyzation, recordings were carried out simultaneously from an ipsilateral MTN primary pulpal neuron, whole IAN and DIG. The responses to gradually increasing intensity of single pulse stimuli (20 μsec duration) to the mandibular canine TP were graded and the thresholds were carefully determined. The fastest component of the IAN compound action potential due to the largest pulpal afferent fibers apperared at 2.7 mA. Stimulus intensity had to be increased to 1.4 times threshold to cause firing of the MTN pulpal neuron and short-latency digastric reflex activity (7.0 msec latency). This MTN pulpal neuron with the same threshold for the digastric reflex was histologically confirmed to locate in the mesencephalic trigeminal tract a little away from other primary afferent neurons with relatively large soma aggregating in clusters. The main finding of the present study is that the feline MTN primary neurons supply the TP with Aβ afferent fibers. These MTN pulpal neurons might play a role of mechanoreception, as indicated by Dong et al. (1985) in the feline intradental afferents with soma in the TG.