MORPHOMETRIC STUDY OF CHORDA TYMPANI-DERIVED FIBERS ALONG THEIR COURSE IN THE LINGUAL NERVE

Abstract

The topographical organization of chorda tympani-derived fibers (Ch-fibers) was investigated macroscopically and histologically along their entire course through the lingual nerve in 81 adult Japanese cadavers (113 sides), in order to obtain basic data to explain the taste disturbance occurring after oral anesthesia. Ch-fibers frequently (approx. 80%) merged and intermingled with nerve bundles composed of fibers of the lingual nerve proper (L-fibers) above the level of the mandibular foramen (intermingled type). However, on 23 sides (20.4%) the Ch-fibers could be separated under a stereomicroscope from the L-fiber bundles during their course behind the oral cavity (separated type). Moreover, on 12 of the 23 sides, the Ch-fibers were clearly separated from the L-fibers, since the Ch-fibers formed an independent nerve bundle attached to the surface of the L-fiber bundles throughout almost their entire course to the tongue. Irrespective of whether they were intermingled or separated, however, the Ch-fibers traveled downward, maintaining their superficial and posterolateral topographical organization in the lingual nerve. In 8 of the 12 clearly separated cases, the perimeter of the myelinated fibers was analyzed at three levels: the hard palate, the mandibular foramen and immediately above the angle of the mandible. At each level, the perimeter spectrum of Ch-fibers was significantly smaller (mean: 16.6 +/- 5.4 microns, e.q. at the level of the mandibular foramen) than that of the L-fibers (mean: 20.2 +/- 6.2 microns, at the same level) (p < 0.01). Moreover, the lingual nerve consistently had 2-3 buccal branches supplying the lining of the oral cavity posterior to the internal oblique line of the mandibular ramus. These buccal branches did not contain Ch-fibers, but were composed of only L-fibers, possibly including pain fibers. These observations suggest that physicians performing nerve block to and around the mandibular foramen tend to regard the inferior alveolar nerve and the lingual nerve as targets of the block, irrespective of whether or not this is intended, since the buccal branch of the lingual nerve should also be anesthetized in such cases. Moreover, local anesthetics seem to have a rapid effect on Ch-fibers, because of their superficial position in the lingual nerve and their smaller diameter than L-fibers. Consequently, Ch-fibers along the lingual nerve seem to be easily damaged by local anesthesia of the oral region. Greater care should therefore be exercised to avoid producing a taste disturbance when performing oral anesthesia in clinical practice.