The Neurohistology of the Proximal Portion of the Human Ear Canal1

The external auditory canal of man serves primarily as a tube for the passage of sound waves to the tympanic membrane. The latter structure stretches across the lumen at the proximal extremity of the canal where it separates the external and middle ear. This extremely fragile sound-sensitive membrane is protected by the unique anatomical structure of the canal. The entrance to the canal is guarded by stiff hairs called tragi. The lumen is small and surrounded by relatively unyielding cartilaginous and bony walls. The stickiness of the cerumen which coats the wall further impedes the penetration of any small foreign body that might enter the canal. While the protection afforded by gross structure and the presence of cerumen is considerable, it is obvious that the nervous system must also play an important protective role. Our studies have shown a pattern of nerve distribution which appears unique to the area. This presentation describes this pattern in the bony -canal and tympanic membrane of man. METHODS AND MATERIALS Specimens were obtained from the bony (proximal) portion of the ear canals of cadavers at autopsy examination. In order to rule out the possibility of postmortem autolysis of nerve fibers, several specimens were obtained from living subjects under anaesthesia for other surgical procedures, such as mastoidectomy.! Tympanic membranes were secured from cadavers. Tissue from the back, arm and face was used for comparative purposes and to control idiosyncrasies in the staining technic. Utilizing both fresh and formalin fixed tissue, frozen sections 25 micra in thickness were cut and placed in 4 % neutral formalin until final processing. The Winklemann technic for silver impregnation of peripheral nerve endings was used throughout. In accordance with the suggested technic, variations in the time and temperature of impregnation and of subsequent reduction were employed to obtain the optimal contrast between the nerve fibers and their surrounding tissue. Optimal results were obtained by incubation of the tissue for five hours in the impregnating solution which was maintained at a temperature ranging from 50-55° C. Reduction was accomplished in one to three minutes at a temperature of 27° C. Tissue from other parts of the body was treated simultaneously in an identical fashion. t Obtained through the courtesy of Dr. Edward H. Campbell and his associates in the Department of Otolaryngology. 103 - *;'f “' '~. ir'vr-Y'i-'"' Fig. 1. The skin lining the bony portion of the external auditory canal: shown to illustrate the abundance of large nerve trunks. Winklemann silver stain. X 200. Fig. 2. The tympanic membrane showing several large nerve bundles. Winklemann silver stain. X 178. Fig 3 (a) The skin of the bony canal with a cross section of a large nerve bundle deep in the dermis. A nerve can be seen branching off from the bundle. Winklemann silver stain. X 142. (b) A higher magnification of (a). Winklemann silver stam. X 480. Fig. 4. The course of a nerve from the deeper dermis toward the dermal-epidermal junction. Winklemann silver stain. X 112. ferity” . ' '-• . ' Fig. 5. Large nerve bundle running through the upper dermis of the bony canal just beneath the basal layer of the epidermis. Winklemann silver stain. X 210. Fig. 6. Large nerve bundle running through the tympanic membrane just beneath and parallel to the epidermis. Winklemann silver stain. X 190. Ym 7.......High mlgnification of the epidermal-dermal junction in the skin of the bony canal. Just beneath the basal layer of the epidermis (arrow) small non-myelinated fibers can be seen. Winklemann silver stain. X 675. Fig. 8. Non-myelinated nerve fibers beneath the epidermis of the tympanic membrane. Exposures at three levels were made on the same film in order to demonstrate as much of the fiber as possible. Winklemann silver stain. X 235. Fig. 9. Myelinated nerve fibers (arrow) in the superficial dermis. Note their close spatial relationship to the basal layer of the epidermis. Winklemann silver stain. X 570. 108 NEUROHISTOLOGY OF PROXIMAL PORTION OF HUMAN EAR CANAL 109 OBSERVATIONS The most striking observation on low power examination of tissue removed from the bony canal and tympanic membrane is the abundance of large nerve trunks (Figs. 1 and 2). These trunks originate, in the case of the bony canal, deep in the dermis (Figs. 3a and 3b) and course upward (Fig. 4) where they may be seen running just beneath the dermal-epidermal junction (Fig. 5). large trunks enter at the periphery of the tympanic membrane and pass just beneath and parallel to the epithelial covering (Fig. 6). This proximity of large nerve trunks to the epidermal-dermal junction is not seen in control sections from the arm, face and back. Non-myelinated fibers could be demonstrated in both the bony canal (Fig. 7), and the tympanic membrane (Fig. 8), but they were few in number. In several instances, myelinated fibers were seen in apposition to the basal cells of the epidermis (Fig. 9). No intraepidermal nerve fibers were seen. DISCUSSION While previous workers (1, 2, 3, 4) have outlined the gross innervation of these areas by the auriculo-temporal branch of the trigemenus and the auricular branch of the vagus, the presence of large nerve bundles and fibers in such superficial position has not been emphasized. Because of their serpentine course, it is difficult or impossible to follow nerve fibers in the ordinary thin (7-10 micra) sections. The older nerve impregnation technics prohibited the use of thicker sections because the contrast between the nerve tissue and the background was poor. The Winklemann technic (5, 6) increases the contrast between the silver impregnated nerve tissue and the connective tissue background permitting the use of thick sections and thus the visualization of large segments of nerve fibers and of nerve trunks. The pattern of nerve distribution in the skin lining the bony canal and in the typanic membrane is unique. In both areas one may see an abundance of large nerve trunks. Smaller trunks and single large myelinated nerve fibers are seen directly beneath the epidermis. These observations were not duplicated in control sections taken from the back, arm, and face. Non-myelinated fibers are present throughout the dermis but, again compared with the controls, in relatively small numbers. One may speculate that the large myelinated nerve fibers which are so prevalent in the proximal portion of the ear canal subserve touch sensation and thus contribute substantially to the protection of the tympanic membrane. SUMMARY In keeping with their important role in the protection of the tympanic membrane, the nerves of the skin of the bony portion of the human external auditory canal have a unique distribution pattern. The Winklemann technic, which utilizes thicker tissue sections, permits the visualization of many large nerve trunks in the skin of this area that are not found in control sections from other regions. It 110 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY is speculated that these large trunks subserve touch sensation and thus contribute substantially to the protection of the delicate ear drum.