The Human Recurrent Laryngeal Nerve During the Aging Process

Okajimas Folia Anat. Jpn., 76(6): 363-368, March, 2000

The Human Recurrent Laryngeal Nerve during the Aging Process

Takahisa NAKAI, Noboru GOTO, Hiroshi MORIYAMA, Naoki SHIRAISHI and Naoko NONAKA

Department of Oto-rhino-laryngology, Nihon University School of Medicine, 30-1, Oyaguchikamimachi, Itabashi-ku, Tokyo 173-8610, Japan * Department of Anatomy, Showa University School of Medicine, 5-8, Hatanodai 1, Shinagawa-ku, Tokyo 142-8555, Japan

-Received for Publication, September 28, 1999-

Key Words: Larynx, Morphometry, Phonation, Recurrent laryngeal nerve, Vocal cord

Summary: After a morphometric analysis of nerve fibers of the human recurrent laryngeal nerve, which is connected with phonation, the authors discovered several changes occurring in the axon during the aging process. These changes are mainly the disappearance of large axons and a slenderisation of all remaining axons. This is the first time that such observation has been made.

The larynx, an organ of phonation, contains vious reports1,2,4,6,9-1 1) regarding fixation, washing, many small intrinsic muscles: the cricothyroid, dehydration, embedding, sectioning, staining and posterior cricoarytenoid, lateral cricoarytenoid, morphometry (For more details, see the reference transverse arytenoid, oblique arytenoid, aryepi- section1,2,4,6,9-11)). For the measurement of axons, glotticus, thyroarytenoid, vocalis and thyro- the systemic sampling method was employed to epiglotticus. These muscles are innervated by two select a site at the center of the nerves. branches of the vagus nerve: the external branch of the superior laryngeal nerve innervates the cricothyroid, and the recurrent laryngeal nerve in- Results nervates the remaining intrinsic muscles mentioned above. It is generally agreed that various alter- Microscopic findings ations of motor speech may occur with aging. We High-power microscopic observation (Fig. 1) conducted research to find morphological changes shows the axons stained in dark blue or purple, and in the recurrent laryngeal nerve. This paper reports the surrounding myelin sheaths in light blue green. our findings concerning the changes in the human We can also, occasionally, notice degenerated recurrent laryngeal nerve that occur during the axons and axonal swelling. aging process. Number of myelinated axons The number of myelinated axons within the unit Methods area of 33 x 33 tirn2 was counted as being between 10 and 20 (average 13.75 + 2.65; Table). Small sections (about 10 mm in length) of the right-side human recurrent laryngeal nerve were Transverse area of myelinated axons removed from 16 cadavers (10 males and 6 females) The transverse area of myelinated axons ranged aged 60-93 (average age: 75.1 years). The causes of from 5.03 to 18.09 (average: 11.19 + 3.48) 111112 death indicated no direct or indirect connection (Table 1). A scatter diagram with regression line with the nervous system (central and peripheral). between the mean transverse area and age revealed We employed the same methods as in our pre- that the mean transverse area of myelinated axons

Correspondence: Noboru GOTO, M.D., Prof. and Chairman, Dept. of Anat., Showa Univ. Sch. of Med.

363 364 T. Nakai et al.

Table 1. Axonal numbers, areas and perimeters in the human right recurrent laryngeal nerve*

* Within the unit area of 33 x 33 pm2 . SD: standard deviation. The values are given in pm2 for the area and in pm for the perimeter.

in the recurrent laryngeal nerve decreased with age analyses of this nerve in relation to the aging pro- (Fig. 2). cess. This may be the first such report. Our results indicate that the main features of the aging process Perimeter of myelinated axons are a disappearance of large axons and a slender- The mean perimeter of myelinated axons was isation of the remaining axons. We hope this study 11.95 ± 1.60 gm (ranging from 8.54 to 13.89 gm; may prove to be useful in finding an explanation of Table 1). A scatter diagram with regression line the changes that affect phonation during the aging between the perimeter of axons and age revealed process. that the perimeter of myelinated axons in the re- We would like to add an explanation of the rea- current laryngeal nerve also decreased with age son why we chose to collect the specimens from the (Fig. 3). right side recurrent laryngeal nerve. It differs each other on two sides of the body concerning the ori- gin, course and composition of nerve fibers. This is Discussion because on the left side, this nerve may include more cardiac branches at the beginning part, which Phonation is a very important means of commu- may have had an adverse effect on our results. nication for human beings. It can be evaluated from The methods we employed for tissue preparation the view points of two aspects: quality and inten- are particularly suitable for morphometric research sity. It is known that phonation undergoes physio- on the nervous system, as they afford clear dis- logical change with aging. For example, when aged crimination of the various structures"), as well as persons speak, their voice may be weak (this is minimum shrinkage (less than 10% in length)4). As often the case), sometimes hoarse, just a bit we mentioned before, these methods were used in cracked, quivery, occasionally whispery or, rarely, our previous paper, which are listed in the refer- aphonic, even if they are not suffering from any ence section1,2,4,6,9-11). diseases which may affect phonation In addition to the nerve fiber analysis conducted Although there are several reports studying here research should be done on the fibers of the macroscopic anatomy81, vascular anatomy5), clinical intrinsic muscles of the larynx, such as the vocalis, anatomy') and neuromuscular junctions') regarding in order to enhance our understanding of phona- the human recurrent laryngeal nerve, as far as we tion in general. know there have been no reports on nerve fiber Recurrent Laryngeal Nerve and Aging 365

Acknowledgments 5) Moreau S, Goullet de Rugy M, Babin E, Salame E, Delmas P and Valdazo A. The recurrent laryngeal nerve: Related vascular anatomy. Laryngoscope 1998;108:1351-1353. The authors would like to thank Mr. M. Shibata, 6) Nagai Y, Goto N, Goto J, Kaneko Y and Suzaki H. Mor- Department of Anatomy, Showa University School phometric nerve fiber analysis and aging process of the of Medicine for his technical assistance. human vestibular nerve. Okajimas Folia Anat Jpn 1999; 76:95-100. 7) Premachandra DJ, Radcliffe GJ and Stearns MP. Intra- operative identification of the recurrent laryngeal nerve References and demonstration of its function. Laryngoscope 1990; 100:94-96. 1) Fujii M and Goto N. Nerve fibre analysis of the facial 8) Schweizer V and Dorfl J. The anatomy of the inferior nerve. Ann Otol Rhinol Laryngol 1989;98:732-735. laryngeal nerve. Clin Otolaryngol 1997; 22:362-369. 2) Fujii M, Goto N and Kikuchi K. Nerve fibre analysis and 9) Takeshita Y, Matsumoto K, Goto N and Shibata M. Nerve the ageing process of the vestibulocochlear nerve. Ann fibre analysis and ageing process of the human oculomotor Otol Rhinol Laryngol 1990;99:863-870. nerve. Showa Univ J Med Sci 1996;8:55-61. 3) Gambino DR, Malmgren LT and Gacek RR. Age-related 10) Tanaka J, Goto N, Nagai Y and Moriyama H. Myelinated changes in the neuromuscular junctions in the human pos- nerve fiber analysis of the human greater splanchnic nerve. terior cricoarytenoid muscles: A quantitative study. Lar- Okajimas Folia Anat Jpn 1999; 76:117-122. yngoscope 1990;100:262-268. 11) Yanagisawa K, Goto N, Kimura T and Tanaka J. Myeli- 4) Goto N. Discriminative staining methods for the nervous nated nerve fiber analysis and ageing process of the deep system: Luxol fast blue-periodic acid-Schiff-hematoxylin peroneal nerve in man. Showa Univ J Med 1994; 54:249- triple stain and subsidiary staining methods. Stain Technol 254 (in Japanese). 1987;62:305-315. 366 T. Nakai et al. Plate I

Explanation of Figures

Plate I

Fig. 1. High-power view of the human recurrent laryngeal nerve 77 year-old man, LPH stain, bar = 10 Am. Axons are stained in dark blue or purple, and surrounding myelin sheaths in light blue-green. Degenerated axons (*), axonal swelling(arrow) and a venule containing erythrocytes (white arrow) can be observed in the photo. Recurrent Laryngeal Nerve and Aging 367 Plate H

Plate II

Fig. 2. Mean axonal areas (11112)of the recurrent laryngeal nerve.

Fig. 3. Mean axonal perimeters (pm) of the recurrent laryngeal nerve.