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Symposium on the Role of the Vestibular Organs in Space Exploration (1970)

Chapter: THE FIRST-ORDER VESTIBULAR NEURON

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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Page 131
Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Page 132
Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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Suggested Citation:"THE FIRST-ORDER VESTIBULAR NEURON." National Research Council. 1970. Symposium on the Role of the Vestibular Organs in Space Exploration. Washington, DC: The National Academies Press. doi: 10.17226/18593.
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The First-Order Vestibular Neuron HANS ENGSTROM Uppsala, Sweden SUMMARY The vestibular part of the statoacoustic nerve contains both afferent and efferent fibers, the former being much more numerous than the latter. The afferent neurons have bipolar ganglion cells located in one single ganglion cell in the inner meatus. The ganglion cells belonging to the vestibular nerve are considerably larger than those of the spiral ganglion and they also differ slightly in structure. Several of the vestibular fibers are thicker than the cochlear fibers. The majority of the vestibular ganglion cells are surrounded by a multilayered myelin sheath. In this sheath some regions are found where the myelin is very regular, but in most areas the myelin is quite irregular with alternating regions of loose and semicompact myelin. The majority of the ganglion cells are mye- linated, but a small percentage (2 to 5 percent) have only a single or a double layer of Schwann-cell cytoplasm. These cells differ considerably from the myelinated ones, not only in structure for they are also much smaller. The way they are related to the sensory cells is not yet known, nor is their function known. Similar unmyelinated cells are found in about 10 percent of the spiral ganglion of the cochlea, and it has not yet been possible to certify their sensory-cell relation in the cochlea either. The efferent fibers are rather thin compared with the afferent ones. They have their ganglion cells in the brainstem. Their peripheral endings form a rich plexus in the vestibular epithelia where they form many en passant synapses with sensory cells, nerve calyces, and nerve fibers. The vestibular nerve contains a large number of unmyelinated fibers found intermingled with the afferent and efferent nerve fibers. During the last 10 to 15 years the fundamental principles of the structural organization of vestib- ular sensory regions have been described. In studies by Wersall (ref. 1); by Wersall and Flock (ref. 2); by Ades and Engstrc'im (ref. 3); by Eng- strom, Lindeman, and Ades (ref. 4); by Smith (ref. 5); by Spoendlin (ref. 6), and others, it has been shown that all vestibular sensory epithelia have two kinds of sensory cells, called type I and type II. Their structural features have been repeat- edly described in earlier symposia of this series (figs. 1 to 4). In studies by Engstriim, Ades, and Hawkins (ref. 7), by Flock (ref. 8), and recently by Linde- 1 This research has been sponsored in part by the Office of Naval Research, Washington D.C., through the European Office of Aerospace Research, OAR. U.S. Air Force under Contract F61052-68-C-0064. man (refs. 9 and 10), it has been shown that the sensory cells are organized in a very character- istic manner on each macula, and on each crista ampullaris. Because of this characteristic ori- entation of the sensory cells, we talk about a morphological polarization of the cells. Several authors have shown that the morphological polari- zation of the sensory cells is paralleled by a func- tional polarization. This means that two nearby regions in one macula utriculi, for instance, may give different responses to the same stimulus. It is rather generally agreed upon that the otoliths or the statoconium membranes over the •maculae (figs. 5 to 7) and the cupulae over the cristae act upon the sensory cells by shearing movements with respect to the surface of the sensory epithelium. It is also generally accepted that the semicircular canals act as integrating 123

124 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION FIGURE 1.—Schematic drawing showing principles in •*• innervation. The sensory cells are innervated by dendrites from bipolar ganglion cells (yellow). These form the afferent fibers, leading from the sensory cells to the second- ary vestibular ganglion cells. The efferent fibers are red. FIGURE 2. —Schematic drawing of the sensory cells over ^ a crista ampullaris. Sensory cells of type I and type II are distributed over the surface of the crista. FIGURE 4. — Schematic drawing of one sensory cell type I w (left) and type II (right). They are both innervated by afferent fibers in direct contact with the sensory cells. Around the type I cell the afferent ending is shaped like a calyx. Efferent (red) fibers end at the surface of the type II cell, but at the surface of the calyx around the type I cell. FIGURE 3.— Schematic drawing of sensory cells and stato-"* conia from a macula.

FIRST-ORDER VESTIBULAR NEURON 125 FIGURE 5.—Statoconiafrom macula sacculi of a guinea pig. 4 The hexagonal prisms are very distinct. They consist of calcite. This is a micrograph taken with a scanning electron microscope. This technique permits a study of unsectioned material. (Cf. also figs. 6 and 7.) FIGURE 6. — Bundles of sensory hairs at the surface of macula utriculi of a guinea pig as seen in the scanning electron microscope. The different length of the hairs in each bundle on one cell can be very beautifully seen. In this case we have used a drying technique. If freeze-drying technique is used, the hairs will stand straight up. In this case the varying length can be better visualized through the drying technique. FIGURE 1.— Detail from the previous figure showing the sen- ^ sory hnirs on one cell. The scanning electron microscope gives an excellent method for the study of the vestibular sensory regions, and we have also used it extensively now for a study of the cochlea. The technique permits a study of a whole macula and the magnification can be varied in a range between 20 X to 20 000 X.

126 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION accelerometers while the maculae are regarded, at least mainly, as linear integrating accelerom- eters. Information from these accelerometers is propagated through the vestibular nerve fibers from the peripheral sense organ over the vestib- ular ganglion cells to vestibular nuclei where synaptic contacts are established with the second- order neurons. The stimulus response can be recorded as action potentials in the vestibular nerve. Even at rest these nerve fibers have a low-frequency activity with a rather steady firing rate. The stimulus acting upon the vestibular sensory regions thus in reality modifies the discharge, and the frequency change follows distinctive patterns as described by many authors and reported to this group a few years ago by Lowenstein (ref. 11). The peripheral sensory cells and the contact between the peripheral terminations of the vestibular nerve fibers have been carefully studied with the aid of electron microscopy (figs. 8 to 11). The basic information regarding the vestibular nerve as a whole and its branches, however, has been long known. The early literature regarding the nerve can be found summarized in the beautiful publication by Retzius (ref. 12), but the most-referred-to publi- cation concerning the vestibular nerve is Lorente de No's paper of 1926 (ref. 13) in which he describes the distribution of both cochlear and vestibular nerve fibers. His findings have been discussed in a recent paper by Ballantyne and Engstriim (ref. 14). It is quite evident that the rapid development of techniques for studying fine structure and the simultaneous development of microsurgery for both experimental and clinical purposes offer the opportunity to study the vestibular nerve further. Very little has been published in this field, and the present publication will describe some of the features of the first-order neurons of the vestibular nerve. The acoustic nerve in man contains about 35000 to 50000 nerve fibers, while only 14000 to 24 000 nerve fibers form the vestibular nerve. This nerve has two main subdivisions: the upper and lower vestibular branches. The superior vestibular nerve innervates the cristae of the *- "•« 5 :&:' ••V ' ^.£y •«. v*-V • .V * > O .-K"' '« FIGURE 8. — Schematic drawing of different forms of contact between sensory cells and nerve enilings. Some cells have a direct contact with the afferent endings (NE 1) and efferent endings (NE 2). while at type I cells the efferent endings contact the nerve calyx (NC) only. N: nerve fibers. KC: kinocelium. FIGURE 9. — Synaptic region (Sy) between an afferent nerve ending (NE 1) and a hair cell, type II. At the synaptic junction many small imaginations look like prestages to synaptic bars.

FIRST-ORDER VESTIBULAR NEURON 127 5 FIGURE 10. — Two afferent nerve endings (NE 1) at the base of a type II cell from macula utriculi of a squirrel monkey. Observe the large number of mitochondria in the nerve endings. r •»^r-:-> i f »«* ^ '•"*,*'. ''v. * if'}. " « ' . „••• '>-^ . FIGURE 11. — Nerve calyx (NC) situated between a hair cell type I to the left and type II to the right. There are many synaptic regions (Sy) between the nerve calyx and the type II cell. Squirrel monkey, macula utriculi. superior vertical and lateral ampullae, the utricu- lar macula, and a small part of the macula sacculi. The inferior branch innervates the posterior vertical ampulla and the major portion of the macula sacculi. The diameters of the nerve fibers vary con- siderably in the statoacoustic nerve. In general, many of the vestibular fibers are slightly thicker than the acoustic fibers. The cochlear fibers have a diameter varying between I/A and 9/A (refs. 15 and 16). The vestibular fibers have a diameter between I/A and 13/A, but only around 10 percent exceed 6/A in diameter. The vestibu- lar nerve also contains a large number of non- myelinated fibers (refs. 1 and 17, figs. 12 and 13). These are intermingled with the myelinated fibers. There are different densities of these fibers in different parts of the nerve. The major portion of the fibers in the vestibular nerve is of afferent nature, but it is well known that a small number of nerve fibers (according to Gacek, 400 fibers in the cat, ref. 18) carry efferent impulses. Gacek in 1961 (ref. 19) was able to demonstrate by experimental studies that there is a system of myelinated efferents with their origin in the lateral vestibular nucleus, and Carpenter (ref. 20) stated, "The medial, superior and parts of the descending vestibular nuclei on both sides project efferent fibers \ FIGURE 12. —Nerve fiber of afferent nature (Nl leaving the sen- sory epithelium of a macula utriculi in a squirrel monkey. Observe how the supporting cell forms a sheath around the unmyelinuted nerve. This sheath ends when Schwann cells begin.

128 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION . •• • ' - • . 4 . ' FIGURE 13. — Afferent nerve fiber at a macula utriculi below the epithelium. A few layers of irregular myelin can be seen. Squirrel monkey. JP FIGURE 14. — Efferent, granulated ending (NE 2) forming a synapse to the sensory cell. .• HC - FIGURE 15.— Efferent, granulated nerve ending (NE 2) at a type 11 cell. The small white arrows indicate a subsynaptic cistern. The black arrow points to a synaptic bar close to FIGURE 16. - Hair cell type I completely surrounded by a nerve an afferent ending. Macula utriculi, squirrel monkey. calyx (NC). Squirrel monkeY, macula utriculi.

FIRST-ORDER VESTIBULAR NEURON 129 FIGURE 17. — Very long synaptic bars in type I cells from a macula sacculi of a squirrel monkey. These extended synaptic structures are sticking far into the sensory cell from the synaptic region. Synaptic vesicles in large numbers can be seen. to the labyrinth via the vestibular nerve." Studies by us and by Lindeman (refs. 9 and 10) indicate that these fibers form a very widespread network in the lower half of the vestibular sensory epithelia (figs. 14 to 16). At that level the nerve fibers make contact with several sensory cells. The nerve endings are of a typical presynaptic type. They contain large amounts of synaptic vesicles (figs. 17 and 18). Further reference to this system can be found in Lindeman's papers (refs. 9 and 10). The majority of the nerve fibers in the vestib- ular nerve, those forming the first-order neuron, are of afferent nature. It has just been stated that they vary considerably in size and it has been known for a long time that their ganglion cells also vary in size. This was carefully studied by Lorente de No (ref. 13) who separated groups of ganglion cells according to size and in relation to specific regions in the sensory epithelium. He wrote of magnocellular and parvicellular regions. The various sensory regions of the vestibular labyrinth, according to FIGURE 18.—A, B, C, and D are found between sensory cells and afferent nerve endings. In A, B, and C the synaptic vesicles are found inside the sensory cell. Type E is an efferent ending with large numbers of synaptic vesicles and a subsynaptic cisterna. Lorente de No, were projected onto the different parts of the vestibular ganglion. A discussion of this problem can be found in the paper by Ballantyne and Engstrom (ref. 14). During recent years the tonotopical organization has been doubted by several authors, but as stated by Ballantyne and Engstriim, the fibers have a complicated course which is extremely difficult to follow. In several recent publications (refs. 8 and 9), it has been shown that the vestibular sensory regions have a structural and functional sub- division which was not known earlier. The presence of these different regions and their different functional importance make it necessary to restudy the topographical arrangement of vestibular nerve fibers. In this relation, recent studies by Lindeman (ref. 10) are of great interest. He was able to show that the striola regions of

130 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION maculae and the central area of the cristae are especially vulnerable to ototoxic antibiotics. These areas as far as we know are innervated by the thickest fibers. We hope it will be pos- sible to follow the chain of neural degeneration after a localized degeneration to the striolar region. Likewise, studies by Winther (ref. 21) are of interest. We knew before that the central area of the cristae is most vulnerable to ototoxic antibiotics. Winther has shown that X-rays, on the other hand, preferentially damage the . I —^-' *J FIGURE 19. — Afferent nerve fiber (NI at the region where myelin (MY) begins to appear. Observe that one Schwann cell forms very irregular myelin. peripheral regions of the cristae. In this way we can get two sets of inverse degenerations. Another approach to this problem employs the laser technique. Stable and collaborators (ref. 22) have now developed techniques by which it is possible to pinpoint damage to very minute regions in the labyrinth. It has been stated that the afferent fibers of the vestibular nerve are myelinated and their bipolar ganglion cells are also surrounded by a myelin sheath. That cochlear and vestibular ganglion cells have a myelin coating has been known for a long time. In a recent publication, Kellerhals et al. (ref. 23) have surveyed this literature, and we refer the reader to that monograph. In this book and earlier studies by Rosenbluth (ref. 24), it has been shown that the cochlear ganglion contains two types of ganglion cells. Ballantyne and Engstriim (ref. 14) have recently shown that the vestibular ganglion cells have different sizes and that these ganglion cells are rather mixed. FIGURE 20. — Detail from figure 19 showing the myelin forma- tion and a Schwann cell (SO with irregular myelin. FIGURE 21.—Two cross-sectioned nerve fibers immediatelv below the macula utriculi of a squirrel monkey.

FIRST-ORDER VESTIBULAR NEURON 131 It has been very difficult to verify any such sub- division as described by Lorente de No (ref. 13), but it is possible that it still exists. This matter is now being studied in embryological material. It is, however, quite clear that there exist two distinctly separate types of ganglion cells in the vestibular ganglion as has been found earlier in the cochlear ganglion. Of these, one variety has a thick myelin sheath, the other has no such multilayered myelin, only one thin Schwann-cell layer around the cell (figs. 19 to 25). We call the first type myelinated ganglion cell (figs. 26 to 28), the other unmyelinated. In the cochlea of the guinea pig, the latter type comprises about 10 percent. The number of unmyelinated cells is lower in the vestibular ganglion, approximately 2 to 5 percent as far as we have been able to find. has appeared. FIGURE 23.-Regular myelin FIGURE 24. — Myelinated nerve (My) from guinea pig close to FIGURE 22. — Region below macula utriculi with myelin the vestibular ganglion. The Schwann cell surrounds and formation. Schwann-cell cytoplasm envelops the axon. forms the myelin.

132 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION ••,.;.. 9 I. -;' Ku'UKE 21,—Ganfr1ion cells from the vestibular panglion of (I Cttt. FIGURE 25. — Very thick myelin sheath around a vestibular nerve fiber. Macula utriculi nerve, squirrel monkey. I FlcURE 26. — V cstibular jgnngtion and nen'e fibers from a cat. FIGURE 2S. — (ianplion cell of rather fibrous type. The nucleus (Nu), mitochondria (Ml, and endoplasmic reticulum (Krl are seen. Guinea pip.

FIRST-ORDER VESTIBULAR NEURON 133 The myelin sheath has a very irregular appear- ance in certain areas; in others, the myelin is of a more normal, regular type. This arrangement has been discussed by Kellerhals et al. (ref. 23) for the cochlear ganglion, and the vestibular ganglion myelinization has been described by Ballantyne and Engstnim (ref. 14). Both the peripheral dendrite and the central neurite are provided with a myelin sheath. This sheath, however, is quite regular and of the normal ap- pearance seen in other nerves. It has just been said that the ganglion cell myelin sheath is very irregular, and it is of great clinical interest to remember that this region of irregular myelin seems to be the origin of acoustic neuromas. Such neuromas are generally found in a region corresponding to the vestibular ganglion and they often develop inside the vestibular nerve. A few neuromas are also found in the acoustic nerve and then generally close to the spiral ganglion or at the region of demyelinization in the osseous spiral lamina. Centrally to the ganglion cells the afferent fibers continue their course to the second- order neurons in the vestibular nuclei. We have made several attempts at a reconstruction of the course of the fibers, but we have found it extremely difficult to follow the fibers because of their tortuous course and the intermingling of fibers of different diameters. In previous discussions concerning the dis- tribution of the nerve fibers in the vestibular sensory regions, studies by several authors have shown that special parts of the epithelium are innervated by the thickest fibers. In general, these regions are the striolae on the maculae and the centers of the cristae. Recent studies by Lindeman have taken up this problem through numerical analysis and have shown that the density of sensory cells is higher at the slopes of the cristae than at the top and also that the density of type I sensory cells is higher per surface unit along the slopes. The distribution of nerve fibers seems to be such that the thick fibers mainly run to type I cells at the central FIGURE 29. — Crista ampullaris with nerve fibers. The nerve calvces are also verv distinct. sensory regions. The medium-sized fibers, on the other hand, go mainly to peripheral regions where they may innervate several type I cells. They sometimes have a rather long intraepithelial course. Lindeman (ref. 9) has found 10 type I cells get ramifications from one single nerve fiber. Sometimes the same fiber may innervate both type I and type II cells. The innervation of type II cells seems to be mainly by medium- sized or thin fibers. Of great interest from a functional point of view is, of course, the high density of sensory cells and nerve fibers at the slopes of the cristae (fig. 29) and its functional implications. It may be that physiological interest has been too much focused at the top of the cristae instead of at the peripheral regions.

134 THE ROLE OF THE VESTIBULAR ORGANS IN SPACE EXPLORATION REFERENCES 1. WERSAt.L, J.: The Minute Structure of the Crista Ampullaris in the Guinea Pig as Revealed by the Electron Microscope. Acta Oto-Laryngol.. vol. 44, 1954, pp. 359-369. 2. WERSALL. J.; AND FLOCK, A.: Functional Anatomy of the Vestibular and Lateral Line Organs. Contributions to Sensory Physiology, W. D. Neff, ed.. Academic Press, 1965, pp. 39-61. 3. ADES, H. W.; AND ENGSTROM, H.: Form and Innerva- tion of Vestibular Sensory Epithelia. The Role of the Vestibular Organs in the Exploration of Space, NASA SP-77, 1965, pp. 23-41. 4. ENGSTROM, H.; LINDEMAN, H. H.; AND ADES, H. W.: Anatomical Features of the Auricular Sensory Organs. Second Symposium on the Role of the Vestibular Organs in Space Exploration, NASA SP-115, 1966, pp. 33-44. 5. SMITH, C. A.: Microscopic Structure of the Utricle. Ann. Otol., vol. 65,1956, pp. 450-469. 6. SPOENDLIN, H. H.: Organization of the Sensory Hairs in the Gravity Receptors in Utricle and Saccule of the Squirrel Monkey. Z. Zellforsch., vol. 62, 1964, pp. 701-716. 7. ENGSTROM, H.; ADES, H. W.; AND HAWKINS, J. E., JR.: Structure and Function of the Sensory Hairs of the Inner Ear. J. Acoust. Soc. Am., suppl. 34, 1962, pp. 1356-1362. 8. FLOCK, A.: Electron Microscopic and Electrophysiological Studies on the Lateral Line Canal Organ. Acta Oto- Laryngol., suppl. 199,1965. pp. 1-90. 9. LINDEMAN, H.: Studies on the Morphology of the Sensory Regions of the Vestibular Apparatus. Ergebn. Anat. Entwgesch., 1969. 10. LlNDEMAN, H. H.: Regional Differences in Structure of the Vestibular Sensory Regions. J. Laryngol., vol. 83, 1969, pp. 1-17. 11. LOWENSTEIN, OTTO: The Functional Significance of the Ultrastructure of the Vestibular End Organs. Second Symposium on the Role of the Vestibular Organs in Space Exploration, NASA SP-115, 1966, pp. 73-87. 12. RETZIUS, G.: Das Gehiirorgan der Wirbelthiere. Das Gehororgan der Reptilien, der Vdgel, und der Sauge- thiere. Samson & Wallin (Stockholm), 1884. 13. LORENTE DE N6, R.: Etudes sur 1'Anatomie et la Physio- logic du Labyrinthe de 1'Oreille et du VIII' Nerf. Trav. Lab. Rech. Biol. (Madr.), vol. 24, 1926, pp. 53-153. 14. BALLANTYNE, J.; AND ENGSTROM, H.: Morphology of the Vestibular Ganglion Cells. J. Laryngol., vol. 83. 1969, pp. 19-42. 15. ENGSTROM, H.; AND REXED, B.: Cber die Kaliberverhalt- nisse der Nervenfasern im N. stato-acusticus des Menschen. Z. Mikr.-anat. Forsch., vol. 47, 1940, pp. 448-455. 16. GACEK, R. R.; AND RASMUSSEN, G. L.: Fiber Analysis of the Statoacoustic Nerve in Guinea Pig. Cat. and Monkey. Anat. Record, vol. 139, 1961, pp. 455-463. 17. PALUMBI, G.: Innervation of Human Inner Ear in Light of New Histomorphological Studies. Sc. Med. Ital., vol. 3, 1954, pp. 351-367. 18. GACEK, R. R.: Efferent Component of the Vestibular Nerve. Neural Mechanisms of the Auditory and Vestibular Systems, G. L. Rasmussen and W. F. Windle, eds., Charles C Thomas, 1960, pp. 276-284. 19. GACEK, R. R.: The Efferent Cochlear Bundle in Man. Arch. Otolaryngol., vol. 74, 1961, pp. 690-694. 20. CARPENTER, M. B.: Experimental Anatomical-Physio- logical Studies of the Vestibular Nerve and Cerebellar Connections. Neural Mechanisms of the Auditory and Vestibular Systems, G. L. Rasmussen and W. F. Windle, eds., Charles C Thomas, 1960, pp. 297-323. 21. WlNTHER, F.: Acute Degenerative Changes in the Inner Ear Sensory Cells of the Guinea Pig Following Local X-Ray Irradiation. Acta Oto-Laryngol., vol. 67. 1969, pp. 262-275. 22. STABLE, J.; AND HOBERG, L.: Laser and the Labyrinth. Some Preliminary Experiments on Pigeons. Acta Oto-Laryngol., vol. 60,1965, pp. 367-374. 23. KELLERHALS, B.; ENGSTROM, H.; AND ADES, H. W.: Die Morphologie des Ganglion Spirale Cochleae. Acta Oto-Laryngol., suppl. 226,1968, pp. 1-78. 24. RoSENBLUTH, J.: The Fine Structure of Acoustic Ganglia in the Rat. J. Cell Biol., vol. 12, 1962, pp. 329-359.

FIRST-ORDER VESTIBULAR NEURON 135 DISCUSSION Lowenstein: Do you have any information about the pos- terior ampulla nerve? Where does this join the pars superior complex of the vestibular ganglions? It is due to develop- mental reasons that that nerve passes initially through the pars inferior nerve. It has always puzzled me how one can find the reunion of the three ampullary nerves in the vestibular ganglion. KiigNtrum: We have dissected the vestibular nerve es- pecially in the cat, but also in man and some other animals. We have also many serial-sectioned labyrinths. It is normal that the nerve fibers are twisted. The posterior ampullary nerve joins the saccular nerve to form the lower vestibular nerve. After a slight further twist this nerve meets the superior vestibular nerve. We have tried to dissect the vestibular branches free together with their ganglion cells, but due to the complicated course of the different components, it is extremely difficult to follow the nerve fibers during their whole length. Steinberg: In one of your slides you seemed to show a progressive change in the structure of the ribbon synapse of the type II primary sensory cell. As you know, this type of synapse is also prominent in the vertebrate retina, at both the receptor and bipolar cells. Do you mean that this struc- ture actually opens into the extracellular space? Engstrom: I am not quite clear whether you mean the invagination shown in my schematic drawings. In the cochlea we have good evidence that invaginations form an intermediate stage in the formation of synaptic bars, but I admit that the evidence is not conclusive. Large numbers of similar invaginations of nerve-ending cytoplasm into in- foldings of the sensory cells have been observed in vestibular and cochlear sensory regions. Some such formations can be found among our illustrations. Precht: My question concerns the type I sensory cell. From your illustrations it appears that one fiber, one large fiber, supplies one sensory cell. Is that a simplification or is this a true one-to-one relation between this particular type I sensory cell and the primary fibers? Engstrom: This is a simplification. In the text it states that, at the top of the crista, there is a low factor relation, approximately one to three or one to five. But down along the sides, Lindeman has counted up to 10 cells innervated by one nerve fiber; therefore, one medium-sized nerve fiber at the slope of the crista is innervating 10 sensory cells of type I. I might mention also, as I have earlier, that a type I nerve fiber may innervate a type II cell. It might even be that a sensory cell of type I has a nerve calyx around itself, and that nerve calyx has a synaptic bar onto the type I cell and a synaptic bar to an adjacent cell of type II on the side. Thus, there must be an intermediate stage sometimes between these cells. And 1 guess there is a development from the type II cells. It has been stated, especially by Wersall, that, in reality, it was a mistake when he named these type I and the other type II. It should have been the opposite way. Lowy: You mentioned that the spike generation occurred at the beginning of the myelinated part of the vestibular nerve. Is that based on direct experimental evidence or an analogy with other systems like Pacinian corpuscles? Engstrbm: I was referring to other reports, not to my own experiments. Newsom: Would you expand a little on the comment you made about the radiosensitivity of the particular portion of that cell? Engstrom: These are experiments made by Winther who has published a paper (ref. 21 of text) on this problem. He studied irradiation of the inner ear and discovered that by irradiating the head of a guinea pig and giving a dosage to its inner ear. He then followed the degeneration taking place and found just the opposite to what we expected. The damage occurred mainly at the slopes of the crista; all other damage we have been able to cause has been at the top. Thus, ototoxic antibiotics are always acting upon the top of the crista, but irradiation is clearly mainly acting upon the sides. Thus, there is a kind of inverse situation in this case. At the same time he has shown that, by giving a high dosage of irradiation, he obtained a very clear damage to the base of the cochlea. So the region of the slopes of the cristae gets severe damage by irradiation, and it is much more than Winther expected at the beginning of his studies. We have been very amazed at both the extent of damage and the form the damage takes, but it is quite clear and indisputable absolutely that, with the high dosage, he obtained an inverse form of degeneration.

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