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The efferent cochlear bundle in the caiman and pigeon
EXPERIMENTAL
The
NEUROLOGY
Efferent
3, 225-239 (1961)
Cochlear
Bundle
in the
Caiman
and
Pigeon ROBERT L. BOORD~ Laboratory of Neuroanatomical Sciences, National Institute of Neurological Diseases and Blindness, National Institutes of Health, U. S. Public Health Service, Department of Health, Education, and Welfare, Bethesda, Maryland Received
October
26, 1960
An efferent component to the reptilian and avian cochlear nerve is demonstrated by experimental anatomical methods and its central and peripheral course described. The efferent cochlear bundle decussates in the medulla oblongata dorsal to the medial longitudinal fasciculi at a level corresponding to the facial genua. This decussation was cut, in the caiman and pigeon, with a small knife fitted to a stereotaxic instrument. Following appropriate postoperative survival periods, serial sections of the brain stems and statoacustic nerves were stained for axonal and myelin degeneration by the Nauta-Gygax, Protargol silver, and Sudan black B methods. Cochlear efferent fibers of either side cross the raphe, pass laterad to traverse the motor root of the facial nerve and become dispersed among vestibular root fibers. Peripherally they traverse the superior and inferior vestibular ganglia and join the cochlear nerve via the vestibulocochlear anastomosis. The parent bundle extends throughout the length of the cochlear ganglion and distributes fibers which are traceable as far as the organ of Corti. Some efferent fibers continue beyond the cochlear ganglion to terminate in the macula lagenae. The cells of origin and the ultimate termination of these efferent fibers remain to be determined. The similarity of the central and peripheral topographic relationships of the efferent cochlear bundle in reptiles, birds, and mammals suggest that this bundle is homologous among those vertebrates which possess a cochlea. This structural constancy adds support to the contention that the efferent innervation of the cochlea may play an important role in audition and implies that the efferent cochlear bundle is a basic part of the neural apparatus involved in the hearing process. 1 I am greatly indebted to Dr. G. L. Rasmussen for his continued help and supervision throughout the course of this study and for the privilege of carrying out the investigation in his laboratory. This investigation was carried out during the tenure of a Predoctoral Fellowship (No. BF-9003) from the National Institute of Neurological Diseases and Blindness, United States Public Health Service. 225
226
BOORD Introduction
The purpose of this study is to determine whether a bundle comparable to the olivocochlear bundle of Rasmussen,previously described in the cat and other mammals (9, lo), is present in birds and reptiles. In these submammalian forms the cochlea is uncoiled and lacks such specialized mammalian features as internal and external hair cells, tunnels, and complex spiral fiber arrangements. Previous studies in mammals concerning the question of the ultimate termination of efferent cochlear fibers have been seriously handicapped by the intricate complexity of innervation of the organ of Corti. Much of this difficulty could be obviated by the study of a more simply constructed hearing organ, such as found in birds and reptiles, provided that an equivalent efferent cochlear component is present in these forms. Therefore, the anatomical demonstration of an efferent cochlear bundle in the pigeon and caiman (South American alligator) would be of fundamental importance in extending previous mammalian research and providing phylogenetic evidence on which to base future morphological and functional considerations. Materials
and
Methods
Eighteen adult domestic pigeons ranging in weight from 2.50to 380 gm and thirteen young South American spectacledcaimans (Caiman sklerops) ranging in total length from 51 to 82 cm and in weight from 400 to 1920 gm survived the operative experiments. Of this number favorable lesions were placed in eleven pigeons and four caimans. For normal study, serial sections of the brain stems and inner ears of both specieswere prepared by a Protargol silver method. Dissections of the eighth nerve complex of Sudan black B in toto stained specimensproved helpful in the study of both normal and experimental histological preparations. Pigeons were anesthetized by the slow administration of Veterinary Nembutal (Abbott) in the brachial vein. Approximately 0.3 ml (18 mg) diluted to 0.6 ml with normal saline per kilogram of body weight usually induced surgical anesthesia. For the caiman, Nembutal was found less effective than hypothermia. Caimans were first subjected to a temperature of 5” C in a refrigerator after which surgical anesthesia was maintained by covering them with a plastic bag filled with cracked ice. The lesion site objective was the decussatingfibers in the dorsal raphe at the level of the facial genua which was anticipated as being homologous to the olivocochlear decussation in mammals. In both the caiman and
EFFERENT
COCHLEAR BUNDLE
227
pigeon these fibers were transected with a small knife fitted to a stereotaxic instrument having a head holder modified to accommodateindividually the pigeon and the caiman. The angle of dorsal approach and the distance from the dura to the descussationwere previously determined on formalin-fixed heads while oriented in the stereotaxic apparatus. Following postoperative survival periods ranging from 2 days and 3 hours to 4 days for the pigeon, and from 2 to 5 weeks for the caiman, these animals were anesthetized and perfused after the method of Koenig, Groat, and Windle (6). The dorsal aspect of the brain was then exposed by removing the calvarium and the headsimmersedin 10 per cent neutral formalin. Twelve to 24 hours later, the brain and bony auditory capsules, containing the inner ears, were removed and further fixed 3 to 5 days in 10 per cent neutral formalin. Brain stems were cut at 24-40 p on the freezing microtome and serial sections stained by the method of Nauta and Gygax (8) as modified by Nauta (7). The peripheral portion of the eighth nerve was handled by three different techniques in order to exhibit the presence of fragmented axons or degenerating myelin. Interrupting decussatingfibers resulted in bilateral degeneration of the efferent fibers in the acoustic nerve; therefore, it was possible to study both ears by utilizing a different method on each. Axonal fragmentation was demonstrated by Protargol silver methods in paraffin serial sections of the inner ears following decalcification, and myelin disintegration exhibited by the Sudan black B method of Rasmussen (11). Favorable results were also obtained by the application of the Nauta-Gygax technique to the peripheral nervous system. This method proved of value in substantiating Protargol and Sudan black degeneration as well as providing a greater degree of differentiation between normal and degenerating axons. Results
A semidiagrammaticdrawing of the medullary and peripheral course of the efferent cochlear bundle in the caiman and pigeon is depicted in Fig. 1. A more detailed description, supported with photomicrographs of the cochlear efferents, is given below. CROSSEDLIMBOF
EFFERENT
COCHLEAR BUNDLE IN MEDULLA
OBLONGATA
Caiman. The level of the decussation is at the rostra1 border of the abducens nuclei from which axons pass over the medial longitudinal
228
BOORD
fasciculi as a compact bundle and proceed beneath the facial genua (Fig. 2A). Efferent cochlear fibers accompany the facial motor root of either side for some distance before traversing it to enter the incoming vestibular root (Fig. 2B) where the bundle becomesdispersed into one larger and two or three smaller fascicles. These fasciclescourse peripherad
FIG. 1. Diagrammatic presentation of the central and peripheral course and relationships of the efferent cochlear bundle in the caiman (left) and pigeon (right) based on observations of experimental material and reconstructed. Efferent cochlear fibers are depicted as thicker beaded dashes. Avc, vestihulocochlear anastomosis ; Flm, fasciculus longitudinalis medialis; G, genu n. facialis; Gvi, ganglion vestibulare (pars inferior) ; Gvs, ganglion vestibulare (pars superior) ; Ml, macula lagenae; Nu I, nucleus laminaris; Nu OS, nucleus olivaris superior; Nu VI, nucleus abducentis; R n VII, motor root of seventh nerve; Rv, root of vestibular nerve; Tr s \‘, tractus spinahs n. trigemini.
among vestibular fibers and emerge from the medulla with the vestibular root (Fig. 3A). Pigeon. As in the caiman, the bundle decussatesimmediately rostra1 to A, transverse section through the medulla at the level of the FIG. 2. Caiman. facial genua showing efferent cochlear fibers, as a degenerated bundle (arrow), coursing from the mid-line and proceeding laterad beneath the facial genu (G). The raphe lies immediately to the left of the field shown in this photomicrograph. Nauta stain, 150 X. B, transverse section through the medulla showing degenerated efferent cochlear fibers (arrows) traversing the motor root of the facial nerve (R n VII) and coursing dorsolaterad to enter the incoming vestibular root (Rv). The dark-staining structures at the lower right represent reticular fibers of blood vessels. Nauta stain, 150 X.
EFFERENT
COCHLEAR
BUNDLE
229
230
BOORD
the abducens nuclei but pursues a more ventral course from the raphe in order to intersect the facial genua (Fig. 4A). The efferent cochlear fibers accompany the facial root for only a short distance before dispersing themselves among the widely scattered vestibular root fibers (Fig. 4B). As may be seen in Fig. SA, they leave the brain stem as small fascicles. PERIPHERAL
COURSE OF EFFERENT
COCHLEAR
BUNDLE
Caiman. After emerging from the medulla in the incoming vestibular root, efferent cochlear fibers traverse the superior vestibular ganglion and course caudoventrally to enter the inferior vestibular nerve. At a level where posterior ampullar and saccular fibers separate from the cochlear nerve, they collect into a special fascicle known as the vestibulocochlear anastomosis. This anastomosisextends from the saccular ganglion to the cochlear nerve and consists predominantly of efferent fibers. The main bundle, upon reaching the cochlear ganglion, bifurcates (Fig. 3B) and coursesalong the outer border of the ganglion throughout its entire length. En route, efferent fibers leave to accompany afferent radial fibers. Degenerated axons have been followed as far as the receptor epithelium but their ultimate termination could not be determined. The efferent fibers which continue beyond the apical end of the cochlea terminate in the macula lagenae, a sensory area commonly classified as a vestibular receptor organ. Pigeon. Efferent cochlear fibers, after leaving the brain stem, traverse the superior vestibular ganglion. As in the caiman, they proceed in a ventrocaudal direction to course successively through the inferior vestibular ganglion, the saccular nerve, and the vestibulocochlear anastomosis to’ join the cochlear nerve (Figs. SB, 6A, 6B). The anastomosis is clearly identified in histological sections cut parallel to its course between the saccular and cochlear nerves (Fig. 6). The presenceof degenerated efferent fibers entering the cochlear nerve via the vestibulocochlear anastomosisis shown in Fig. 6B. Efferent fibers FIG. 3. Caiman. A, transverse section through the medulla showing dark degenerated fascides of efferent cochlear fibers (arrows) coursing peripherad in the vestibular root (Rv). Other abbreviations: Nu 1, nucleus laminaris; R n VII, motor root of the seventh nerve; Tr s V, tractus spinalis n. trigemini. Nauta stain, 27 X. B, longitudinal section through the posterior ramus of the statoacoustic nerve showing the degenerated efferent cochlear bundle bifurcating (arrows) distal to the cochlear ganglion (Gc). Many normal impregnated axons appear in the upper portions of this field. Nauta stain, 150 X.
EFFERENT
COCHLEAE
BUNDLE
231
232
BOORD
course along the external face of the cochlear nerve before reaching the ganglion (Fig. 6C). The efferent bundle courses apicalwards within the cochlear ganglion and distributes whole (unbranched) fibers evenly throughout the length of the organ of Corti as evidenced by the presence of a few scattered degenerated fibers among a much greater number of normal afferent radial fibers (Fig. 7A). A few fragmented axons have been followed as far as the cochlear sensory epithelium in Protargol silver preparations (Fig. 7B), but not to their terminations. As is the case in the caiman, the efferent fibers which continue beyond the cochlear ganglion terminate in the macula lagenae. Discussion
The topographic relationships of the outgoing limb and the peripheral extension of the efferent cochlear bundle in the pigeon and caiman are basically similar to those found in mammals. For example, the point of decussation is in the dorsal raphe at a level corresponding to the facial genua and it is through the fibers of the latter that cochlear efferents pass before emerging from the medulla with the vestibular root. Also, peripherally the bundle enters the cochlear nerve via the vestibulocochlear anastomosis. Although the topographical features of the medulla of the pigeon and caiman vary considerably, the basic relationships of the efferent cochlear bundle persist. A common feature among reptilian, avian, and mammalian forms is the interdigitation of the intramedullary efferent bundle with fibers of the facial root. This relationship is clearly revealed in embryonic and fetal mammalian brains as appearing chronologically at the stage of development when the facial nucleus is situated near the floor of the fourth ventricle (13, 14). Later development and subsequent displacement of the facial nerve in different animals fail to alter this relationship because the efferent
FIG. 4. Pigeon. A, transverse section through the medulla at the rostra1 level of the facial genua showing efferent cochlear fibers as a dark degenerated bundle (arrows) coursing from the mid-line lesion and passing ventrolaterad to the facial genu (G). The lesion is in the raphe bordering the medial longitudinal fasciculus (Flm). Nauta stain, 100 X. B, transverse section through the medulla revealing fascicles of degenerated efferent cochlear fibers (arrows) coursing peripherad, i.e., toward the trapezoid body (C tr), among incoming vestibular root fibers (Rv). Nauta stain, 150 X.
EFFERENT
COCHLEAR
BUNDLE
233
234
BOORD
fibers are intertwined with those of the facial root. The facial genua of the pigeon are displaced in an atypical position and as a consequence, the crossed efferent bundles, caught in the facial nerve roots, are drawn sharply ventrad. On the other hand, the genua of the caiman remain dorsally situated immediately beneath the floor of the fourth ventricle as in the cat and other mammals; hence, the efferent fibers pass more directly laterad from the raphe to become associated with facial fibers. Not all of the fibers which constitute the vestibulocochlear anastomosis degenerate following transection of the medullary decussation. A similar condition exists among mammals as shown by Rasmussen (10) who suggests that the persistent nondegenerated ones may represent afferent cochlear fibers. In the pigeon and caiman, it is possible that some of these normal fibers are afferent fibers to the lagena especially if the vestibulocochlear anastomosis represents, in part, the lagenar nerve. Bocca (1) and Federici (3) for example, contend that the vestibulocochlear anastomosis contains fibers from the lagena. In the present study, no attempt was made to determine the origin of these efferent fibers but the answer concerning their ultimate termination was sought. The various histological techniques used, however, proved inadequate for settling this point. Work thus far on mammals indicates that they terminate along with afferent fibers on the receptor cells (2, 4, 12, and others) but this important point has not yet been established beyond doubt. In view of the undetermined origin of this fiber tract, it is referred to simply as the efferent cochlear bundle rather than the olivocochlear bundle. On the basis of what has been established in the mammal (9), one may assume that its origin is associated with the rudimentary superior olivary complex. The experimental findings of Gillaspy (5) in the alligator support this view. She found the occurrence of chromatolysis in the cells of the contralateral superior olive following transection of the seventh and eighth cranial nerves. FIG. 5. Pigeon. A, transverse section through the medulla at the level of the vestibular root and ganglion. Degenerated fascicles of efferent cochlear fibers (arrows) are leaving the medulla among incoming vestibular root fibers and entering the superior part of the vestibular ganglion (Gvs). The superior vestibular ganglion is partially detached from the brain stem. Nauta stain, 150 X. B, degenerated efferent cochlear fibers in the vestibulocochlear anastomosis (arrow). The efferents can be followed through the saccular nerve (Ns) and ganglion (Gs) to near its entrance into the cochlear nerve (NC). Nauta stain, 17.5 X.
EFFERENT
COCHLEAR
BUNDLE
235
236
BOORD
FIG. 6. Pigeon. Upper, photomicrograph cf a section through the longitudinal extent of the vestibulocochlear anastomosis (Avc) to show its course and relationships. From a pigeon in which the efferent cochlear bundle had been transected at its decussation in the medulla. Gc, cochlear ganglion; Gvi, ganglion vestibulare (pars inferior) ; NC, cochlear nerve; Ns, N. saccularis. Protargol silver preparation, 100 X. A, from area indicated in the upper photomicrograph at 600 X. Axonal fragments of efferent cochlear fibers (arrow) leaving the saccular nerve to enter the vestibulacochlear anastomosis. B, from area indicated in the upper photomicrograph at 600 X. Degeneration in the vestibulocochlear anastomosis (arrow) at its entrance into the cochlear nerve. C, from area indicated in the upper photomicrograph at 600 X. Axonal debris (arrow) representing efferent cochlear fibers within the cochlear ganglion.
EFFERENT
COCHLEAR
BUNDLE
237
238
BOORD
FIG. 7. Pigeon. A, longitudinal section through the radial fibers of the cocblear nerve showing swollen myelin sheath globules of a degenerating efferent cocblear nerve fiber (arrow) ; others are normal afferent radial fibers. The organ of Corti lies immediately to the left of the field shown in this photomicrograph. Gc, cochlear ganglion. Sudan black B stain, 600 X. B, longitudinal section through the organ of Corti (OC) showing a portion of a fragmented fiber (arrow) bordering the cochlear sensory epithelium. Protargol silver stain, 750 X.
EFFERENT
COCHLEAR
BUNDLE
239
References 1. 2. 3.
4. 5. 6. 7.
8.
9. IO. 11.
12.
13. 14.
BOCCA, E., Notes on the innervation of the cochlea. II. A.M.A. Arch. Otolaryngol. 66: 690-703, 1953. ENGSTR~M, H., On the double innervation of the sensory epithelia of the inner ear. Acta Oto-Laryngol. 49: 109-118, 1958. FEDERICI, F., tliber die peripherische Ausbreitung des VIII Schldelnervenpaares bei den Vijgeln und iiber die Bedeutung der Lagena. Amt. Anz. 61: 449465, 1926. FERNANDEZ, C., The innervation of the cochlea (Guinea Pig). tiryngoscope 61: 1152-1172, 1951. GILLASPY, CARRIE C., Superior olive in the alligator. PYOC. Sot. Expt?. BioE. Med. 96: 492-494, 1958. KOENIG, H., R. A. GROAT, and W. F. WINDLE, A physiological approach to perfusion-fixation of tissues with formalin. Stain Tecknol. 20: 13-22, 1945. NAUTA, W. J. H., Silver impregnation of degenerating axons. In “New Research Techniques of Neuroanatomy,” W. F. Windle (Ed.), Springfield, Illinois, Thomas, 1957 (ref. pp. 17-26). NAUTA, W. J. H., and P. ,4. GYGAX, Silver impregnation of degenerating axons in the central nervous system: a modified technique. Stain Technol. 99: 91-93, 1954. RASMUSSEN, G. L., The olivary peduncle and other fiber projections of the superior olivary complex. J. Camp. Neurol. 94: 141-219, 1946. RASMUSSEN, G. L., Further observations of the efferent cochlear bundle. J. Camp. Neural. 99: 61-74, 1953. RASMUSSEN, G. L., A method of staining the statoacustic nerve in bulk with Sudan black B. Amt. Record (in press). SCNUKNECHT, H. F., J. A. CHURCHILL, and ROSEMARY DORAN, The localization of acetylcholinesterase in the cochlea. A.M.A. Arch. Otolaryngol. 69: 549-559, 1959. SHANER, R. F., The development of the nuclei and tracts related to the acoustic nerve in the pig. J. Camp. Neural. 99: S-19, 1934. WINDLE, W. F., Neurofibrillar development in the central nervous system of cat embryos between 8 and 12 mm. long. J. Camp. Neural. 69: 643-723, 1933.
The
NEUROLOGY
Efferent
3, 225-239 (1961)
Cochlear
Bundle
in the
Caiman
and
Pigeon ROBERT L. BOORD~ Laboratory of Neuroanatomical Sciences, National Institute of Neurological Diseases and Blindness, National Institutes of Health, U. S. Public Health Service, Department of Health, Education, and Welfare, Bethesda, Maryland Received
October
26, 1960
An efferent component to the reptilian and avian cochlear nerve is demonstrated by experimental anatomical methods and its central and peripheral course described. The efferent cochlear bundle decussates in the medulla oblongata dorsal to the medial longitudinal fasciculi at a level corresponding to the facial genua. This decussation was cut, in the caiman and pigeon, with a small knife fitted to a stereotaxic instrument. Following appropriate postoperative survival periods, serial sections of the brain stems and statoacustic nerves were stained for axonal and myelin degeneration by the Nauta-Gygax, Protargol silver, and Sudan black B methods. Cochlear efferent fibers of either side cross the raphe, pass laterad to traverse the motor root of the facial nerve and become dispersed among vestibular root fibers. Peripherally they traverse the superior and inferior vestibular ganglia and join the cochlear nerve via the vestibulocochlear anastomosis. The parent bundle extends throughout the length of the cochlear ganglion and distributes fibers which are traceable as far as the organ of Corti. Some efferent fibers continue beyond the cochlear ganglion to terminate in the macula lagenae. The cells of origin and the ultimate termination of these efferent fibers remain to be determined. The similarity of the central and peripheral topographic relationships of the efferent cochlear bundle in reptiles, birds, and mammals suggest that this bundle is homologous among those vertebrates which possess a cochlea. This structural constancy adds support to the contention that the efferent innervation of the cochlea may play an important role in audition and implies that the efferent cochlear bundle is a basic part of the neural apparatus involved in the hearing process. 1 I am greatly indebted to Dr. G. L. Rasmussen for his continued help and supervision throughout the course of this study and for the privilege of carrying out the investigation in his laboratory. This investigation was carried out during the tenure of a Predoctoral Fellowship (No. BF-9003) from the National Institute of Neurological Diseases and Blindness, United States Public Health Service. 225
226
BOORD Introduction
The purpose of this study is to determine whether a bundle comparable to the olivocochlear bundle of Rasmussen,previously described in the cat and other mammals (9, lo), is present in birds and reptiles. In these submammalian forms the cochlea is uncoiled and lacks such specialized mammalian features as internal and external hair cells, tunnels, and complex spiral fiber arrangements. Previous studies in mammals concerning the question of the ultimate termination of efferent cochlear fibers have been seriously handicapped by the intricate complexity of innervation of the organ of Corti. Much of this difficulty could be obviated by the study of a more simply constructed hearing organ, such as found in birds and reptiles, provided that an equivalent efferent cochlear component is present in these forms. Therefore, the anatomical demonstration of an efferent cochlear bundle in the pigeon and caiman (South American alligator) would be of fundamental importance in extending previous mammalian research and providing phylogenetic evidence on which to base future morphological and functional considerations. Materials
and
Methods
Eighteen adult domestic pigeons ranging in weight from 2.50to 380 gm and thirteen young South American spectacledcaimans (Caiman sklerops) ranging in total length from 51 to 82 cm and in weight from 400 to 1920 gm survived the operative experiments. Of this number favorable lesions were placed in eleven pigeons and four caimans. For normal study, serial sections of the brain stems and inner ears of both specieswere prepared by a Protargol silver method. Dissections of the eighth nerve complex of Sudan black B in toto stained specimensproved helpful in the study of both normal and experimental histological preparations. Pigeons were anesthetized by the slow administration of Veterinary Nembutal (Abbott) in the brachial vein. Approximately 0.3 ml (18 mg) diluted to 0.6 ml with normal saline per kilogram of body weight usually induced surgical anesthesia. For the caiman, Nembutal was found less effective than hypothermia. Caimans were first subjected to a temperature of 5” C in a refrigerator after which surgical anesthesia was maintained by covering them with a plastic bag filled with cracked ice. The lesion site objective was the decussatingfibers in the dorsal raphe at the level of the facial genua which was anticipated as being homologous to the olivocochlear decussation in mammals. In both the caiman and
EFFERENT
COCHLEAR BUNDLE
227
pigeon these fibers were transected with a small knife fitted to a stereotaxic instrument having a head holder modified to accommodateindividually the pigeon and the caiman. The angle of dorsal approach and the distance from the dura to the descussationwere previously determined on formalin-fixed heads while oriented in the stereotaxic apparatus. Following postoperative survival periods ranging from 2 days and 3 hours to 4 days for the pigeon, and from 2 to 5 weeks for the caiman, these animals were anesthetized and perfused after the method of Koenig, Groat, and Windle (6). The dorsal aspect of the brain was then exposed by removing the calvarium and the headsimmersedin 10 per cent neutral formalin. Twelve to 24 hours later, the brain and bony auditory capsules, containing the inner ears, were removed and further fixed 3 to 5 days in 10 per cent neutral formalin. Brain stems were cut at 24-40 p on the freezing microtome and serial sections stained by the method of Nauta and Gygax (8) as modified by Nauta (7). The peripheral portion of the eighth nerve was handled by three different techniques in order to exhibit the presence of fragmented axons or degenerating myelin. Interrupting decussatingfibers resulted in bilateral degeneration of the efferent fibers in the acoustic nerve; therefore, it was possible to study both ears by utilizing a different method on each. Axonal fragmentation was demonstrated by Protargol silver methods in paraffin serial sections of the inner ears following decalcification, and myelin disintegration exhibited by the Sudan black B method of Rasmussen (11). Favorable results were also obtained by the application of the Nauta-Gygax technique to the peripheral nervous system. This method proved of value in substantiating Protargol and Sudan black degeneration as well as providing a greater degree of differentiation between normal and degenerating axons. Results
A semidiagrammaticdrawing of the medullary and peripheral course of the efferent cochlear bundle in the caiman and pigeon is depicted in Fig. 1. A more detailed description, supported with photomicrographs of the cochlear efferents, is given below. CROSSEDLIMBOF
EFFERENT
COCHLEAR BUNDLE IN MEDULLA
OBLONGATA
Caiman. The level of the decussation is at the rostra1 border of the abducens nuclei from which axons pass over the medial longitudinal
228
BOORD
fasciculi as a compact bundle and proceed beneath the facial genua (Fig. 2A). Efferent cochlear fibers accompany the facial motor root of either side for some distance before traversing it to enter the incoming vestibular root (Fig. 2B) where the bundle becomesdispersed into one larger and two or three smaller fascicles. These fasciclescourse peripherad
FIG. 1. Diagrammatic presentation of the central and peripheral course and relationships of the efferent cochlear bundle in the caiman (left) and pigeon (right) based on observations of experimental material and reconstructed. Efferent cochlear fibers are depicted as thicker beaded dashes. Avc, vestihulocochlear anastomosis ; Flm, fasciculus longitudinalis medialis; G, genu n. facialis; Gvi, ganglion vestibulare (pars inferior) ; Gvs, ganglion vestibulare (pars superior) ; Ml, macula lagenae; Nu I, nucleus laminaris; Nu OS, nucleus olivaris superior; Nu VI, nucleus abducentis; R n VII, motor root of seventh nerve; Rv, root of vestibular nerve; Tr s \‘, tractus spinahs n. trigemini.
among vestibular fibers and emerge from the medulla with the vestibular root (Fig. 3A). Pigeon. As in the caiman, the bundle decussatesimmediately rostra1 to A, transverse section through the medulla at the level of the FIG. 2. Caiman. facial genua showing efferent cochlear fibers, as a degenerated bundle (arrow), coursing from the mid-line and proceeding laterad beneath the facial genu (G). The raphe lies immediately to the left of the field shown in this photomicrograph. Nauta stain, 150 X. B, transverse section through the medulla showing degenerated efferent cochlear fibers (arrows) traversing the motor root of the facial nerve (R n VII) and coursing dorsolaterad to enter the incoming vestibular root (Rv). The dark-staining structures at the lower right represent reticular fibers of blood vessels. Nauta stain, 150 X.
EFFERENT
COCHLEAR
BUNDLE
229
230
BOORD
the abducens nuclei but pursues a more ventral course from the raphe in order to intersect the facial genua (Fig. 4A). The efferent cochlear fibers accompany the facial root for only a short distance before dispersing themselves among the widely scattered vestibular root fibers (Fig. 4B). As may be seen in Fig. SA, they leave the brain stem as small fascicles. PERIPHERAL
COURSE OF EFFERENT
COCHLEAR
BUNDLE
Caiman. After emerging from the medulla in the incoming vestibular root, efferent cochlear fibers traverse the superior vestibular ganglion and course caudoventrally to enter the inferior vestibular nerve. At a level where posterior ampullar and saccular fibers separate from the cochlear nerve, they collect into a special fascicle known as the vestibulocochlear anastomosis. This anastomosisextends from the saccular ganglion to the cochlear nerve and consists predominantly of efferent fibers. The main bundle, upon reaching the cochlear ganglion, bifurcates (Fig. 3B) and coursesalong the outer border of the ganglion throughout its entire length. En route, efferent fibers leave to accompany afferent radial fibers. Degenerated axons have been followed as far as the receptor epithelium but their ultimate termination could not be determined. The efferent fibers which continue beyond the apical end of the cochlea terminate in the macula lagenae, a sensory area commonly classified as a vestibular receptor organ. Pigeon. Efferent cochlear fibers, after leaving the brain stem, traverse the superior vestibular ganglion. As in the caiman, they proceed in a ventrocaudal direction to course successively through the inferior vestibular ganglion, the saccular nerve, and the vestibulocochlear anastomosis to’ join the cochlear nerve (Figs. SB, 6A, 6B). The anastomosis is clearly identified in histological sections cut parallel to its course between the saccular and cochlear nerves (Fig. 6). The presenceof degenerated efferent fibers entering the cochlear nerve via the vestibulocochlear anastomosisis shown in Fig. 6B. Efferent fibers FIG. 3. Caiman. A, transverse section through the medulla showing dark degenerated fascides of efferent cochlear fibers (arrows) coursing peripherad in the vestibular root (Rv). Other abbreviations: Nu 1, nucleus laminaris; R n VII, motor root of the seventh nerve; Tr s V, tractus spinalis n. trigemini. Nauta stain, 27 X. B, longitudinal section through the posterior ramus of the statoacoustic nerve showing the degenerated efferent cochlear bundle bifurcating (arrows) distal to the cochlear ganglion (Gc). Many normal impregnated axons appear in the upper portions of this field. Nauta stain, 150 X.
EFFERENT
COCHLEAE
BUNDLE
231
232
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course along the external face of the cochlear nerve before reaching the ganglion (Fig. 6C). The efferent bundle courses apicalwards within the cochlear ganglion and distributes whole (unbranched) fibers evenly throughout the length of the organ of Corti as evidenced by the presence of a few scattered degenerated fibers among a much greater number of normal afferent radial fibers (Fig. 7A). A few fragmented axons have been followed as far as the cochlear sensory epithelium in Protargol silver preparations (Fig. 7B), but not to their terminations. As is the case in the caiman, the efferent fibers which continue beyond the cochlear ganglion terminate in the macula lagenae. Discussion
The topographic relationships of the outgoing limb and the peripheral extension of the efferent cochlear bundle in the pigeon and caiman are basically similar to those found in mammals. For example, the point of decussation is in the dorsal raphe at a level corresponding to the facial genua and it is through the fibers of the latter that cochlear efferents pass before emerging from the medulla with the vestibular root. Also, peripherally the bundle enters the cochlear nerve via the vestibulocochlear anastomosis. Although the topographical features of the medulla of the pigeon and caiman vary considerably, the basic relationships of the efferent cochlear bundle persist. A common feature among reptilian, avian, and mammalian forms is the interdigitation of the intramedullary efferent bundle with fibers of the facial root. This relationship is clearly revealed in embryonic and fetal mammalian brains as appearing chronologically at the stage of development when the facial nucleus is situated near the floor of the fourth ventricle (13, 14). Later development and subsequent displacement of the facial nerve in different animals fail to alter this relationship because the efferent
FIG. 4. Pigeon. A, transverse section through the medulla at the rostra1 level of the facial genua showing efferent cochlear fibers as a dark degenerated bundle (arrows) coursing from the mid-line lesion and passing ventrolaterad to the facial genu (G). The lesion is in the raphe bordering the medial longitudinal fasciculus (Flm). Nauta stain, 100 X. B, transverse section through the medulla revealing fascicles of degenerated efferent cochlear fibers (arrows) coursing peripherad, i.e., toward the trapezoid body (C tr), among incoming vestibular root fibers (Rv). Nauta stain, 150 X.
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fibers are intertwined with those of the facial root. The facial genua of the pigeon are displaced in an atypical position and as a consequence, the crossed efferent bundles, caught in the facial nerve roots, are drawn sharply ventrad. On the other hand, the genua of the caiman remain dorsally situated immediately beneath the floor of the fourth ventricle as in the cat and other mammals; hence, the efferent fibers pass more directly laterad from the raphe to become associated with facial fibers. Not all of the fibers which constitute the vestibulocochlear anastomosis degenerate following transection of the medullary decussation. A similar condition exists among mammals as shown by Rasmussen (10) who suggests that the persistent nondegenerated ones may represent afferent cochlear fibers. In the pigeon and caiman, it is possible that some of these normal fibers are afferent fibers to the lagena especially if the vestibulocochlear anastomosis represents, in part, the lagenar nerve. Bocca (1) and Federici (3) for example, contend that the vestibulocochlear anastomosis contains fibers from the lagena. In the present study, no attempt was made to determine the origin of these efferent fibers but the answer concerning their ultimate termination was sought. The various histological techniques used, however, proved inadequate for settling this point. Work thus far on mammals indicates that they terminate along with afferent fibers on the receptor cells (2, 4, 12, and others) but this important point has not yet been established beyond doubt. In view of the undetermined origin of this fiber tract, it is referred to simply as the efferent cochlear bundle rather than the olivocochlear bundle. On the basis of what has been established in the mammal (9), one may assume that its origin is associated with the rudimentary superior olivary complex. The experimental findings of Gillaspy (5) in the alligator support this view. She found the occurrence of chromatolysis in the cells of the contralateral superior olive following transection of the seventh and eighth cranial nerves. FIG. 5. Pigeon. A, transverse section through the medulla at the level of the vestibular root and ganglion. Degenerated fascicles of efferent cochlear fibers (arrows) are leaving the medulla among incoming vestibular root fibers and entering the superior part of the vestibular ganglion (Gvs). The superior vestibular ganglion is partially detached from the brain stem. Nauta stain, 150 X. B, degenerated efferent cochlear fibers in the vestibulocochlear anastomosis (arrow). The efferents can be followed through the saccular nerve (Ns) and ganglion (Gs) to near its entrance into the cochlear nerve (NC). Nauta stain, 17.5 X.
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FIG. 6. Pigeon. Upper, photomicrograph cf a section through the longitudinal extent of the vestibulocochlear anastomosis (Avc) to show its course and relationships. From a pigeon in which the efferent cochlear bundle had been transected at its decussation in the medulla. Gc, cochlear ganglion; Gvi, ganglion vestibulare (pars inferior) ; NC, cochlear nerve; Ns, N. saccularis. Protargol silver preparation, 100 X. A, from area indicated in the upper photomicrograph at 600 X. Axonal fragments of efferent cochlear fibers (arrow) leaving the saccular nerve to enter the vestibulacochlear anastomosis. B, from area indicated in the upper photomicrograph at 600 X. Degeneration in the vestibulocochlear anastomosis (arrow) at its entrance into the cochlear nerve. C, from area indicated in the upper photomicrograph at 600 X. Axonal debris (arrow) representing efferent cochlear fibers within the cochlear ganglion.
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FIG. 7. Pigeon. A, longitudinal section through the radial fibers of the cocblear nerve showing swollen myelin sheath globules of a degenerating efferent cocblear nerve fiber (arrow) ; others are normal afferent radial fibers. The organ of Corti lies immediately to the left of the field shown in this photomicrograph. Gc, cochlear ganglion. Sudan black B stain, 600 X. B, longitudinal section through the organ of Corti (OC) showing a portion of a fragmented fiber (arrow) bordering the cochlear sensory epithelium. Protargol silver stain, 750 X.
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References 1. 2. 3.
4. 5. 6. 7.
8.
9. IO. 11.
12.
13. 14.
BOCCA, E., Notes on the innervation of the cochlea. II. A.M.A. Arch. Otolaryngol. 66: 690-703, 1953. ENGSTR~M, H., On the double innervation of the sensory epithelia of the inner ear. Acta Oto-Laryngol. 49: 109-118, 1958. FEDERICI, F., tliber die peripherische Ausbreitung des VIII Schldelnervenpaares bei den Vijgeln und iiber die Bedeutung der Lagena. Amt. Anz. 61: 449465, 1926. FERNANDEZ, C., The innervation of the cochlea (Guinea Pig). tiryngoscope 61: 1152-1172, 1951. GILLASPY, CARRIE C., Superior olive in the alligator. PYOC. Sot. Expt?. BioE. Med. 96: 492-494, 1958. KOENIG, H., R. A. GROAT, and W. F. WINDLE, A physiological approach to perfusion-fixation of tissues with formalin. Stain Tecknol. 20: 13-22, 1945. NAUTA, W. J. H., Silver impregnation of degenerating axons. In “New Research Techniques of Neuroanatomy,” W. F. Windle (Ed.), Springfield, Illinois, Thomas, 1957 (ref. pp. 17-26). NAUTA, W. J. H., and P. ,4. GYGAX, Silver impregnation of degenerating axons in the central nervous system: a modified technique. Stain Technol. 99: 91-93, 1954. RASMUSSEN, G. L., The olivary peduncle and other fiber projections of the superior olivary complex. J. Camp. Neurol. 94: 141-219, 1946. RASMUSSEN, G. L., Further observations of the efferent cochlear bundle. J. Camp. Neural. 99: 61-74, 1953. RASMUSSEN, G. L., A method of staining the statoacustic nerve in bulk with Sudan black B. Amt. Record (in press). SCNUKNECHT, H. F., J. A. CHURCHILL, and ROSEMARY DORAN, The localization of acetylcholinesterase in the cochlea. A.M.A. Arch. Otolaryngol. 69: 549-559, 1959. SHANER, R. F., The development of the nuclei and tracts related to the acoustic nerve in the pig. J. Camp. Neural. 99: S-19, 1934. WINDLE, W. F., Neurofibrillar development in the central nervous system of cat embryos between 8 and 12 mm. long. J. Camp. Neural. 69: 643-723, 1933.