Degeneration behaviour of the cochlear nerve

The cochlear nerve in various forms of deafness

Operative specimens of cochlear nerve of 56 patients with profound deafness of varying origin were examined under the light microscope and the number of myelinated nerve fibers in each specimen was counted. The transverse fascicular area of each nerve was also determined. When the resulting neuron populations were evaluated with regard to the cochlear implant program the only truly unsuitable ears for artificial stimulation were found to be those with cholesteatoma involving the cochlear nerve and ears with local pathology in the internal auditory canal severe enough to cause atrophy of the cochlear nerve.

Transganglionic transport of D-aspartate from cochlear nucleus to cochlea--a quantitative autoradiographic study

This study concerns the connections of the inner and outer hair cells and the different types of ganglion cells of the mammalian cochlea with the central nervous system by making use of their putative neurotransmitters. D-[3H]Aspartate (D-ASP), a putative marker for glutamatergic neurons, was injected into the cochlear nucleus of cats and guinea pigs and the cochleas prepared for light microscopic autoradiography after varying survival times. A quantitative, statistical autoradiographic method is described. Grain counts per unit area were made for each of 14 tissue compartments in the cochlea and normalized to permit comparisons between cases. An operationally defined background labeling level was computed for each case and a statistical test for significance applied to the neuron-containing tissue compartments. With increasing survival times, significant labeling appeared successively in the cochlear nerve root, in each type of spiral ganglion cell, and in the neuron-containing tissue compartments of the organ of Corti. The findings are consistent with uptake of D-ASP and retrograde transport by cochlear nerve axons from the cochlear nucleus to the perikarya and peripheral processes of the spiral ganglion. We conclude that axons of all spiral ganglion cells project to the cochlear nucleus and that this nucleus is directly connected with both the inner and outer hair cells. Transganglionic transport of D-ASP from the cochlear nucleus is consistent with the hypothesis that the cochlear nerve axons use glutamate or aspartate as a neurotransmitter.

Substance P-like immunoreactivity in the rabbit inner ear

The auditory and vestibular sense-organs of the rabbit were examined for the presence of substance P(SP) by an immunohistochemical technique using a monoclonal SP-antibody. Between 30 and 40 per cent of the cells of the vestibular ganglia and about 50 per cent of the spiral ganglion cells innervating the extreme basal part of the cochlea showed SP-like immunoreactivity. The neural elements in the vestibular sensory epithelia, notably the calyx-formed nerve terminals of type I sensory cells of the maculae, also showed strong SP-like immunoreactivity. The findings suggest the possibility of a modulator or transmitter role for SP in the inner ear function.

Fine structure of cochlear innervation in the cat

Examination of adult and juvenile cat cochleas by electron microscopy and semi-serial sections permitted identification of the cytological features characteristic of the afferent and efferent nerve fiber populations identified in Golgi impregnations of the cochlea. This study demonstrated the distribution of synaptic contacts made by these fiber populations. As in the Golgi findings, radial and outer spiral afferent fibers were identified in well separated zones of the inner spiral bundle. The trunks of the outer spiral fibers, containing many microtubules and few neurofilaments, at first coursed spirally below the inner hair cells on the proximal face of the inner pillar, turned abruptly between adjacent pillar cells and entered the tunnel without branching. Radial afferents, containing many neurofilaments and a few microtubules, coursed through the inner spiral bundle, maintaining a radial or oblique orientation and proceeded directly toward the inner hair cells. Efferent fibers in the region of the inner spiral bundle were distinguishable by size, by orientation, and, to a lesser extent, by cytology. Small (1 micron) efferent fibers, containing few neurofilaments, an occasional microtubule, and mitochondria, occurred in the inner and tunnel spiral bundles and formed large varicosities, which contacted radial afferents. A separate population of much thicker efferents, containing many neurofilaments, mitochondria and dense-cored vesicles, but no microtubules, did not enter the inner spiral bundle but coursed directly to the level of the tunnel spiral bundle on the proximal face of the inner pillar cells. These fibers crossed the tunnel at the level of the tunnel spiral bundle and, upon reaching the outer hair cells, formed large synaptic contacts on outer hair cells and on outer spiral fibers as well. Some of these efferent fibers also synapse on afferent fibers while crossing the tunnel. The findings agree with previous observations with the Golgi method showing that entirely separate populations of spiral ganglion cells innervate the inner and outer hair cells. Likewise, there are efferent fibers innervating only inner or outer hair cells, but the probability of efferent fibers to both inner and outer hair cells cannot be ruled out.

Morphometry of intracellularly labeled neurons of the auditory nerve: correlations with functional properties

Single auditory-nerve fibers were injected with horseradish peroxidase after their tuning properties, characteristic frequencies, and spontaneous discharge rates were measured. From these functional properties virtually all other aspects of auditory-nerve response can be predicted. Labeled fibers were reconstructed from the point of peripheral termination on cochlear hair cells to the point at which they enter the cochlear nucleus. Several morphological properties were measured at the light-microscopic level, including axonal diameter, axonal length, internodal distances, cell-body area, and cell-body shape. All of these parameters were correlated, though some weakly, with characteristic frequency. However, only axonal diameter was correlated with spontaneous discharge rate.

Differential olivocochlear projections from lateral versus medial zones of the superior olivary complex

An anterograde tracer (35S-methionine) was injected unilaterally in the superior olivary complex (SOC) at regions previously demonstrated by retrograde labeling to contain olivocochlear (OC) cell bodies. Quantitative analysis of cochlear autoradiographs from these cats demonstrates that there are two OC systems. The lateral OC system has cell bodies lateral to the medial superior olivary nucleus (MSO) and projects to the inner hair cell (IHC) region bilaterally (mostly ipsilaterally). The medial OC system has cell bodies medial, ventral, and anterior to the MSO and projects to the outer hair cell (OHC) region bilaterally (mostly contralaterally). A single medial OC neuron innervates many small patches of OHCs with substantial gaps between the patches. Medial OC neurons also appear to project to the IHC region to a small extent. A review of the literature with the medial-lateral division of OC efferents in mind reveals many differences between these two systems. In particular, lateral OC axons are unmyelinated and innervate the dendrites of radial afferent fibers under IHCs, whereas medial OC axons are myelinated and directly innervate OHCs. Although both systems appear to be cholinergic, the lateral OC system also shows met-enkephalin-like immunoreactivity. The synapses of the medial OC system are formed in development before those of the lateral OC system and they degenerate more slowly after the OC axons are cut. The many differences between these two OC systems suggest that they are functionally separate systems.

Influence of temperature on tuning of primary-like units in the guinea pig cochlear nucleus

The effect of temperature changes on the response area of primary-like single units recorded extracellularly in the anteroventral cochlear nucleus of the guinea pig was studied for cochlear temperatures in the range 27.4-40.3 degrees C, while maintaining a normal rectal temperature of 38.5 degrees C. A new approach to the cochlear nucleus was developed which involved opening of the temporal bone overlying the flocculus cavity and aspirating a small portion of the paraflocculus. Irrespective of characteristic frequency CF (0.8-16 kHz), hypothermia caused a reversible elevation of CF thresholds (1.6 dB/degrees C), a small loss of the sharpness of threshold tuning and reductions of the saturation and mean spontaneous rates. The CFs were unaffected, in contrast to single units in birds and cold-blooded animals. From a comparison of the effects of temperature on different species it is proposed that the processes that define CF and sharpness of tuning are different within a cochlea, and that these processes can be differentially influenced.

Cytodifferentiation of cochlear hair cells

Cytodifferentiation of a limited number of cochlear hair cells in the mouse starts on the 14th gestational day. One day later, the number of identified hair cells increases considerably. Cytodifferentiation apparently occurs in a gradient from the hair cell surface to the base. First, the irregular microvilli covering the future hair cell surfaces begin to show a regular pattern but are of the same thickness and length as microvilli on supporting cells. Second, a polarization of sensory hairs occurs with a stepwise increase in stereocilia length in the different rows toward the kinocilium. Finally, the cuticle is formed, giving an anchorage for sensory hair rootlets in the hair cell. At birth some hair cells can be found with immature surface morphologic features, e.g., stereocilia of the same length on the entire hair cell surface and lack of a cuticular plate. The onset of hair cell differentiation takes place without morphologic contact with ingrowing nerve fibers.

Cochlear ganglion cells in the alligator lizard

Two types of ganglion cells are present in the cochlear ganglion of the alligator lizard. Myelinated cells are the predominant type but a small population of unmyelinated cells is also present. Ganglion cells are reconstructed and morphometrically analyzed from serial light and electron micrographs. The results are interpreted to indicate that the unmyelinated ganglion cells are degenerating neurons. No normal population of unmyelinated nerve fibers was found. The anatomy of the cochlear nerve of the lizard is described and compared with the mammalian auditory nerve.

Artificial activation and degeneration of the cochlear nerve in guinea pigs

Degeneration of the cochlear nerve was produced in guinea pigs after treatment with high doses of ototoxic antibiotics completely destroying the cochlear receptor. An effective electric stimulation of the inner ear inducing evoked potentials at the auditory cortex was applied over a period of 3.5 months in half of the animals, while the other half was kept for control. The artificial stimulation had no effect on the sensitivity of the inner ear to electric stimulation and did not modify the extent of degeneration of the cochlear nerve.

Postnatal development of spiral ganglion cells in the rat

The postnatal development of the spiral ganglion in the albino rat was studied using light and electron microscopy. The morphological characteristics distinguishing type 1 from type 2 spiral ganglion cells were defined, and the critical period for distinguishing the two types of neurons was identified. At birth, the spiral ganglion consists of a homogeneous population of small, densely packed, spherical cells that have large cytoplasmic-to-nuclear ratios. During the first postnatal week, the cells mature slowly. At this period the myelin sheath around the cell bodies generally consists of only a few layers of loose myelin. Long glial fingers extend around the cell processes and soma, particularly the filopodial extensions of the somatic membrane. Type 2 spiral ganglion cells can be distinguished at postnatal day 8. Viewed with the phase-contrast microscope these cells are smaller and have more darkly staining nuclei and more lightly staining cytoplasm than the type 1 cells. The most characteristic ultrastructural features of the type 2 neurons are the densely packed neurofilaments in the cytoplasm and lack of compact myelin around the cell soma. By day 14, spiral ganglion cells are morphologically mature, although the myelin sheath continues to thicken. The results are discussed in relation to the electrophysiological development of the auditory system and the morphological maturation of the organ of Corti.

Early auditory evoked potentials (EAEPs) in the rabbit. Normative data and effects of lesions in the cerebello-pontine angle

Early auditory evoked potentials (EAEPs) were measured in rabbits. After establishing normative data an operative method was developed to expose the cerebello-pontine angle. Compression of the VIIIth nerve and a reduction of blood supply to the cochlea via the labyrinthine artery altered the EAEP systematically. In all cases of an VIIIth nerve lesion wave 1 continued to be elicited whereas waves 2-5 disappeared. Circulatory disturbances had an effect upon wave 1. The results indicate that the generator of wave 1 is the unit cochlea/acoustic nerve and of waves 2-5 the auditory pathway within the brain stem. The resultant data help to understand wave alterations in patients with acoustic neuromas.

Serial section reconstruction of the neural poles of hair cells in the human organ of Corti. I. Inner hair cells

Study of the anatomy of the cochlea, and in particular the morphology of synaptic relationships between hair cells and cochlear neurons, is essential for elucidation of the mechanisms of transduction of mechanical acoustic signals into electrical neural events. Because considerable gaps remain in our understanding of the microscopic anatomy of these synapses, particularly in the human, a reconstruction of neural pole of inner hair cells of the human organ of Corti was performed. The data are based on 526 serial sections from the basal turn (10 mm region) and 356 serial sections from the middle turn (26 mm region). This provided complete data on 3 and partial data on 5 inner hair cells. Afferent terminals on inner hair cells were variable in size, ranging 1 to 20 micrometers in diameter. Branching of large fibers to produce multiple terminals innervating from 1 to 3 inner hair cells was common. Each inner hair cell received approximately 6 to 8 different nerve terminals. In addition, each terminal possessed a variable number of synaptic contacts. Junctional membrane specialization consisted of synapses, desmosomes, coated vesicles and arrays of microtubules and membrane cisternae. Specialization at synapses consisted of asymmetrical membrane thickening. At inner hair cells the postsynaptic membrane was thicker than the presynaptic membrane. Eighty-three percent of synapses had presynaptic bodies. Vesiculated efferent terminals synapsed on afferent fibers at the base of inner hair cells, but never directly on the inner hair cell. These anatomical data demonstrate distinct differences between the human and animal inner ear, which are important in the interpretation of neurophysiological data in animals and the formulation of hypotheses that involve assumptions crossing species.

A study of cochlear innervation in the young cat with the Golgi method

Individual afferent and efferent nerve fibers were identified and traced in Golgi-impregnated cochleas of cats from newborn to one month old. Afferent radial fibers project radially without varicosities to terminate at the base of one or two inner hair cells. Outer spiral fibers have both radial and spiral orientations within the organ of Corti, do not form varicosities while crossing the base of the tunnel, and spiral for long distances in the outer spiral bundles. They contact many outer hair cells of more than one row both en passant and by small terminal branchlets. Two separate groups of efferent fibers are identifiable. Thin efferent fibers with many large varicosities spiral for long distances in the inner and tunnel spiral bundles; varicosities in the inner spiral bundle may contact radial afferent fibers or hair cells, depending on age. Thick radial efferent fibers course radially through the tunnel spiral bundle and across the upper part of the tunnel, often in fascicles. They contact a few outer hair cell bases by large terminals. The spiral expanse of the terminals is limited. These fibers are most common in the more basal turns of the organ. The present results confirm the anatomical separation of radial and spiral afferent fiber systems and identify two separate efferent populations beyond the neonatal period in the cat. The major features of afferent innervation discernible in Golgi-impregnated cochleas are present at birth, although some simplification of afferent fibers probably occurs during the first postnatal week. In contrast, the efferent fiber pattern undergoes important changes during the first few weeks after birth. In mature animals, the fine spiral efferents probably contact only afferent fibers, whereas the thick radial efferents may contact both outer hair cells and spiral afferent fibers. The possibility that some individual efferents branch to both inner and outer hair cell regions in the older cats cannot be ruled out.

Degeneration of cochlear neurons after amikacin intoxication in the rat

Intoxication with high doses of the aminoglycoside antibiotic amikacin in a supranormal sensitive period in the rat induces complete destruction of the inner and outer hair cells in the organ of Corti in all turns, whereas the supporting cells remain partially preserved in the upper turns. With increasing survival time, the number of ganglion cells in the spiral ganglion decreases progressively, reaching a minimum of about 10% surviving cells after 12 months. Both type I and type II neurons are subject to retrograde degeneration, although type-II cells degenerate more slowly than type-I cells. The presence or absence of supporting cells in the organ of Corti does not seem to influence neuronal degeneration. This retrograde degeneration is similar in all animals so far studied but its time course is different from different species. Retrograde degeneration after destruction of Corti's organ is a long-lasting process and is never completed at once. This must be taken into consideration in the treatment of total deafness with electric stimulation of surviving neurons.

Early development and maturation of the spiral ganglion

The maturation of the spiral ganglion and its peripheral pathways to the cochlea has been morphologically analysed from birth to the 14th day postnatally, i.e. to the time from the onset and during the maturation of cochlear potentials (mouse). The spiral ganglion cells and the neurons between the ganglion and habenula perforata must attain myelination before action potentials can be elicited. Earlier observations that a certain maturation of the organ of Corti is a prerequisite for the development of electrophysiological potentials is probably only coincidental in time with the structural development of the ganglion and the neurons.

Preferential amino acid uptake identifies Type II spiral ganglion neurons in the gerbil

The localization of 3H-labeled amino acids was compared by light and electron microscopic autoradiography in the spiral ganglion of the gerbil, following in vivo intracochlear incubations. Following incubations with taurine, a population of heavily labeled neurons could be distinguished from lightly labeled neurons. The heavily labeled population comprised 5-6% of spiral ganglion neurons, and included the least myelinated cells. Ultrastructurally, the heavily labeled neurons were characterized by a loosely coiled Schwann cell sheath covering the cell body, and the presence of abundant cytoplasmic microfilaments. The unlabeled cells showed a typical perikaryal myelin sheath and cytoplasm rich in rough endoplasmic reticulum. It is concluded that the spiral ganglion of the gerbil contains both Type I and Type II neurons, and that Type II neurons preferentially incorporate the amino acid taurine. Type II neurons are therefore biochemically as well as morphologically distinct from Type I neurons.

Changes in rapidly transported proteins in the auditory nerve after hair cell loss

In an effort to further characterize the proteins of the auditory nerve, the effects of hair cell loss on rapidly transported proteins of the auditory nerve were studied. The effects were studied in the waltzing guinea pig, a genetically deaf animal which displays an age-dependent loss of sensory cells, and in normal guinea pigs treated with neomycin. In both cases hair cell loss is followed by a slow degeneration of spiral ganglion cells and corresponding auditory nerve fibers. Rapidly transported proteins in the cochlear nucleus were analyzed by two dimensional electrofocusing/electrophoresis 3 h after cochlear injection of [35S]methionine. In both the waltzing guinea pig and the neomycin-treated animals, a significant increase in labeling of two series of polypeptides (average molecular weights of 27,000 and 36,000 daltons) was apparent. Quantitation of the 36,000 dalton protein by extracting from dried gels showed a 2-fold increase in the 10-day-old waltzing guinea pig and a 6-fold increase in the 80-day-old waltzing guinea pig. Further analysis shows these proteins to be membrane-associated glycoproteins.

Type II cochlear ganglion cells in the chinchilla

In order to ascertain whether Type II cochlear ganglion cells project to the brain, we have studied the retrograde transport of horseradish peroxidase (HRP) from the cochlear nucleus to the spiral ganglion of the chinchilla. In this animal there exist two types of ganglion neurons, which closely correspond to those previously described in guinea pigs, cats and rats. As in the guinea pig, the majority population (Type I) consists of relatively large, myelinated neurons. The minority population (Type II, 10% of the total population) consists of small, mostly unmyelinated cells, with filamentous cytoplasm and finely grained nuclear chromatin. Type II neurons tend to be clustered toward the peripheral side of Rosenthal's canal, often in close proximity to the intraganglionic spiral bundle. By 24 h after injections of HRP into the cochlear nucleus, incubation of the cochlear ganglion in diaminobenzidine/H2O2 reveals abundant HRP label in both Type I and Type II neurons. Type II neurons, however, tend to be labelled less intensely than Type I neurons. Control experiments, consisting of spillage of HRP solution over the cochlear nucleus, were carried out to determine how much HRP might be picked up by neurons after HRP diffusion. Comparison of cochleae from injected animals and from the control animals suggests that most of the label that was found in ganglion neurons after cochlear nucleus injections represents axonally transported HRP. We conclude, at least tentatively, that Type II neurons project to the brain. The fact that less label is found in Type II neurons that in Type I neurons suggests that the former have thinner axons and/or finer terminals in the cochlear nucleus.

Spiral ganglion changes after massive aminoglycoside treatment in the guinea pig. Counts and ultrastructure

Morphological changes of the eighth nerve were observed in the guinea pig between 1 month and 1 year after treatment with large doses of the antibiotic amikacin which resulted in complete cochlear hair cell destruction. The neural retrograde degeneration was found to be relatively fast, with a considerable loss (30 to 55%) of ganglion cells one month after treatment, continuously increasing (up to 85) after one year. Gross changes in the habenula perforata and in the spiral ganglion are described, together with ultrastructural alterations of organelles important for the cell metabolism and axonal transport. The rapid degeneration and the morphological findings suggest a direct influence of toxic substances on the ganglion cells.

Uptake and transport of horseradish peroxidase by cochlear spiral ganglion neurons

Cochlear spiral ganglion neurons in the cat have been classified as type I and type II. The type II neurons are of special interest since they are reported to provide the afferent innervation to three-quarters of the cochlear hair cells, and also because of a recent assertion that they do not project to the brain. In the present study the neuronal marker horseradish peroxidase (HRP) was utilized to re-examine these issues. HRP was injected into the intranuclear cochlear nerve root of adult cats. 18 to 48 h post-injection HRP reaction product was observed in both type I and type II neurons. The majority of labeled cells were characterized by granular reaction product, while a smaller number of cells were also diffusely filled with HRP, resulting in intense staining of the cell soma and its processes. These data indicate that the central axons of type II neurons are constituents of the cochlear nerve root at the level of the brainstem and thus do project centrally. In the organ of Corti HRP-filled fibers to both inner and outer hair cells were observed and demonstrated in electron microscopy to be afferent neurons. This intra-axonal accumulation of the HRP by outer hair cell afferents constitutes direct evidence of the functional projection of these neurons to the cochlear nucleus.

Hair-cell innervation by spiral ganglion cells in adult cats

A horseradish peroxidase technique was used to trace the peripheral terminations of two types of ganglion cells in adult cats. It was found that large, usually bipolar ganglion cells end on inner hair cells and small, usually pseudomonopolar ganglion cells end on outer hair cells. Thus, a virtually complete segregation of afferent neural inputs from the two types of hair cells was directly confirmed.

Cochlear nerve fiber populations in 25 patients with profound deafness due to Menière's disease or sudden deafness

Surgical biopsy specimens of the cochlear nerve from 19 patients with Menière's disease and six patients with sudden deafness were studied. The total number of myelinated nerve fibres in the transverse fascicular area of each nerve were counted and the results were correlated with the clinical and preoperative audiological data. The total number of cochlear neurons in patients with Menière's disease ranged from 5,760 to 27,722 and in patients with sudden deafness from 8,500 to 22,714. The long duration of the disease or absent speech discrimination appeared not to be connected with lower numbers of neurons than preserved speech discrimination or short history of the disease.

The effect of postsynaptic deprivation on the presynaptic chief cell in the rat carotid body

After the surgical removal of the postsynaptic sensory sinus nerve, the presynaptic specializations in the chief cell of the rat carotid body chemoreceptor rapidly disappear. When the sinus nerve is allowed to regenerate, the presynaptic specializations reappear almost simultaneously with the re-establishment of apposition of the nerve to chief cells and the number of the synapses is restored to a normal range by the third week after deafferentation. This finding indicates that the maintenance of the presynaptic specializations in the chief cell depends on the persistence of the postsynaptic element, and further suggest the neurotrophic maintenance of the presynaptic specializations may be exerted by the postsynaptic sinus nerve. The sustentacular cells may play a role in governing the specificity of reinnervation by the sinus nerve.

Human vestibular nerve morphology and labyrinthectomy

Surgical labyrinthectomy leads to extensive and degenerative changes in the membranous labyrinth and is often followed by fibrosis and ossification of the vestibule and neuroma formation. After labyrinthectomy the distal processes of the vestibular nerve degenerate, but the nerve trunk within the internal auditory canal, including Scarpa's ganglion, does not show an obvious loss of neurons. Electron microscopic examination of the cells of Scarpa's ganglion revealed many changes similar to those of chromatolysis, indicating increased cell metabolism and probable regenerative changes. Many nerve fibers appeared to be in the process of regeneration. Some ganglion cells were fibrotic and showed ultrastructural features similar to those of cells that had undergone atrophy but survived in a sublethal state.

An anatomical and physiological study of regeneration of the eighth nerve in the leopard frog

The ability of auditory fibers in the anuran's VIIIth nerve to regenerate back into the central nervous system after their axons have been served was studied electrophysiologically and anatomically. Single unit recordings in the regenerated portion of the nerve indicate that: (1) fibers from both the amphibian and basilar papillae regenerate; (2) tuning curves of regenerated fibers, as in normal fibers, are 'V' shaped and retain their sharp frequency selectivity; (3) latency and threshold measurements of regenerated fibers are similar to values obtained in intact nerves. Filling the posterior branch of the regenerated VIIIth nerve with horseradish peroxidase indicates that fibers in this branch remain together within the nerve during regeneration and succeed in terminating in their correct target nuclei and no others.

Morphology of Human cochlear nerve after labyrinthectomy

Five temporal bones and three surgical cochlear nerve specimens from patients who previously underwent labyrinthectomy were studied by light and electron microscopy. All cochleas had nearly total loss of sensory cells, extensive or total loss of both cochlear nerve fibers in the osseous spiral lamina, and spiral ganglion cells. In spite of this severe distal degeneration of the cochlear nerve, it processes within the internal auditory canal appeared normal in four of the five temporal bones, and the majority of the myelinated nerve fibers had survived in all three surgical specimens. Most of the central processes of the primary neurons of the cochlear nerve appear to have resisted retrograde degeneration when it was induced by a lesion primarily causing degeneration of the organ of Corti. This peculiar situation of a few nerve cell bodies and disproportionately large numbers of central axons is discussed.

Hereditary deafness in the cat. An electron microscopic study of the spiral ganglion

The spiral ganglion from white cats with hereditary deafness has been studied with the transmission electron microscope, and comparisons made with hearing animals at different ages. Ganglion cell loss occurs secondary to destruction of the organ of Corti, but only after the lapse of several months. Prior to neuronal loss, the type I ganglion cells lose their myelin sheaths and concurrently develop an increased content of neurofilaments. Type I neurons transform into type II through an intermediate type III stage. This process of neurofilamentous degeneration occurs slowly, and phagocytosis is therefore an inconspicuous feature.

[Anatomical observations in the spiral ganglion of human newborns (author's transl)]

Four temporal bones of human newborns were studied light- and electron-microscopically. Different parts of the spiral ganglion were examined but absolutely no myelinization of the large (type I) and small (type II) ganglion cells could be seen. Between the ganglion cells dense bundles of mixed nerves are detected, some of the unmyelinated nerves show synaptic-like contact zones to the surface of type II-cells. The small type II cells are mainly located at the periphery of the ganglion cell complex. Between the spiral ganglion and the osseous spiral lamina large bundles of unmyelinated nervecomplexes are crossing the mainly myelinated radial fibers. The nerve fibers running from the modiolus to Rosenthal's channel contain bout 12% unmyelinated nerves whereas the nerve fascicles crossing the osseous spiral lamina reveal a population of 22-25% unmyelinated nerve fibers. About 2% of the small unmyelinated nerve fibers (diameter 200-500 nm) which run between the ganglion cells contain dense core vesicles.

Ultrastructural study of the human spiral ganglion

Electron microscopic study of the human spiral ganglion was conducted on 17 ears from 12 individuals aged 9 months to 92 years. Two types of neurons, large and small, with distinct cytological characteristics were found. Both types of neurons were myelinated and unmyelinated; however, a majority of the population was unmyelinated (94%). The distribution of myelinated neurons varied greatly among individuals, though a slight increase in their population was noted in aged individuals, the highest count being 28% in a specimen from an individual 75 years old. The small neurons constituted 6% of the population, and their cytoplasms were highly filamentous in both myelinated and unmyelinated types. The findings of this study provide no evidence as to the functional significance of the myelination of spiral ganglion cells.

[Function returning without and with recovery nystagmus after sudden unilateral isolated vestibular loss (author's transl)]

The restoration of peripher labyrinthine excitability after sudden unilateral isolated vestibular loss shows a different behaviour: It can happen with or without recovery nystagmus with a numerical nearly equal frequency. In a follow-up of 44 patients with unilateral vestibular loss of vascular genetics the objective and subjective characteristics of both states are described. In explanation of it a lesion of variable etiology in the region of the vestibular artery and a supposition of a not yet or already occured compensation of the disturbed tonus balance in the nuclear area are not sufficient. Rather the origin of the different clinical states of recovery phenomenon must be presumed in all probability as the result of a primary and secondary retrograde degeneration of the vestibular neurons. Thereby the distance of the lesion from the Scarpa ganglion is presumably of decisive importance.

Nerve fiber synapses on spiral ganglion cells in the human cochlea

Nerve fiber synapses were observed on the small myelinated and unmyelinated spiral ganglion cells of the human cochlea. The nerve fibers penetrated the junctions of the ensheathing satellite cells and myelin lamellae, and partly invaginated into the perikarya to establish axosomatic synapses. One small myelinated neuron with a nerve fiber synapse demonstrated multipolar cell processes. However, most small neurons with identical ultrastructural characteristics did not show these synapses. Axodendritic synapses were also seen on the dendritic processes of the small neurons and between varicose nerves and unmyelinated nerve fibers some distance from the small neurons. While these observations conform in some aspects with the concept of parasympathetic nerve fibers and neurons in the cochlea, they are also compatible with the idea that the small neurons may have an auditory function influenced by the synaptic contacts of efferent fibers from the olivocochlear bundle.

Neural connections of the outer haircell system

The afferent innervation of the outer hair cells consists of a special type of neuron, distinguished by its characteristic structure and particular degeneration behavior. The connections of these neurons to the sensory cell, the central nervous system and the other cochlear neurons have been studied from different aspects and under various experimental conditions. There is evidence that the neurons are not effectively connected to the central nervous system, have no functional interconnections to other neurons and their functional significance in the adult animal is only rudimentary.

The cochlear nerve in the cat: topography, cochleotopy, and fiber spectrum

The topographical and cytological features of the cochlear nerve in normal adult cats were studied by microdissection, light microscopy, transmission and scanning electron microscopy. The 2-mm long cochlear nerve trunk is situated within the internal acoustic meatus. The nerve is wrapped like a roll with the more basal fibers situated peripheral to the more apical ones. The fibers are fasciculated according to their target in the cochlea and follow a helical course with the sharpest spiralling in the distal portion of the fiber. Apical and basal coil fibers are of approximately the same length measured from the spiral lamina to the rostral border of the anteroventral cochlear nucleus. The nerve contains about 90% thick, myelinated fibers, about 10% unmyelinated axons, and a few thin, but heavily myelinated axons. The first group shows a unimodal distribution of axonal diameters, with a gradual increase in average diameter from basal to apical coil fibers. The number of axonal microtubules and neurofilaments have a high linear correlation to axonal circumference without principal cochleotopic differences. The myelin sheaths are thicker and the internodal segments shorter than expected from similarly sized peripheral fibers in rodents. The possible nature and origin of the three fiber categories are discussed.

Different patterns of cochlear innervation during the development of the kitten

The postnatal development of neuro-epithelial junctions inside the kitten cochlea has been investigated by electron microscopy, and correlated with previous electrophysiological results. Two main stages of development are described. During the first postnatal days, outer hair cells look very immature with only a few afferent endings adjoining them. The inner hair cells, on the other hand, are surrounded by numerous endings with mature afferent and efferent synapses. Thus, when the efferent olivo-cochlear system begins to function during the first postnatal day s, it is able to modify only inner hair cell responses. The second postnatal week is characterized by maturation of the large efferent endings below the outer hair cells. At the same time, direct efferent connections become sparce at the level of inner hair cells. The maturation of hearing, at the receptor level, seems to proceed in two steps, one related to inner hair cells and corresponding to a gross and primitive hearing, the other related to outer hair cells and corresponding to more precise and discriminative hearing abilities.

Horseradish peroxidase acute ototoxicity and the uptake and movement of the peroxidase in the auditory system of the guinea pig

When guinea pig cochlea was perfused in vivo with a solution of 1% horseradish peroxidase (HRP) in artificial perilymph, the enzyme was found in the basilar membrane, spiral limbus, some outer and inner hair cells and some supporting cells and it was gradually cleared away with time. Acute signs of cell damage included swelling, vacuolization and diffuse labeling of some hair cells, but stereocilia remained normal in configuration. Albino melanocytes of the spiral ligament were also damaged, and vacuolization of Reissner's membrane occurred after 10% HRP. Both concentrations caused a gradual decline in CM, showing that HRP is acutely ototoxic but its mode of action is unknown. No retrograde transport of HRP to spiral ganglion cells or to brain stem neurons occurred, but some brain stem neurons took up HRP from the neuropil following diffusion from the cochlea.

Scanning electron microscopy of the nerves within the organ of Corti

A method of studying the innervation inside the organ of Corti is presented. Rabbits, guinea pigs and chinchillas were fixed with perfusion of the perilymphatic spaces. The cochleas were dissected according to the surface specimen technique and the specimens critical point dried, coated with gold and studied in the SEM. The final dissection into the planes of the fluid spaces of the organ of Corti was done in the dry state using especially sharpened watch makers forceps and razor blade knives. The course of the afferent and efferent innervation is described and differences between the species illustrated. Small high nerve endings on the outer hair cell degenerate after cutting the efferent nerve supply by dividing the vestibular nerve indicating their efferent nature.

[To Wittmaack's theory of "hearing without the organ of Corti" (author's transl)]

1911 Wittmaack had shown by experiments in cats that the organ of Corti is independent from the cochlear nerve but dependent from a normal internal auditory artery. Similar findings he could observe in his temporal bone collection in cases of acoustic nerve tumors. On the other hand many human cases with the compression phenomenon of the organ of Corti had intact cochlear nerve fibers and neurons, and nearly normal hearing. Thus Wittmaack concluded that some parameters of hearing are transmitted by the nerve endings in the organ of Corti without the haircells. This hypothesis is discussed by means of examples of Wittmaack's temporal bone collection in the light of our present knowledge.

Auditory prostheses in perspective

The results of the evaluation of subjects presently fitted with internal auditory prostheses are summarized. Based on these results, the role of single-channel prostheses as a treatment for acquired deafness is discussed in terms of the auditory expectation for a single channel and of subject selection.

Ultrastructural changes of the nerve elements following disruption of the organ of Corti. I. Nerve elements in the organ of Corti

3-137 days after disruption of the guinea pig organ of Corti by perilymphatic perfusion with 20% streptomycin (SM), ultrastructural changes of the nerve fibers in the organ were observed. Most of nerve fibers began to degenerate after a latent period of 4 days. On the other hand, a number of fibers survived reactively enlarged and later developed into myelinated and unmyelinated fibers by becoming enclosed in Schwann cells which entered the organ of Corti through the habenula perforata. Regeneration and sprouting of the surviving nerve fibers also occurred. The fibers became mature, but atrophied after 60 days and then gradually disappeared. The regenerating fibers were mainly of the myelinated and unmyelinated efferent type. Retrograde degeneration occurred in both afferent and efferent fibers. In the less damaged organ of Corti perfused with 2% SM or Ringer's solution, Schwann cell invasion was not found.

Electrocochleography and cochlear pathology

In experimentally damaged inner ears the structural alterations were correlated to electrocochleographic responses of the ear. Sectioning of the cochlear nerve with degeneration of the type I neurons but intact sensory cells results in normal cochlear microphonics but very weak and atypical nerve responses. By contrast, damage of the organ of Corti with retrograde degeneration abolished primarily the cochlear microphonics, whereas the compound VIII nerve action potential is barely affected when only the outer hair cells are gone and even a small number of surviving inner hair cells is still compatible with a relatively strong compound action potential of the cochlear nerve.

The innervation of the organ of Corti. A scanning electron microscopic study

The innervation within the organ of Corti of guinea pigs, chinchillas and rabbits was studied in the scanning electron microscope. Specimens were fixed, dissected, critical point dried and coated with gold. Final dissection of the specimens was done in the dry state in the planes of the former fluid spaces. The courses of the efferent and afferent nerve fibres are described. The efferent fibres reach the outer hair cells by crossing the tunnel space, run between the outer pillars and divide several times in the space of Nuel. The afferent fibres cross the floor of the tunnel and turn basalwards in the outer spiral bundles where they run for at least 0.5-0.8 mm before connecting with the outer hair cells. Small nerve endings on every fifth to tenth outer hair cell disappear after cutting the efferent nerves, thus indicating their efferent nature.

Effects of removal of the statoacoustic ganglion complex upon the growing otocyst

An experiment was designed to answer the question as to whether or not the neural elements of the statoacoustic ganglion complex have a trophic effect upon the histodifferentiation of the sensory structures of the embryonic mouse inner ear anlage as it develops in vitro. The embryonic inner ear anlage with associated otic mesenchyme and statoacoustic ganglion complex was excised from 11, 12, and 13-day CBA/C57 mouse embryos. The inner ear explants of each gestational age group were further divided into two groups: the first group "A" (with) statoacoustic ganglion was explanted to the organ culture system without further surgical intervention; the second group "B" (without) statoacoustic ganglion underwent further surgical manipulation during which their statoacoustic ganglion complexes were dissected away prior to explantation to in vitro. The explanted embryonic inner ears were allowed to develope in organ culture until the equivalent of gestation day 21 in vivo was reached for each group; then all cultures were fixed and histologically processed and stained by a nerve fiber stain, in combination with a stain for glucoprotein membranes. Each specimen was code labeled and scored for histodifferentiation of sensory structures. Light microscopic observations confirmed that in group "A" cultures, statoacoustic ganglion neurons and their nerve fibers were present in association with the developed sensory structures; neither ganglion cell neurons nor their nerve fibers were found to be present in the sensory structures that developed in the group "B" organ culture specimens. Quantification revealed no consistent trend of greater occurrence of any sensory structure in the groups of explants analyzed. The presence of such a trend would have signified the probable existence of a trophic effect of the statoacoustic ganglion neural elements upon development of inner ear sensory structures in the group "A" explants of the 11, 12, and 13-day embryo inner ear organ culture specimens when compared to the aganglionic group "B" cultures. Microscopic comparison of the sensory structures and their sensory hair cells that developed in the organ cultures revealed no differences in the quality of the histodifferentiation of eithergroup "A" or goup "B" explants. A base to apex pattern of histodifferentiation of the organ of Corti sensory structures, which has been described to occur in vivo, was noted to occur in the in vitro developed cochlear ducts of all of the explanted inner ears without respect to whether neural elements were present ("A") or absent ("B") during development. It was concluded from the quantification of histodifferentiation data and the above observation on the pattern of differentiation of Corti's organ that no trophic effect of neural elements of the statoacoustic ganglion complex influencing the histodifferentiation of sensory structures of 11, 12, and 13-gestation day mouse embryo inner ear explants as they differentiate in vitro could be demonstrated.

Ultrastructural cochlear changes following acoustic hyperstimulation and ototoxicity

Using guinea pigs and chinchillas as experimental animals, modes and patterns of sensory cell damage by acoustic hyperstimulation and kanamycin intoxication were compared. In general, outer hair cells were more vulnerable to both acoustic trauma and ototoxicity (particularly in the basal turn) than inner hair cells. However, in kanamycin ototoxicity, the inner hair cells were more vulnerable in the apical coil. Nerve endings and nerve fibers generally were resistant to both acoustic trauma and kanamycin intoxication, and their degeneration appears to be secondary to the sensory cell degeneration. A large number of unmyelinated nerve fibers were seen in both the organ of Corti and the osseous spiral lamina even three months after the organ of Corti had been completely degenerated by ototoxicity. The total number of unmyelinated and myelinated nerve fibers in the osseous spiral lamina far exceeded the scanty surviving ganglion cells in Rosenthal's canal, indicating the possibility of regeneration of these fibers following kanamycin intoxication. The remaining few ganglion cells were mainly type II or type III cells, and a majority of the type I ganglion cells appeared to be degenerated. Signs of strial damage were observed in both acoustic trauma and ototoxicity, but their pattern did not correlate well with that of sensory cell degeneration.

Electron microscopic cochlear observations in bilateral Ménière's disease

Electron microscopic study of the cochleas of an individual with bilateral Ménière's disease revealed the presence of many abnormal sensory cells in the apical regions of the cochleas. The pathological alterations were greater in the left ear with the greater hearing loss. There were some giant cilia, fusion of cilia, and loss of cilia. The outer hair cells contained diffuse cuticular bodies near or basal to the nuclei. Many outer hair cells were retracted away from the reticular membrane. The population of the nerve endings appeared normal. The inner hair cells of the apical turns appeared essentially normal. The spiral ganglia were normal in number and morphology at the apical turns, and a majority of their cell bodies were of the unmyelinated or partly myelinated types. The stria vascularis showed atrophy; however, the magnitude of this change was consistent with that known to occur in the aging ear. In the distended areas Reissner's membrane showed areas devoid of mesothelial cells, as well as atrophic epithelial cells. The blood vessels were no different from other human cochlear vessels. These abnormalities seen in the stereocilia, the outer hair cells, and Reissner's membrane are a matter of fact. We must admit, however, that not enough electron microscopic studies have been performed on ears from aging individuals or ears with other pathologies to state that these changes are unique to Ménière's disease. Furthermore, there is currently insufficient knowledge to predict whether these changes affect auditory function.

Nerve fibers in the human organ of Corti

In the human cochlea, the following characteristics of nerve fiber arrangements were observed. 1. Where there was only the basilar fiber in the tunnel of Corti, the internal and tunnel spiral bundle were not observed. In such cases, the number of nerve fibers in the external spiral bundle was small. 2. In the tunnel of Corti, the basilar fibers (afferent) took a course at the base and crossing the tunnel slanting to the basal end. 3. The medial fibers were present with the internal spiral bundle and tunnel spiral bundle. These fibers are possibly efferent. 4. The external spiral bundle increases its fiber count when internal and tunnel spiral bundles were present in the organ of Corti. 5. The external spiral bundle was composed of the basilar and medial fibers. The former were less and the latter much more numerous in the bundle. 6. 85-90% of the afferent fibers went to the internal hair cells, while the remainder went to the external hair cells. 7. The nerve fibers were found in the organ of Corti lateral to the external spiral bundles. The fibers were running in the Hensen's and Claudius' cell area. 8. In some cases medial fibers showed a torpedo-like deformity in the tunnel of Corti. When this was observed, it was found distributed throughout the entire cochlea. Hearing disorders due to this pathology of the efferent fiber was suggested.