Morphology of labeled efferent fibers in the guinea pig cochlea

Efferent axons to the guinea pig cochlea were labeled by extracellular injections of horseradish peroxidase into the intraganglionic spiral bundle within the spiral ganglion. The terminal fibers formed by these axons were classified according to their patterns of termination within the basal turn of the cochlea. A class of terminal fibers designated "autonomic" forms a highly branched plexus in the osseous spiral lamina but does not enter the organ of Corti. The termination of single autonomics includes blood vessels as well as areas of the osseous spiral lamina not adjacent to blood vessels. Two major classes of efferent axons from the olivocochlear bundle enter the cochlea by way of the vestibulocochlear anastomosis and terminate either in areas near inner hair cells (IHC efferents) or onto outer hair cells (OHC efferents). The IHC efferents have thin axons throughout their course within the cochlea and can be divided into two subclasses. The most numerous subclass of IHC efferents (unidirectional) enters the inner spiral bundle and turns to spiral in only one direction for less than 1 mm and then forms a discrete termination including many en passant and terminal swellings that are within both the inner and tunnel spiral bundles. A less common subclass of IHC efferents (bidirectional) bifurcates upon entry into the inner spiral bundle to send branches both apically and basally. These terminal fibers take spiral courses that are greater than 1 mm in extent, often course in the tunnel spiral bundle for a large portion of the spiral, and form terminals throughout their extended spiral course. None of the IHC efferent fibers send branches to cross the tunnel to innervate the outer hair cells. A second major class of olivocochlear fibers, OHC efferent fibers, forms large boutons on the outer hair cells, and although they sometimes spiral beneath the IHCs for some length, they do not give off terminals to this region. The OHC efferent axons are thick and myelinated as they enter the cochlea, and they branch near the spiral ganglion to form several terminal fibers. Some of these terminal fibers are thin as they travel from the intraganglionic spiral bundle across the osseous spiral lamina to the organ of Corti, whereas others are thick and obviously myelinated as far peripheral as the habenula.(ABSTRACT TRUNCATED AT 400 WORDS)     

GABA visualized by immunocytochemistry in the guinea pig cochlea in axons and endings of efferent neurons

Antiserum raised against GABA coupled with glutaraldehyde to bovine serum albumin was applied to the guinea pig cochlea. Immunoreactivity was visualized as horseradish peroxidase reaction product in surface preparations of the organ of Corti using immunocytochemical techniques. Bright-field, differential interference contrast and video-enhanced contrast light microscopy were used. GABA-like immunoreactivity was found in axons and endings of efferent neurons in all turns of the cochlear spiral, but predominantly in the third turn and first half of the fourth turn. In these apical turns, immunoreactivity was seen in the efferent components: inner spiral bundle, tunnel spiral bundle, tunnel-crossing fibers, large nerve endings synapsing on outer hair cell bases, nerve endings high up on outer hair cells, nerve endings or varicosities close to outer hair cells, and outer spiral fibers. Some immunoreactive large nerve endings at outer hair cells were found in the apical half of the fourth turn. This study shows that axons and endings of efferent neurons in the organ of Corti of guinea pig contain GABA-like immunoreactivity with a distribution similar to that of GAD-like immunoreactivity as shown in a previous study. In both studies, many efferent nerve axons and endings were unstained, even in regions of maximal density of immunoreactivity in the apical turns. The evidence indicates that a subpopulation of efferent neurons projecting to the organ of Corti is GABAergic and very likely different from the lateral and the medial olivocochlear efferent systems.     

Synaptic arrangements between inner hair cells and tunnel fibers in the mouse cochlea

Hair cells, the sensory cells of the organ of Corti, receive afferent innervation from the spiral ganglion neurons and efferent innervation from the superior olivary complex. The inner and outer hair cells are innervated by distinctive fiber systems. Our electron microscopical studies demonstrate, however, that inner hair cells, in addition to their own innervation, are also synaptically engaged with the fibers destined specifically to innervate outer hair cells, within both the afferent and efferent innervation. Serial sections of the afferent tunnel fibers (destined to innervate outer hair cells) in the apical turn demonstrate that, while crossing toward the tunnel of Corti, they receive en passant synapses from inner hair cells. Each inner hair cell (in a series of five in the apical turn) was innervated by two tunnel fibers, one on each side. We show here for the first time that, in the adult, the afferent tunnel fibers receive a ribbon synapse from inner hair cells and form reciprocal contacts on their spines. Vesiculated efferent fibers from the inner pillar bundle (which carries the innervation to outer hair cells) form triadic synapses with inner hair cells and their synaptic afferent dendrites; the vesiculated terminals of the lateral olivocochlear fibers from the inner spiral bundle synapse extensively on the afferent tunnel fibers, forming triadic synapses with both afferent tunnel fibers and their synaptic inner hair cells. This intense synaptic activity involving inner hair cells and both afferent and efferent tunnel fibers, at their crossroad, implies functional connections between both inner and outer hair cells in the process of hearing.     

Morphology of labeled afferent fibers in the guinea pig cochlea

Cochlear afferent and efferent fibers in the guinea pig were labeled by focal extracellular injections of horseradish peroxidase into the spiral ganglion of the basal turn. The morphology and pattern of termination of these fibers were studied by light microscopy. Fibers labeled by injections into the peripheral side of the ganglion could be grouped on the basis of their courses and terminations in the cochlea into two classes of afferent fibers, two classes of efferent (olivocochlear) fibers, and other presumably autonomic fibers. This paper describes the characteristics of labeled afferent fibers and their parent ganglion cells. Peripheral afferent fibers were grouped into two major classes: thick (mean diameter 1.7 micron) radial fibers projecting in a primarily radial fashion from the spiral ganglion and terminating on single inner hair cells and thin (mean diameter 0.5 micron) outer spiral fibers that spiral basalward in the organ of Corti to terminate on outer hair cells, usually in one row. For outer spiral fibers, the number of outer hair cells contacted and the length of the terminal region depend on the row of outer hair cells contacted, with third-row fibers forming, on the average, the most extensive region of termination. Within the spiral ganglion, two types of ganglion cells could be distinguished: type-I ganglion cells of large size (mean soma area = 216 microns 2) with a ratio of central process diameter to peripheral process diameter greater than one and type-II ganglion cells of smaller size (mean soma area = 100 microns 2) and a central to peripheral process ratio near one. In three cochleae in which injections were made central to the ganglion, 11 type-I ganglion cells have been traced to radial fibers contacting inner hair cells and eight type-II ganglion cells have been traced to outer spiral fibers contacting outer hair cells. Thus the afferent innervation of the guinea pig cochlea is similar to the pattern described in other mammals, in which there is separate innervation of the inner and outer hair cells by the two types of ganglion cells. The central axons of both types of ganglion cells were traced individually through serial sections of a block of tissue containing the cochlea, the auditory nerve, and the cochlear nucleus. They followed similar courses in the auditory nerve, and the axons followed into the cochlear nucleus bifurcated in similar regions of the interstitial portion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Neuron-specific enolase during the development of the organ of Corti

NSE immunoreactivity has been studied in the organ of Corti of the developing mouse from birth to 21 days. NSE immunohistochemical stain is observed in spiral ganglion cells, in nerve fibers and in nerve endings of inner and outer hair cells, and in both populations of sensory cells. Spiral ganglion cells in lower and central parts of the ganglion stain for NSE at birth, but all nerve cells are stained by day 4. Radial and spiral fibers and the endings on inner hair cells stain at birth, but the nerve endings on outer hair cells develop NSE between days 3 and 6. The inner and outer hair cells are NSE-positive at day 2 but the NSE immunoreactivity in the outer hair cells decreases at the end of the second week until the cells become negative. The NSE stain in the neuronal pathways of the inner and outer hair cell regions increases for about 19 days, showing a predominant accumulation in neuronal endings. The data suggest that the development of NSE expression in the organ of Corti reflects the nascence and maturation of the synaptic contacts. Spiral neurons, their fibers and endings as well as inner and outer hair cells express NSE in the isolated organ of Corti in culture. Variability of stain among the different cell populations indicates a role of local factors in the regulation of NSE expression.     

Electron microscopic observation of calcitonin gene-related peptide-like immunoreactivity in the organ of Corti of the rat

Calcitonin gene-related peptide (CGRP)-like immunoreactive (CGRP-IR) nerve terminals in the organ of Corti of rats were studied by light and electron microscopy. Surface preparation of the organ of Corti were immunostained using anti-CGRP antiserum for avidin-biotin immunohistochemistry. Dense CGRP-IR fiber bundles were observed by light microscopy in the inner spiral bundles, tunnel spiral bundles and outer spiral bundles. Electron microscopic analysis indicated that CGRP-IR fibers belong to efferent nerves. In the inner spiral bundles, the CGRP-IR fibers showed a direct contact mainly with non-immunoreactive afferent fibers. Some CGRP-IR nerve endings in the inner spiral bundles formed contacts directly with inner hair cells. In the outer spiral bundles, CGRP-IR fibers formed synaptic contacts exclusively with the outer hair cells. It should be noted that the number of synapses of CGRP nerve endings with outer hair cells varied depending upon the sub-row: a falling gradient in number occurred along the inner-outer axis. Our results suggest that CGRP acts as an efferent neuromodulator in the organ of Corti.     

Early efferent innervation of the developing rat cochlea studied with a carbocyanine dye.

The olivocochlear pathway in the developing rat was visualized in fixed material. The fluorescent carbocyanine dye 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was applied to the cut central axons of the olivocochlear neurones at the floor of the fourth ventricle, and the termination pattern within the cochlea was examined after dye diffusion. From the day of birth (P0) to postnatal day 2 (P2), efferent innervation of the cochlea was exclusively in the region of the inner hair cells. Between P2 and P11, progressive outgrowth of neuronal processes to the outer hair cell region occurred; possible connections with the outer hair cells were occasionally seen at P4 and approached the mature pattern by P6. The efferent innervation of the organ of Corti appeared to mature progressively from the cochlear base to the apex, with outgrowth to the outer hair cells occurring earlier in the basal turn of the cochlea than in the second and third cochlear turns. Numerous blind axonal endings were observed in the spiral lamina especially at early postnatal ages. These findings may be consistent with a sequential pattern of arrival of efferent axons at the organ of Corti and ongoing death of efferent neurones in the brainstem during this period of development.     

Cochlear innervation in the developing rat: an immunocytochemical study of neurofilament and spectrin proteins

We have studied the innervation of the developing cochlea by immunocytochemical staining of the cytoskeletal proteins, neurofilament (NF), and spectrin (brain spectrin and erythrocyte spectrin). NF immunoreactivity was seen in spiral ganglion cell bodies and their processes and in fibers of the intraganglionic spiral bundle (IGSB) on gestational day 16. NF immunoreactivity with monoclonal antibodies to NF160 and NF68 was present beneath both inner hair cells (the IHC) and outer hair cells (OHCs) on gestational day 20. NF200 immunostaining was located only in the IGSB and in fibers reaching the IHC. The first NF200 immunoreactivity beneath the OHCs was seen in the basal turn at birth. NF labelling began to decrease on postnatal day 9 and its intensity became more like that of the adult. Brain spectrin immunostaining was first seen in the IGSB of the basal turn on gestational day 18. It reached the fibers between the spiral ganglion and the IHC on gestational day 20. Brain spectrin immunoreactivity was first seen beneath the OHCs in the basal turn at birth. It reached all the OHCs of the cochlea by postnatal day 4, and began to decrease 9 days after birth. Erythrocyte spectrin immunostaining was first observed during the second postnatal week, when it labelled spiral ganglion cells. The distribution of NF200 and brain spectrin immunoreactivity suggested that efferent innervation of OHCs is present at birth in the rat, and confirms previous studies showing the early efferent innervation of the OHCs of the mouse and the rat at birth, and the time lag between the appearance of the two spectrin isoforms during development.     

Olivocochlear efferent innervation of the organ of corti in hypothyroid rats

Congenital hypothyroidism induces developmental abnormalities in the auditory receptor, causing deafness due to a poor development of the outer hair cells (OHCs) and a lack of synaptogenesis between these cells and the olivocochlear axons. This efferent innervation is formed by two separate systems: the lateral system, which originates in the lateral superior olive (LSO) and reaches the inner hair cells; and the medial system, which originates in the ventral nucleus of the trapezoid body (VNTB) and innervates the OHCs. A previous study carried out in our laboratory showed that in congenitally hypothyroid animals, the neurons which give rise to the efferent system are normal in number and distribution, although smaller in size. The aim of the present work was to study the efferent fibers in the auditory receptor of hypothyroid animals, by means of stereotaxic injections of biotinylated dextran amine in the nuclei that give rise to the olivocochlear system: LSO and VNTB. In hypothyroid animals, injections in LSO gave rise to lateral olivocochlear fibers lacking their characteristic dense terminal arbors, while injections in the VNTB-labeled fibers terminating in the spiral bundle region, far from the OHCs with which they normally contact. In the latter case, only a small percentage of labeled fibers reached the OHCs area, giving off only two radial branches maximum. Because the number of neurons which develop into the efferent innervation was normal in hypothyroid animals, we conclude that medial fibers may contact a new target.     

The pattern of sensorineural degeneration in the cochlea of the deaf shaker-1 mouse: ultrastructural observations

Experiments were done to extend existing knowledge on the nature and course of postnatal sensory and neural changes in the inner ear of shaker-1 mutant mice. Mice, 3-, 6-, 10-, 12-, 18-, and 30 days old, homozygous for the sh-1 gene, were studied using transmission electron microscopy. The data indicate retarded development coupled with the early onset of progressive degeneration in Corti's organ its nerve supply, and the cells of the spiral ganglion. Especially noteworthy are the following: in 3-day-old mice both outer hair cells and spiral ganglion cells are already abnormal. The latter are especially loosely ensheathed by glial cells and are in direct contact with nerve fibers. Outer hair cells contain vacuoles and lysosomes. By 6 days of age inner hair cells come to be similarly affected. By 18 days most of the afferent nerve supply of the organ of Corti has degenerated. The behavior of efferents within the organ is complex. Efferents arrive late (day 12) at the outer hair cells, they are few in number, form only immature synapses with the cell, and they subsequently degenerate. In contrast, the efferent nerve supply of the inner hair cell appears normal, if not over-abundant. The simultaneous occurrence of organ of Corti and spiral ganglion cell anomalies is discussed in terms of the role of sensorineural interactions in the expression of genetic defects affecting the inner ear. The selective degeneration of efferents to outer hair cells is viewed as being consistent with the hypothesis that there are two independent efferent systems which innervate the two types of cochlear sensory hair cells.     

Immunohistochemical localization of adrenergic receptors in the rat organ of corti and spiral ganglion

Alpha(1)-, beta(1)-, and beta(2)-adrenergic receptors (ARs), which mediate responses to adrenergic input, have been immunohistochemically identified within the organ of Corti and spiral ganglion with polyclonal antibodies of established specificity. Alpha(1)-AR was immunolocalized to sites overlapping supranuclear regions of inner hair cells as well as to nerve fibers approaching the base of inner hair cells, most evident in the basal cochlear turn. A similar preponderance across cochlear turns for alpha(1)-AR in afferent cell bodies in the spiral ganglion pointed to type I afferent dendrites as a possible neural source of alpha(1)-AR beneath the inner hair cell. Foci of immunoreactivity for alpha(1)-AR, putatively neural, were found overlapping supranuclear and basal sites of outer hair cells for all turns. Beta(1)- and beta(2)-ARs were immunolocalized to sites overlapping apical and basal poles of the inner and outer hair cells, putatively neural in part, with immunoreactive nerve fibers observed passing through the habenula perforata. Beta(1)- and beta(2)-ARs were also detected in the cell bodies of Deiters' and Hensen's cells. Within the spiral ganglion, beta(1)- and beta(2)-ARs were immunolocalized to afferent cell bodies, with highest expression in the basal cochlear turn, constituting one possible neural source of receptors within the organ of Corti, specifically on type I afferent dendrites. Beta(1)- and beta(2)-ARs in Hensen's and Deiters' cells would couple to Galphas, known to be present specifically in the supporting cells. Overall, adrenergic modulation of neural/supporting cell function within the organ of Corti represents a newly considered mechanism for modifying afferent signaling.     

Peripherin-like immunoreactivity in type II spiral ganglion cell body and projections

Peripherin, an intermediate filament protein, is present in neuronal subpopulations of both peripheral and central nervous systems. The distribution of peripherin was studied in the adult rat cochlea using immunohistochemistry on whole mount material, in cryostat sections and sections of plastic embedded tissue. In the spiral ganglion, peripherin labeling was restricted to the perikarya of a subpopulation of neurons and their peripheral and central processes. Peripherin positive neurons had the following features: (i) they have a large eccentric nucleus, they were often found in a cluster of 2 or 3 cells, (ii) they were often located near the intraganglionic spiral bundle fibers, (iii) they represented roughly 8% of the whole ganglion population and (iv) on the average they had smaller perikarya than non-immunoreactive cells. Immunostaining on semithin plastic sections revealed positive reactivity on Type II ganglion cells, while Type I neurons were negative. Double labeling using peripherin and three neurofilament (NF) subunit antibodies confirmed the presence of both markers within the same spiral ganglion cell type. Type II neurons have been previously documented as the only subpopulation of the spiral ganglion that presents a strong positive NF immunoreactivity within their perikarya. In the organ of Corti, peripherin-positive fibers formed bundles that course beneath the outer hair cells and send branches that end as boutons contacting the outer hair cells. All these characteristics suggest that peripherin-positive cells are Type II neurons, and that peripherin constitutes a reliable marker for this spiral ganglion subpopulation, as well as their peripheral and central processes.     

Immunocytochemical detection of glutamate decarboxylase in the postnatal developing rat organ of corti

Using a fluorescent avidin-biotin technique, we have immunolocalized the GABA synthesizing enzyme, glutamate decarboxylase, in postnatal developing and adult rat organs of Corti. At birth, the glutamate decarboxylase-like immunoreactivity is already present in the basal turn below the inner hair cells, i.e. within lateral olivocochlear efferent fibers of the inner spiral bundle. In the apical turn, the inner spiral bundle displays an immunoreactivity as early as postnatal day 3. Only the outer hair cells of the upper second turn and apex receive fibers immunostained for glutamate decarboxylase that most probably belong to the medial olivocochlear efferent innervation. They first appear at this level at postnatal day 15. Within these two regions of the organ of Corti, the glutamate decarboxylase-like immunoreactivity reaches an adult-like pattern at postnatal days 17-18. These results strengthen the hypothesis that GABA is a putative neurotransmitter that could be used by subpopulations of the two olivocochlear innervations. They also suggest that GABA either plays a neurotrophic function or participates in the regulation of the first cochlear potentials at the level of lateral efferent synapses.     

GABA-like immunoreactivity in the squirrel monkey organ of corti

The distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity in the squirrel monkey organ of Corti was determined using an antiserum against GABA conjugated to bovine serum albumin. Immunoreactive labeling was seen in the region of the inner spiral bundle, the synaptic region below inner hair cells, in terminals contacting the basal part of outer hair cells, and in tunnel spiral fibers. Examples of each of these immunoreactive components could be observed in all cochlear turns. In the region of inner hair cells, immunoreactive labeling took the form of numerous small puncta randomly distributed below the base of the cells. In the region of outer hair cells, large globular immunoreactive structures reminiscent of terminal endings at the subnuclear level were observed. Since similar structures were seen at the base of outer hair cells in other cochleas processed for AChE, we conclude that GABA-like immunoreactivity was contained in efferent terminals which synapse on outer hair cells. These results strengthen previous evidence for the presence of GABA in the olivocochlear system of the mammalian cochlea.     

Cochlear nerve of the alligator lizard

The innervation of the auditory organ of the alligator lizard is described. Patterns of distribution of the nerve fibers were studied at the light microscopic level with the horseradish peroxidase technique, and the types of synaptic contacts with hair cells were studied at the transmission electron microscopic level with standard techniques. The innervation of the two regions of the basilar papilla differs in the following ways. In the apical region, some fibers send branches along the length of the basilar papilla, and both afferent (non-vesiculated) and efferent (vesiculated) nerve endings are present. In the basal region, all fibers terminate in the immediate area where they enter the papilla without sending branches along the length of the papilla; efferent endings are lacking, and nerve fibers are of a smaller average diameter. The punctate nature of the innervation of hair cells in the basal region is consistent with the hypothesis that the systematic organization according to frequency sensitivity observed in electrophysiological recordings from basal nerve fibers may be related to the length of the stereocilia on the hair cells with which the nerve synapses.     

Developmental segregation in the efferent projections to auditory hair cells in the gerbil

The auditory receptor epithelium of mammals receives efferent innervation from neurons within and surrounding the superior olivary complex of the brainstem (Warr [1975] J. Comp. Neurol. 161:159-181). Disruption of this pathway during early postnatal life, when olivocochlear axons are forming their final connections with auditory hair cells and nerve fibers, can lead to profound and permanent hearing impairments (Walsh et al. [1998] J. Neurosci. 18:3859-3869). Identification of the possible causes for this deterioration in auditory function requires a better understanding of the normal developmental interactions that occur between efferent axons and their target cells within the cochlea. To provide such information, we labeled developing efferent fibers at a constant location within the gerbil cochlea by using the fluorescent carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-carbocyanine perchlorate (DiI). The terminal arbors of these neurons were then reconstructed by using digital confocal microscopy. By postnatal day (P) 2, the efferent arbors associated with inner hair cells (IHCs) and outer hair cells (OHCs) displayed distinctly different morphologies closely resembling those described for adult animals (Brown [1987] J. Comp. Neurol. 260:605-619). Unlike their mature counterparts, however, P2 efferent axons frequently branched to contact both types of auditory hair cells. Unexpectedly, between P4 and P6, both IHC and OHC efferent axons produced additional branches that crossed the tunnel of Corti to invade the OHC zone. By P8, all of these supernumerary connections were eliminated, yielding completely segregated efferent pathways to IHCs and OHCs.     

Immunocytochemical detection of calcitonin gene-related peptide in the postnatal developing rat cochlea

Using the immunofluorescence technique, three populations of fibers staining for calcitonin gene-related peptide are identified in postnatal developing cochleas and in adult rat cochleas. During the maturation of the cochlea, the immunostaining first appears in the basal turn and then extends toward the apex of the cochlea. The first population of immunostained fibers belongs to the lateral olivocochlear innervation. It is observed at postnatal day 4 within the inner spiral bundle of the organ of Corti. The second population of calcitonin gene-related peptide-like immunostained fibers belongs to the medial olivocochlear innervation. It is first identified at postnatal day 6 under the outer hair cells, generally in the first row. In the older stages, this population of fibers progressively extends toward the external row of outer hair cells. Finally, the third population of immunostained fibers belongs to the sympathetic supply of the cochlea. They can be identified at postnatal day 6 around the cochlear artery and its branches, and also within the entire modiolus. Our results confirm the presence of calcitonin gene-related peptide in fibers of the lateral and medial efferent innervations of the cochlea. They indicate an early appearance of calcitonin gene-related peptide in the developing cochlea, before the onset of the cochlear function, suggesting for this peptide either a neurotrophic function or a regulation of the early cochlear potentials at the level of the lateral efferent synapses.     

Innervation of supporting cells in the apical turns of the guinea pig cochlea is from type II afferent fibers

The outer supporting cells in the apical turns of the guinea pig cochlea receive a dense innervation. Our previous study (Fechner et al. [1998] J. Comp. Neurol. 400:299-300) suggested that this innervation of the Deiters' and Hensen's supporting cells was not derived from efferent fibers of the olivocochlear bundle, but its origin has not been further specified. To test the hypothesis that the innervation was afferent in origin, we traced apical afferent fibers that were retrogradely labeled by extracellular injections of horseradish peroxidase. Labeled afferent fibers were of two types: type I fibers contacted inner hair cells, whereas type II fibers crossed the tunnel and contacted outer hair cells. Significantly, most of the type II fibers also formed branches to the outer supporting cells. Although a few olivocochlear efferent fibers formed such branches, counts indicated that the overwhelming majority of the branches were produced by type II afferent fibers. These branches were not produced by basal type II fibers. Apical type II fibers also differed from basal fibers by having shorter lengths, spiraling both apically and basally, and contacting all three rows of outer hair cells. These innervation differences suggest differences in the ways that information from outer hair cells is processed in the apex versus the base of the cochlea.     

Hair cell innervation by spiral ganglion neurons in the mouse

Horseradish peroxidase (HRP) was injected extracellularly into the auditory nerve of adult mice so that the enzyme could infuse individual spiral ganglion neurons. Forty-two well-stained neurons were reconstructed through serial sections from their cell bodies to peripheral terminations in the organ of Corti with the aid of a light microscope and drawing tube. No neuron was observed to innervate both inner and outer hair cells (IHCs and OHCs). Previous observations from neonatal mammals that reported that IHCs and OHCs were innervated by the same neuron are thus presumed to describe a transient developmental phenomenon. Two populations of spiral ganglion neurons were determined on the basis of the differences in receptor innervation. The type I neurons innervated exclusively IHCs by way of thick (1-2 microns) radial fibers, whereas the type II neurons innervated only OHCs by way of thin (approximately 0.5 micron) outer spiral fibers. Certain features of the peripheral process in the vicinity of the cell body were highly correlated with fiber type. This pattern of separate innervation of IHCs and OHCs by type I and type II neurons, respectively, may represent the general plan of afferent organization for the adult mammalian cochlea.     

Selective retrograde transport of nipecotic acid, a GABA analog, labels a subpopulation of gerbil olivocochlear neurons.

Perfusion of the gerbil cochlea with micromolar quantities of 3H-gamma-aminobutyric acid (GABA) results in rapid, selective labeling of 50-60% of the olivocochlear (OC) efferent terminals on afferent dendrites beneath the inner hair cells, and all of the efferent terminals beneath the outer hair cells. In order to identify the neurons from which these GABA-accumulating terminals originate, the cell bodies were localized by using retrograde transport of 3H-nipecotic acid, a metabolically inert GABA analog. With survival times of 6-30 hours after cochlear injection, myelinated OC efferent fibers and cell bodies were well labeled, with the greatest number being labeled at 12-18 hours. All of the labeled neurons belonged to the medial OC system, and no lateral OC neurons were labeled. It is concluded that the GABA-accumulating endings in the gerbil cochlea arise from medial OC neurons, and therefore that medial OC efferent neurons in this species project to both inner and outer hair cell regions.