Nerve fibers in the human organ of Corti

Cochlear Efferent Innervation Is Sparse in Humans and Decreases with Age

The mammalian cochlea is innervated by two cholinergic feedback systems called the medial olivocochlear (MOC) and lateral olivocochlear (LOC) pathways, which send control signals from the brainstem back to the outer hair cells and auditory-nerve fibers, respectively. Despite countless studies of the cochlear projections of these efferent fibers in animal models, comparable data for humans are almost completely lacking. Here, we immunostained the cochlear sensory epithelium from 23 normal-aging humans (14 males and 9 females), 0-86 years of age, with cholinergic markers to quantify the normal density of MOC and LOC projections, and the degree of age-related degeneration. In younger ears, the MOC density peaks in mid-cochlear regions and falls off both apically and basally, whereas the LOC innervation peaks near the apex. In older ears, MOC density decreases dramatically, whereas the LOC density does not. The loss of MOC feedback may contribute to the age-related decrease in word recognition in noise; however, even at its peak, the MOC density is lower than in other mammals, suggesting the MOC pathway is less important for human hearing.SIGNIFICANCE STATEMENT The cochlear epithelium and its sensory innervation are modulated by the olivocochlear (OC) efferent pathway. Although the medial OC (MOC) reflex has been extensively studied in humans, via contralateral sound suppression, the cochlear projections of these cholinergic neurons have not been described in humans. Here, we use immunostaining to quantify the MOC projections to outer hair cells and lateral OC (LOC) projections to the inner hair cell area in humans 0-89 years of age. We show age-related loss of MOC, but not LOC, innervation, which likely contributes to hearing impairments, and a relative paucity of MOC terminals at all ages, which may account for the relative weakness of the human MOC reflex and the difficulty in demonstrating a robust functional role in human experiments.

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.

Support for the central theory of tinnitus generation: a military epidemiological study

Tinnitus is poorly reflected by audiometric (cochlear) data, indicating that central nervous system (CNS) components are involved in its development. This study aimed to provide support for the neurophysiological theory of tinnitus as a result of combined peripheral and central nervous dysfunctions. Our main findings were the sudden. significant, stepwise increase in tinnitus after 10 years of service, as opposed to the almost linear increase in noise-induced hearing loss (NIHL) with age. Furthermore, the absence of a correlation between the incidence of tinnitus and the severity of tinnitus was linked to the NIHL. We suggest that, in tinnitus, the central screening apparatus which normally inhibits conscious awareness of irrelevant, spurious and non-informative internal and external noise shows a possibly fatigue- or age-related deterioration over time. Further support was provided by low blood levels of vitamin B1 and B12. which are essential to CNS function.

Scanning electron microscopy of nerve fibers in human fetal cochlea

The cochleas of four human fetuses ranging 22-25 weeks gestation were studied by scanning electron microscopy (SEM) for the purpose of obtaining a better understanding of the nerve fiber arrangement in the human ear. After critical point drying, the specimens were dissected and the floor of the tunnel of Corti and the outer wall of Nuel's space were exposed for observation. Upper cochlear turns, especially the apical turn, seemed to be still immature. Observed nerve fibers were classified into three types: 1. Spiral fibers: Fibers traveling basalward and following the shape of the cochlea were found in both the tunnel of Corti and Nuel's space and believed to be the afferent nerves responsible for innervating the outer hair cells. 2. Radial fibers: radiating outward from the osseous spiral lamina--one such radial fiber transversing high in the tunnel space (supposedly the efferent nerve servicing the outer hair cells), and another sort of radial fiber (found crossing the tunnel floor), the nature of which was uncertain 3. Irregular fibers: Consisting of thin, randomly running fibers within the cochlea. The destination of these fibers was not determined, but possibly they represent transitory nerve branchings of afferent or more probably efferent nerves, which would later regress during maturation.

Comparative anatomy of the cochlea and auditory nerve in mammals

The numbers and structure of hair cells; afferent, efferent, and reciprocal synapses as seen at the base of hair cells; innervation patterns of first order cochlear neurons; and number and morphology of spiral ganglion cells will be discussed and compared in the guinea pig, rat, cat, monkey and man. Despite many similarities both in the organ of Corti and the spiral ganglion in these species, there are a number of differences which may have important physiologic implications. In the organ of Corti, the major differences among species are the length and width of the basilar membrane, the number of inner and outer hair cells, and the length of hairs on both inner and outer hair cells. Significant differences in the innervation pattern of the inner hair cell among these species include the number of afferent nerve terminals per inner hair cell, the degree of branching of afferent fibers, and the number of synapses per afferent nerve terminal. Among outer hair cells, the number of afferent nerve terminals per outer hair cell, presence or absence of a pre-synaptic body, presence or absence of reciprocal synapses, the number of efferent terminals per outer hair cell, and the presence of dendodendritic synapses in outer spiral bundles may be differences important physiologically. In the spiral ganglion, there are significant differences in the number of spiral ganglion cells, the number of cochlear nerve fibers, the percentage of spiral ganglion cells which are myelinated, and the presence of synapses on spiral ganglion cells.

Anatomy of cochlear innervation

In this review of cochlear innervation, the differences in the innervation of outer and inner hair cells are emphasized. Of the afferent neurons, 90 to 95 per cent are large, myelinated type I neurons, exclusively connected in an essentially radial unbranched manner to the inner hair cells; 5 to 10 per cent are small, mostly unmyelinated type II neurons connected to the outer hair cells with considerable spiral extension and branching. The few small type II neurons, with their thin unmyelinated axons, probably have a minor functional importance for centripetal information transfer. The functional emphasis of the outer hair cell system is likely at the level of the receptor cells where the outer hair cells monitor receptor function. The efferent innervation also consists of at least two types of neurons. Small neurons from the lateral superior olivary nucleus project to the inner hair cell area in a predominantly homolateral fashion, making almost exclusively synaptic contacts with the afferent dendrites associated with the inner hair cells. Larger neurons from the medial nucleus of the trapezoid body and periolivary nucleus provide the abundant efferent nerve supply of the outer hair cells, predominantly contralateral. They have mostly large synaptic contacts, and, in some species exclusively, with the receptor cells, indicating again the functional emphasis of the outer hair cell system at the receptor cell level.

Early stages of innervation and sensory cell differentiation in the human fetal organ of Corti

Early development of the human organ of Corti was investigated at the light and electron microscopic level. In 9-week fetal material the cochlea was completely coiled and nerve fibers penetrated into an undifferentiated Corti's primordium. By week 10 a single cell in radial sections, presumed to be the inner hair cell (IHC), could be found with many nerve fibers surrounding and contacting its base. It is possible to identify outer hair cells by the end of the 11th week while IHC development continues. During the 12th week stereocilia appear on IHCs and synaptic specializations could be found at the junctions between both types of hair cell and afferent dendrites. The first appearance of the efferent fibers beneath IHCs were also observed during week twelve.

Nerve supply to the inner sensory cells in a human cochlea

A choclear apical turn taken from a 66-year-old woman showed an area with marked decrease in number of the inner sensory cells. After observation by scanning electron microscope, the same specimen was thin-sectioned tangentially to the osseous spiral lamina. Numbers of myelinated nerve fibers were counted and the thickness of the fiber was measured by computed measuring equipment. Compared to the area with inner sensory cells intact, the area without inner sensory cells showed a 70% decrease in number of myelinated fibers. These degenerated fibers might have had intimate relationship with the inner sensory cells, probably being the afferent nerve supply to the inner sensory cells.

Extra internal hair cells. A scanning electron microscopic study

The extra internal hair cell (EIHC) of the human cochlea was observed by means of a scanning electron microscope. The EIHC was found not infrequently in all turns of the human cochlea. It was located medial to the IHC row. The inner pillar cells showed an abnormal structure. The anatomical relationships between the displaced IHC and EIHC, and the inner pillar cell were classified into five types. The origin of these anomalies is discussed form an embryological viewpoint.

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.