 Institut für Klinische Physiologie / Inst. of Clinical Physiology |
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Institut für Klinische Physiologie / Inst. of Clinical Physiology |
| Charité > Campus Benjamin Franklin > Inst. Clin. Physiology > Gitter > cochlea anatomy |
Alfred H. Gitter
Anatomyof the cochlea
Cochlea
In the cochlea there are two tubular compartments that contain perilymph (extracellular fluid similar in composition to cerebrospinal fluid), the scala vestibuli and the scala tympani. Between the latter is the cochlear duct or scala media, which is filled with endolymph, a unique extracellular fluid with an ionic composition that is not found elsewhere in the mammalian body. It is rich in potassium and has a relatively low sodium concentration. In the cochlear duct, the organ of Corti rests on the basilar membrane and extends to the tectorial membrane (Fig. 1).
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Fig. 1 Transverse section of the cochlear duct. Within the petrous part of the temporal bone the auditory receptors are housed in a bony, coiled canal (canalis spiralis cochleae). On the inside of the outer, lateral wall (CC) is a connective tissue, the spiral ligament (LI). From the medial bony axis of the cochlea, the modiolus, extends an osseous lamina (lamina spiralis ossea, LO). On that rests the spiral limbus, from which a fibrous structure, the tectorial membrane, extends into the fluid of the cochlear duct, the endolymph. The lateral end of the osseous lamina is continued by the basilar membrane (lamina spiralis membranacea, BM) consisting of connective tissue. Below the basilar membrane is the scala tympani. Reissner's membrane (RM) originates medial on the upper surface of the limbus and stretches to the upper end of the spiral ligament. Above Reissner's membrane is the scala vestibuli. On the medial side of the spiral ligament is the stria vascularis (SV), a multilayered epithelium with many blood capillaries. The epithelium at the surface of the stria vascularis, facing the endolymph, is formed by the marginal cells (MZ). At the lower end of the stria vascularis, the spiral prominence (PS) bulges into the cochlear duct. The organ of Corti is between basilar membrane and tectorial membrane (Fig. 2). Between spiral ligament and organ of Corti is the outer sulcus (SE), and between spiral limbus, tectorial membrane and organ of Corti is the inner sulcus (SI). The cell bodies of the afferent nerves are in the spiral ganglion (GS). |
Endolymph is produced by the stria vascularis in the lateral wall of the cochlear duct, adjacent to the organ of Corti. It is possible, however, that the cells of the stria vascularis perform also other tasks than the homeostasis of ionic composition and positive electrical potential of the endolymph.
Organ of Corti
Cochlear hair cells and supporting cells form the sensory epithelium in the organ of Corti (Fig. 2).
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Fig. 2 The organ of Corti rests on the basilar membrane (BM) and is limited above by the tectorial membrane (TM). It comprises the auditory receptor cells, the hair cells (left: inner hair cell, right: outer hair cell), as well as different supporting cells. From inside (medial) to outside (lateral) of the coiled canal of the cochlea is the inner sulcus (SI), supporting cells enclosing the inner hair cells, two rows of (inner and outer) pillar cells (with the tunnel of Corti, CT, between them), 3 - 5 rows of Deiters' cells that form the basis of the outer hair cells, the cells of Hensen (HZ), and the outer sulcus. Afferent nerve fibers project from bipolar neurons in the spiral ganglion, located in Rosenthal's canal in the modiolus. The inner hair cells receive about 95% of the afferent nerve fibers, the inner spiral fibers. About 5% of the afferent nerve fibers, the outer spiral fibers, traverse the tunnel of Corti near its base and end at outer hair cells. Efferent nerve fibers from the superior olivary complex proceed in the olivocochlear bundle to the inner and outer hair cells. At the apical end of the hair cells extend in parallel rows the sensory hairs or stereocilia (SC). The stereocilia are modified microvilli with longitudinal microfilaments meshed by fimbrin. Adjacent stereocilia of the same cell are linked by different, threadlike (although not necessarily flexible) connectors. At the apical pole of the hair cell's cytoplasm is the cuticular plate (KP), which consists predominantly of unpolymerized actin. At the basis of inner hair cells are large endings of afferent nerve fibers (AN), while efferent nerves (EN) contact the afferent ones. At the basis of outer hair cells end efferent as well as afferent nerves; the latter have smaller endings. |
From the apical side of the hair cells protrudes a bundle of regularly arranged hair-like extensions, which are commonly called stereocilia. They are, however, not related to true cilia (containing tubulin), but to microvilli (containing actin). Thus a more appropriate name may be stereovilli. Two types of hair cells can be distinguished by morphology, function and innervation pattern; they are denoted by their different locations as inner and outer hair cells. The human cochlea contains 3200 pear-shaped inner and 12000 cylindrical outer hair cells. The basal end of the outer hair cell is set on a Deiters' cell, which rests on the basilar membrane. It is possible that Deiters' cells are involved in the regulation of extracellular cation (K+, Ca2+) concentrations. The basilar membrane contains different filaments and is elastic. The intercellular space in the organ of Corti widens around the outer hair cells; it contains a fluid similar to perilymph and the phalangeal processes of Deiters' cells. Inner hair cells, however, are enclosed by supporting cells with only a narrow intercellular space. The stereocilia on the apical pole of the hair cells jut into the the fluid below the tectorial membrane, which is generally assumed to be endolymph (although some researchers hold it to be similar to perilymph). The tips of the longest stereocilia of outer hair cells, but not of inner hair cells, insert into the tectorial membrane above the hair cells.
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