The inner ear (labyrinth) is made up of two parts, one within the other. The bony labyrinth is a series of channels in the petrous portion of the temporal bone and is filled with a fluid called perilymph, which has a relatively low concentration of K+, similar to that of plasma or the cerebrospinal fluid. Inside these bony channels, surrounded by the perilymph, is the membranous labyrinth. The membranous labyrinth more or less duplicates the shape of the bony channels and is filled with a K+-rich fluid called endolymph. The labyrinth has three components: the cochlea that contains hair cells (receptors) for hearing, semicircular canals that contain hair cells that respond to head rotation, and the otolith organs that contain hair cells that respond to changes in gravity and head tilt (Figure 1).
Fig1. The membranous labyrinth of the inner ear has three components: semicircular canals, cochlea, and otolith organs. The semicircular canals are sensitive to angular accelerations that deflect the gelatinous cupula and associated hair cells. In the cochlea, hair cells spiral along the basilar membrane within the organ of Corti. Airborne sounds set the eardrum in motion, which is conveyed to the cochlea by bones of the middle ear. This flexes the membrane up and down. Hair cells in the organ of Corti are stimulated by shearing motion. The otolithic organs (saccule and utricle) are sensitive to linear acceleration in vertical and horizontal planes. Hair cells are attached to the otolithic membrane. Information from the cochlear hair cells is carried by the cochlear division of the auditory (VIII cranial) nerve. Information from the hair cells in the semicircular canals and otolith organs is carried by the vestibular divisions of the auditory nerve.
The cochlea is a coiled tube that, in humans, is 35 mm long and makes two and three quarter turns. The basilar mem brane and Reissner membrane divide it into three chambers or scalae (Figure 2). The upper scala vestibuli and the lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema. At the base of the cochlea, the scala vestibuli ends at the oval window, which is closed by the footplate of the stapes. The scala tympani ends at the round window, a foramen on the medial wall of the middle ear that is closed by the flexible secondary tympanic membrane. The scala media, the middle cochlear chamber, is continuous with the membranous labyrinth and does not communicate with the other two scalae.
Fig2. Schematic of the cochlea and organ of Corti in the membranous labyrinth of the inner ear. Top: The cross section of the cochlea shows the organ of Corti and the three scalae of the cochlea. Bottom: This shows the structure of the organ of Corti as it appears in the basal turn of the cochlea. DC, outer phalangeal cells (Deiters cells) supporting outer hair cells; IPC, inner phalangeal cells supporting inner hair cell. (Reproduced with permission from Pickels JO: An Introduction to the Physiology of Hearing, 2nd ed. Academic Press; 1988.)
The organ of Corti on the basilar membrane extends from the apex to the base of the cochlea and thus has a spiral shape. This structure contains the highly specialized auditory receptors (hair cells) whose processes pierce the tough, membrane-like reticular lamina that is supported by the pillar cells or rods of Corti (Figure 2). The hair cells are arranged in four rows: three rows of outer hair cells lateral to the tunnel formed by the rods of Corti, and one row of inner hair cells medial to the tunnel. There are 20,000 outer hair cells and 3500 inner hair cells in each human cochlea. Covering the rows of hair cells is a thin, viscous, but elastic tectorial membrane in which the tips of the hairs of the outer but not the inner hair cells are embedded. The cell bodies of the sensory neurons that arborize around the bases of the hair cells are located in the spiral ganglion within the modiolus, the bony core around which the cochlea is wound. Ninety to 95% of these sensory neurons innervate the inner hair cells; only 5–10% innervate the more numerous outer hair cells, and each sensory neuron innervates several outer hair cells. By contrast, most of the efferent fibers in the auditory nerve terminate on the outer rather than inner hair cells. The axons of the afferent neurons that innervate the hair cells form the auditory (cochlear) division of the eighth cranial nerve.
In the cochlea, tight junctions between the hair cells and the adjacent phalangeal cells prevent endolymph from reaching the bases of the cells. However, the basilar membrane is relatively permeable to perilymph in the scala tympani, and consequently, the tunnel of the organ of Corti and the bases of the hair cells are bathed in perilymph. Because of similar tight junctions, the arrangement is similar for the hair cells in other parts of the inner ear; that is, the processes of the hair cells are bathed in endolymph, whereas their bases are bathed in perilymph.
On each side of the head, the semicircular canals are perpendicular to each other, so that they are oriented in the three planes of space. A receptor structure, the crista ampullaris, is located in the expanded end (ampulla) of each of the membra nous canals (Figure 1). Each crista consists of hair cells and supporting (sustentacular) cells surmounted by a gelatinous partition (cupula) that closes off the ampulla. The processes of the hair cells are embedded in the cupula, and the bases of the hair cells are in close contact with the afferent fibers of the vestibular division of the eighth cranial nerve.
A pair of otolith organs, the saccule and utricle, are located near the center of the membranous labyrinth. The sensory epithelium of these organs is called the macula. The maculae are vertically oriented in the saccule and horizontally located in the utricle when the head is upright. The maculae contain supporting cells and hair cells, surrounded by an otolithic membrane in which are embedded crystals of calcium carbonate, the otoliths (Figure 1). The otoliths, which are also called otoconia or ear dust, range from 3 to 19 μm in length in humans. The processes of the hair cells are embedded in the membrane. The nerve fibers from the hair cells join those from the cristae in the vestibular division of the eighth cranial nerve.
