Composition of inner ear fluids

The space between the bony and membranous labyrinths is filled with perilymph (Fig. 37.3B). The membranous labyrinth is filled with endolymph, a fluid produced by the marginal cells of the stria vascularis and the dark cells of the vestibule (see review by Wangemann & Schacht 1996) (Fig. 37.2B). Whatever their relative contributions, endolymph probably circulates in the labyrinth; it enters the endolymphatic sac, where it is transferred into the adjacent vascular plexus via the specialized epithelium of the sac. Pinocytotic removal of fluid may also occur in other labyrinthine regions.

Perilymph was initially considered to be an ultrafiltrate of plasma because of its low protein content. However, it more closely resembles cerebrospinal fluid in ionic composition, particularly in the scala tympani. Its composition is not precisely the same in both cochlear scalae: concentrations of potassium, glucose, amino acids and proteins are greater in the scala vestibuli. This has led to the suggestion that perilymph in the scala vestibuli is derived from plasma via the endothelial boundary of the cochlear blood vessels, whereas the perilymph in the scala tympani contains some cerebrospinal fluid derived from the subarachnoid spaces via the cochlear canaliculus. However, the lack of significant bulk flow suggests that perilymph homeostasis is predominantly locally regulated. Perilymph contains approximately 5 mM K⁺, 150 mM Na⁺, 120 mM Cl⁻ and 1.5 mM Ca²⁺. Endolymph contains greater K⁺ (150 mM) and Cl⁻ (130 mM) concentrations and lower Na⁺ (2 mM) and Ca²⁺ (20 μM) concentrations than perilymph. The major differences in ionic composition between the two fluids are important for the function of the inner ear. Displacements of the stereociliary bundles of the sensory cells activate relatively non-specific cationic channels in the stereociliary tips which allow an influx of cations, particularly K⁺ and Ca²⁺, from the endolymph. Hair cells also possess K⁺ channels activated by membrane voltage or intracellular Ca²⁺ concentrations, and these allow efflux of K⁺ into the perilymph which bathes their basal and lateral membranes. In addition, synaptic transmission at the base and sides of hair cells depends on the influx of Ca²⁺ from the perilymph through voltage-dependent calcium channels in order to release neurotransmitter.

Utricle

The utricle is the larger of the two major vestibular sacs. It is an irregular, oblong, dilated sac that occupies the posterosuperior region of the vestibule (Fig. 37.3A), and contacts the elliptical recess (where it is a blind-ended pouch) and the area inferior to it.

The macula of the utricle (or utriculus) is a specialized area of neurosensory epithelium lining the membranous wall, and is the largest of the vestibular sensory areas (Fig. 37.5). It is triangular or heart-shaped in surface view and lies horizontally with its long axis orientated anteroposteriorly and its sharp angle pointing posteriorly (Fig. 37.6). It is flat except at the anterior edge, where it is gently folded in on itself, and it measures 2.8 mm long by 2.2 mm wide. The mature form of the macula is reached early in development, but in the adult a bulge is often present on the anterolateral border; there is sometimes an indentation at the anteromedial border. The epithelial surface is covered by the otolithic membrane (statoconial membrane), a gelatinous structure in which many small crystals, the otoconia (otoliths, statoliths), are embedded. A curved ridge, the ‘snowdrift line’, runs along the length of the otolithic membrane. It corresponds to a narrow crescent of underlying sensory epithelium termed the striola, 0.13 μm wide. The density of sensory hair cells in this strip of epithelium is 20% less than in the rest of the macula. The striola is convex laterally and runs from the medial aspect of the anterior margin in a posterior direction towards, but not reaching, the posterior pole. The part of the macula medial to the striola is called the pars interna and is slightly larger than the pars externa, which is lateral to it. The significance of this area is that the sensory cells are functionally and anatomically polarized towards it (Fig. 37.6). The macula in each utricle is approximately horizontal when the head is in its normal position. Linear acceleration of the head in any horizontal plane will result in the otolithic membrane lagging behind the movement of the membranous labyrinth as a result of the inertia produced by its mass. The membrane thus maximally stimulates one group of hair cells by deflecting their bundles towards the striola whilst inhibiting others by deflecting their bundles away from it. Hence each horizontal movement of the head will produce a specific pattern of firing in the utricular efferents.

Fig. 37.5 Section of the utricular macula from a guinea pig, showing the relative positions of the hair cells and supporting cell nuclei. Semi-thin resin section, toluidine blue stain. (The inner ear is extremely vulnerable to hypoxia and situated in one of the hardest bones in the body, which means that well-fixed human tissue is rarely obtained for histology. Guinea pigs are one of the most frequently used animal models of human hearing and their inner ear ultrastructure is very similar.)

(By courtesy of RM Walsh, DN Furness and CM Hackney, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.)

Fig. 37.6 A, Morphological organization of the saccular and utricular maculae and the relationship of their hair cells to the otolithic membrane. The utricular macula has been tilted in the plane of the page to emphasize that it lies horizontally, whereas the saccular macula lies vertically when the head is in an upright position. Note the different shapes of the maculae, the position of the striola as indicated by a curved line in each case, and the different orientations of their stereociliary bundles. The arrows indicate the excitatory direction of deflection. B, Scanning electron micrograph of a fracture of a utricular macula (guinea pig) showing a type I hair cell (left) and a type II hair cell (right). C, The differing innervation patterns of the two types of hair cell.

(Part B by courtesy of DN Furness, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.)

Saccule

The saccule (or sacculus) is a slightly elongated, globular sac lying in the spherical recess near the opening of the scala vestibuli of the cochlea (Fig. 37.6). As already noted, it is connected to the utricle and endolymphatic duct by the utriculosaccular duct, and to the cochlea by the ductus reuniens, which leaves inferiorly to open into the base of the cochlear duct (Fig. 37.3A).

The saccular macula is an almost elliptical structure, 2.6 mm long and 1.2 mm at its widest point. Its long axis is orientated anteroposteriorly but, in contrast to the utricular macula, the saccular macula lies in a vertical plane on the wall of the saccule. Its elliptical shape is very slightly distorted by a small anterosuperior bulge. Like the utricular macula, it is covered by an otolithic (statoconial) membrane and possesses a striola, 0.13 mm wide, which extends along its long axis as an S-shaped strip about which the sensory cells are functionally and anatomically polarized (Fig. 37.6). The part of the macula above the striola is termed the pars interna, and that below it, the pars externa. The operation of the saccule is similar to that of the utricle. However, because of its vertical orientation, the saccule is particularly sensitive to linear acceleration of the head in the vertical plane, and is, therefore, a major gravitational sensor when the head is in an upright position. It is also particularly sensitive to movement along the anteroposterior axis.

Semicircular canals

The lateral, anterior and posterior semicircular ducts follow the course of their osseous canals. Throughout most of their length they are securely attached, by much of their circumference, to the osseous walls. They are approximately one-quarter of the diameter of their osseous canals (Fig. 37.3B). The medial ends of the anterior and posterior canals fuse to form a single common duct, the crus commune, before entering the utricle. The lateral end of each canal is dilated to form an ampulla, which lies within the ampulla of the osseous canal. The short segment of duct between the ampullae and utricle is the crus ampullaris.

The membranous wall of each ampulla contains a transverse elevation (septum transversum) on the central region of which is a sensory area, the ampullary crest (crista). This is a saddle-shaped ridge that lies transversely across the duct. It is broadly concave on its free edge along most of its length and has a concave gutter (planum semilunatum) at either end between the ridge and the duct wall. Sectioned across the ridge, the crests of the lateral and anterior semicircular canals have smoothly rounded corners; the posterior crest is more angular. A vertical plate of gelatinous extracellular material, the cupula, is attached along the free edge of the crest (Fig. 37.7). It projects far into the lumen of the ampulla so that movements of endolymph within the duct readily deflect the cupula and therefore also the stereocilia of the sensory cells that are inserted into its base. The three semicircular canals thus detect angular accelerations during tilting or turning movements of the head in any direction.

Fig. 37.7 Section of an ampullary crest.

Microstructure of the vestibular system

The maculae and crests detect the orientation of the head with respect to gravity and changes in head movement by means of the mechanosensitive hair cells that are interspersed among the non-sensory supporting cells in their sensory epithelia. These hair cells are in contact with afferent and efferent endings of vestibular nerve fibres by synapses at their base. The entire epithelium lies on a bed of thick, fibrous connective tissue containing myelinated vestibular nerve fibres and blood vessels. The axons lose their myelin sheaths as they perforate the basal lamina of the sensory epithelium. There are two types of sensory hair cell in the vestibular system, type I and type II.

Type I vestibular sensory cells measure 25 μm in length, with a free surface of 6–7 μm in diameter. The basal part of the cell does not reach the basal lamina of the epithelium. Each cell is typically bottle-shaped, with a narrow neck and a rather broad, rounded basal portion containing the nucleus (Fig. 37.6). The apical surface is characterized by 30–50 stereocilia (large, regularly-arranged, modified microvilli about 0.25 μm across) and a single kinocilium (with the typical ‘9 + 2′ arrangement of microtubules characteristic of true cilia). The kinocilium is considerably longer than the stereocilia, and may attain 40 μm, whereas the stereocilia are of graded lengths. They are characteristically arranged in regular rows behind the kinocilium in descending order of height, the longest being next to the kinocilium (Fig. 37.8). The kinocilium emerges basally from a typical basal body, with a centriole immediately beneath it. Close to the inner surface of their basal two-thirds, every cell contains numerous synaptic ribbons with associated synaptic vesicles (see p. 45). The postsynaptic surface of an afferent nerve ending encloses the greater part of the sensory cell body in the form of a cup (chalice or calyx). Efferent nerve fibres make synapses with the external surface of the calyx, rather than directly with the sensory cell.

Fig. 37.8 Scanning electron micrograph of a stereociliary bundle from the utricle (guinea pig). The stereocilia are arranged in rows of increasing height towards the tallest element, the kinocilium. Deflection in the direction of the kinocilium results in depolarization of the hair cell.

(By courtesy of DN Furness, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.)

There is much greater variation in the sizes of type II sensory cells (Fig. 37.9). Some are up to 45 μm long, and almost span the entire thickness of the sensory epithelium, whereas others are shorter than type I cells. They are mostly cylindrical, but otherwise resemble type I cells in their contents and the presence of an apical kinocilium and stereocilia. However, their kinocilia and stereocilia tend to be shorter and less variable in length. The most striking difference between type I and II cells is their efferent nerve terminals: type II cells receive several efferent nerve boutons containing a mixture of small clear and dense-core vesicles around their bases. Afferent endings are small expansions rather than chalices.

Fig. 37.9 Human vestibular hair cells (transmission electron micrograph). A, Type I cell (VR) bearing an apical group of stereocilia (ST) seen in a vertical section through the macula. Note that the hair cell is bottle-shaped, and that much of it is enclosed in the calyceal ending (C) of an afferent nerve terminal. SC, supporting cells. B, Human type II vestibular hair cell: a bouton-type afferent nerve terminal is in contact with the basal part.

(By courtesy of H Felix, M Gleeson and L-G Johnsson, ENT Department, University of Zurich and GKT School of Medicine, London.)

Each sensory cell is structurally and functionally polarized (Fig. 37.6). Deflection of the hair bundle towards the kinocilium results in depolarization of the hair cell, and increases the rate of neurotransmitter release from its base. Deflection away from the kinocilium hyperpolarizes the hair cell and reduces the release of neurotransmitter. The hair cells have specific orientations within each sensory organ. In the maculae, they are arranged symmetrically on either side of the striola. In the utricle, the kinocilia are positioned on the side of the sensory cell nearest to the striola. In the saccule, they are furthest from it. In the ampullary crests, the cells are orientated with their rows of stereocilia at right angles to the long axis of the semicircular duct. In the lateral crest the kinocilia are on the side towards the utricle, whereas in the anterior and posterior crests they are away from it. These different arrangements are important functionally, because any given acceleration of the head maximally depolarizes one group of hair cells and maximally inhibits a complementary set, thus providing a unique representation of the magnitude and orientation of any movement (for further details, see Furness 2002).

The type I and II sensory cells are set within a matrix of supporting cells that reach from the base of the epithelium to its surface, and form rosettes round the sensory cells, as seen in surface view. Although their form is irregular, they can easily be recognized by the position of their nuclei, which tend to lie below the level of sensory cell nuclei and just above the basal lamina (Fig. 37.5). The apices of the supporting cells are attached by tight junctions to neighbouring cells to produce the reticular lamina, a composite layer which forms a plate that is relatively impermeable to cations other than via the mechanosensitive transduction channels of the hair cells.

The otolithic membrane is a layer of extracellular material with a complex structure. It can be divided into two strata. The external layer is composed of otoliths or otoconia, which are barrel-shaped crystals of calcium carbonate with angular ends, up to 30 μm long, and heterogeneous in distribution (Fig. 37.6). They are attached to a more basal gelatinous layer into which the stereocilia and kinocilia of the sensory cells are inserted (Fig. 37.8). The gelatinous material consists largely of glycosaminoglycans associated with fibrous protein.

Endolymphatic duct and sac

The endolymphatic duct runs in the osseous vestibular aqueduct and becomes dilated distally to form the endolymphatic sac. This is a structure of variable size, which may extend through an aperture on the posterior surface of the petrous bone to end between the two layers of the dura on the posterior surface of the petrous temporal bone near the sigmoid sinus (Fig. 37.3A). The surface cells throughout the entire endolymphatic duct resemble those lining the non-specialized parts of the membranous labyrinth and consist of squamous or low cuboidal epithelium. The epithelial lining and subepithelial connective tissue become more complex where the duct dilates to form the endolymphatic sac. An intermediate or rugose segment and a distal sac can be distinguished. In the intermediate segment, the epithelium consists of light and dark cylindrical cells. Light cells are regular in form and have numerous long surface microvilli with endocytic invaginations between them and large clear vesicles in their apical region. In contrast, dark cells are wedge-shaped and have a narrow base, few apical microvilli and dense, fibrillar cytoplasm.

The endolymphatic sac has important roles in the maintenance of vestibular function. Endolymph produced elsewhere in the labyrinth is absorbed in this region, probably mainly by the light cells. Damage to the sac, or blockage of its connection to the rest of the labyrinth, causes endolymph to accumulate; this produces hydrops, which affects both vestibular and cochlear function. The epithelium is also permeable to leukocytes, including macrophages which can remove cellular debris from the endolymph, and to various cells of the immune system that contribute antibodies to this fluid.

A unique positive electrical potential exists in the endolymphatic spaces, varying from +77 mV in the cochlear duct near the stria vascularis to +44 mV in the utricle. This is additional to the normal resting potentials of the receptor cells so that there is a very considerable potential difference across their membranes. This undoubtedly contributes to the extreme sensitivity to mechanical deformation of the labyrinthine sensory receptors.

Basilar membrane

The basilar membrane stretches from the tympanic lip of the osseous spiral lamina to the basal crest of the spiral ligament (Fig. 37.2B and C). It consists of two zones. The thin zona arcuata stretches from the spiral limbus to the bases of the outer pillar cells and supports the organ of Corti. It is composed of compact bundles of small (8–10 nm diameter) collagenous filaments, mainly radial in orientation. The outer thicker zona pectinata starts beneath the bases of the outer pillar cells and is attached to the crista basilaris. The basilar membrane is trilaminar in the zona pectinata, but the upper and lower layers fuse at its attachment to the crista basilaris. The length of the basilar membrane is 35 mm; its width increases from 0.21 mm basally to 0.36 mm at its apex, accompanied by corresponding narrowing of the osseous spiral lamina and a decrease in the thickness of the basal crest. The lower or tympanic surface of the basilar membrane is covered by a layer of vascular connective tissue and elongated perilymphatic cells. One vessel, the spiral vessel (vas spirale), is larger; it lies immediately below the tunnel of Corti.

Organ of Corti

The organ of Corti consists of a series of epithelial structures that lie on the zona arcuata of the basilar membrane (Fig. 37.2B and C). The more central of these structures are two rows of cells, the internal and external pillar cells. The bases of the pillar cells are expanded, and rest contiguously on the basilar membrane, but their rod-like cell bodies are widely separated. The two rows incline towards each other and come into contact again at the heads of the pillars, enclosing between them and the basilar membrane the tunnel of Corti, which has a triangular cross-section (Fig. 37.2C). Internal to the inner pillar cells is a single row of inner hair cells. External to the outer pillar cells are three or four rows of outer hair cells. The bases of the outer hair cells are cupped by supporting cells called outer phalangeal (Deiters’) cells, except for a gap where cochlear axons synapse with them. The apical ends of the hair cells and apical processes of the supporting cells form a regular mosaic called the reticular lamina, which is covered by the tectorial membrane, a gel-like structure projecting from the spiral limbus. The reticular lamina is impervious to ions and thus maintains the electrochemical gradient between the fluids surrounding the apices and the basolateral membranes of the sensory hair cells. A narrow gap separates the tectorial membrane from the reticular lamina except where the apical stereocilia of the outer hair cells project to make contact with it.

In addition to the tunnel of Corti, other intercommunicating spaces, the spaces of Nuel, surround the outer hair cells. This entire intercommunicating complex of spaces of Nuel and tunnel of Corti is filled with perilymph, which diffuses through the matrix of the basilar membrane. The fluid in these spaces is also sometimes called the cortilymph; it is possible that minor alterations in perilymphatic composition occur within it, because it is exposed to the activities of synaptic endings and specialized excitable cells.

Each pillar cell has a base or crus, an elongated scapus (rod) and an upper end or caput (head) (Fig. 37.2C); each crus and caput are in contact, but the scapi are separated by the tunnel of Corti. Electron microscopy shows many microtubules, 30 nm in diameter, arranged in linked parallel bundles of 2000 or more in the scapus, originating in the crus and diverging above the scapus to terminate in the head region. The nucleus is situated in the foot-like expansion resting on the basal lamina.

There are almost 6000 internal pillar cells. Their bases rest on the basilar membrane near the tympanic lip of the internal spiral sulcus, and their bodies form an angle of approximately 60° with the basilar membrane. Their heads resemble the proximal end of the ulna, with deep concavities for the heads of the outer pillar cells, which they overhang. There are almost 4000 external pillar cells. They are longer and more oblique than the internal pillar cells, and form an angle of approximately 40° with the basilar membrane. Their heads fit into the concavities on the heads of the inner pillar cells and project externally as thin processes that contact the processes of the Deiters’ cells. The distances between the bases of the internal and external pillar cells increase from the cochlear base to its apex, whereas the angles they make with the basilar membrane diminish.

Cochlear hair cells are the sensory transducers of the cochlea: collectively, they detect the amplitude and frequency of the sound waves that enter the cochlea. All cochlear hair cells have a common pattern of organization. They are elongated cells with a group of modified apical microvilli or stereocilia (which contain parallel arrays of actin filaments) and a group of synaptic contacts with cochlear axons at their rounded bases (Fig. 37.10B). The inner hair cells form a single row along the inner edge of inner pillar cells (and the spiral tunnel), whereas the outer hair cells are arranged in three or, in some regions of the human cochlea, in four or even five rows, interspersed with supporting cells (Fig. 37.10A). These two groups have distinctive roles in sound reception; the differences in their detailed structure reflect this functional divergence. There are 3500 inner hair cells and 12,000 outer hair cells. The two sets of hair cells lean towards each other apically at about the same angles as the neighbouring inner and outer pillar cells. The geometric arrangement of these cells is very precise, and this pattern is closely related to the sensory performance of the cochlea.

Fig. 37.10 Scanning electron micrographs of the surface of the organ of Corti (guinea pig). A, The reticular lamina: three rows of V-shaped stereociliary bundles can be seen protruding from the apices of the outer hair cells. They are separated from the single row of inner hair cells (which have relatively linear stereociliary bundles) by the apices of the inner pillar cells. B, The stereociliary bundle of one outer hair cell, showing three rows of stereocilia increasing in height: deflection of the stereocilia in the direction of the tallest row results in depolarization of the hair cell. Microvilli can be seen on the surface of Deiters’ cells (front right).

(A and B by courtesy of DN Furness, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.)

The inner hair cells are pear-shaped and slightly curved; the narrower end is directed towards the surface of the organ of Corti and the wider basal end is positioned some distance above the inner end of the basilar membrane (Fig. 37.2C). The inner hair cells are surrounded by inner border cells and by inner phalangeal cells which are attached externally to the heads of the inner pillar cells. The flat apical surface of each inner hair cell is elliptical when viewed from above, its long axis directed in the direction of the row of hair cells (Fig. 37.10A). The breadth of the apex exceeds that of the inner pillar cells so that each inner hair cell is related to more than one inner pillar cell. The apex bears 50–60 stereocilia, arranged in several ranks of progressively ascending height, the tallest on the strial side. The tips of the shorter rows are connected diagonally to the sides of the adjacent taller stereocilia by thin filaments called tip links; each stereocilium is also connected to all its neighbours by lateral links. The length of a stereociliary row varies along the length of the cochlea, being longest at the apex and shortest at its base. The stereociliary bases insert into a transverse lamina of dense fibrillar material, the cuticular plate, which lies immediately beneath the apical surface of each inner hair cell. The cuticular plate includes a small aperture containing a basal body. During development, a kinocilium containing microtubules is anchored here, a condition which persists in vestibular hair cells.

At its base, each inner hair cell forms ten or more synaptic contacts with afferent endings, each being marked by a presynaptic structure similar to the ribbon synapses of the retina. Occasionally, an efferent synapse makes direct contact with a hair cell base but these are usually presynaptic to the terminal expansions of afferent endings, rather than to the hair cell itself.

Outer hair cells are long cylindrical cells which are nearly twice as tall as the inner hair cells (Figs 37.2C, 37.6, 37.10A). There is a gradation of length: the outermost row is longest in any one cochlear region, and those of the cochlear apex are taller than those of the base. They are surrounded by the apical or phalangeal processes of the Deiters’ cells or, on the internal side of the inner row, by the heads of the outer pillar cells. The stereocilia, which may number up to 100 per cell, are arranged in three rows of graded heights; the longest is on the outer side. The rows are arranged in the form of a V or W depending on cochlear region, the points of the angles directed externally. The stereocilia are also graded in length according to cochlear region: those of the cochlear base are shortest. Like those of inner hair cells, the stereocilia possess tip links and other filamentous connections with their neighbours, and are inserted at their narrow bases into a cuticular plate. The tallest stereocilia are embedded in shallow impressions on the underside of the tectorial membrane.

The rounded nucleus is positioned near the base of the cell. Below the nucleus are a few ribbon-like synapses associated with afferent endings of the cochlear nerve. The latter are fewer in number and smaller than the cluster of efferent boutons that contact the base of the cell. The neurotransmitter at the afferent synapse in both inner and outer hair cells is glutamate whereas that of the efferent endings is acetylcholine, although other neurotransmitters or neuromodulators have been demonstrated.

Cochlear hair cells respond with phenomenal speed and sensitivity to sound vibrations that cause submicron deflections of their stereociliary hair bundles. Outer hair cells not only detect these vibrations but also generate force to increase auditory sensitivity and frequency discrimination. (See Fettiplace & Hackney 2006, for further details about the sensory and motor functions of auditory hair cells.)

Deiters’ or phalangeal cells lie between the rows of outer hair cells. Their expanded bases lie on the basilar membrane and their apical ends partially envelop the bases of the outer hair cells (Figs 37.2C and 37.11). Each has a finger-like (phalangeal) process that extends diagonally upwards between the hair cells to the reticular membrane, where it forms a plate-like expansion that fills the gaps between hair cell apices.

Fig. 37.11 Scanning electron micrograph of a portion of the organ of Corti (guinea pig) dissected to expose the outer row of outer hair cells and their attendant Deiters’ cells with narrow phalangeal processes. The stereociliary bundles of two rows of outer hair cells are visible above the reticular lamina.

(By courtesy of DN Furness, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.)

Five or six rows of columnar supporting cells or external limiting cells, such as Hensen’s cells and Claudius’ cells lie external to the Deiters’ cells (Fig. 37.2C).

The apices of the hair cells and supporting cells that form the reticular lamina are linked by tight junctions, desmosomes and extensive gap junctions which couple them electrically. This arrangement is significant for two reasons. The reticular lamina creates a highly impermeable barrier to the passage of ions other than via the mechanotransducer channels in the stereociliary membranes. It also forms a rigid support between the apices of the hair cells, coupling them mechanically to the movements of the underlying basilar membrane, which causes lateral shearing movements between the stereocilia and the overlying tectorial membrane. If there is hair cell loss as a result of trauma such as excessive noise or ototoxic drugs, the supporting cells expand rapidly to fill the gap, disturbing the regular pattern of the reticular lamina (phalangeal scars), but restoring its function.

Tectorial membrane

The tectorial membrane overlies the sulcus spiralis internus and organ of Corti and is a stiff, gelatinous plate (Fig. 37.2B and C). It contains collagen types II, V and IX, interspersed with glycoproteins (tectorins), which contribute approximately half of the total protein.

In transverse section, the tectorial membrane has a characteristic shape. The underside is nearly flat and the upper surface is convex, and it is thin on the modiolar side where it is attached to the vestibular labium of the spiral limbus. Its outer part forms a thickened ridge, overhanging the edge of the reticular lamina. The lower surface is relatively smooth, except where the stereocilia of the outer hair cells are embedded in the membrane, leaving a pattern of W- or V-shaped impressions, an S-shaped ridge called Hensen’s stripe which projects towards the stereocilia of the inner hair cells. The interdental cells of the spiral limbus are believed to secrete the membrane.

Labyrinthine artery

The labyrinthine artery arises from the basilar artery, or sometimes from the anterior inferior cerebellar artery. It divides at the bottom of the internal auditory meatus into cochlear and vestibular branches. The cochlear branch divides into 12–14 twigs, which traverse the canals in the modiolus and are distributed as a capillary plexus to the spiral lamina, basilar membrane, stria vascularis and other cochlear structures. Vestibular arterial branches supply the utricle, saccule and semicircular ducts.

Vestibular nerve

The cell bodies of the bipolar neurones that contribute to the vestibular nerve lie in the vestibular ganglion, which is situated in the trunk of the nerve within the lateral end of the internal auditory meatus (Fig. 37.12B). Their peripheral processes innervate the maculae of the utricle and saccule and the ampullary crests of the semicircular canals (see below). Their axons travel to the CNS in the vestibular nerve, which enters the brain stem at the cerebellopontine angle, and terminates in the vestibular nuclear complex. Neurones in this complex project to motor nuclei in the brain stem and upper spinal cord, and to the cerebellum and thalamus. Thalamic efferent projections pass to a cortical vestibular area which is probably located near the intraparietal sulcus in area 2 of the primary somatosensory cortex.

Anatomy of balance and posture

The vestibular labyrinths on each side of the head are arranged symmetrically with respect to each other. Vestibular sensory pathways are concerned with perception of the position of the head in space and movement of the head; they also establish important connections for reflex movements that govern the equilibrium of the body and the fixity of gaze.

The vestibular system consists of two otolithic organs, the utricle and the saccule, and three semicircular canals. The otolithic organs detect linear acceleration due to gravitational pull (gravitoinertial acceleration) and the direction of other linear accelerations such as the up and down movements of the head that occur in running. They also respond when the head is tilted relative to gravity, so called pitch (forward and backward tilting) and roll (side-to-side tilting) movements. The semicircular canals detect angular accelerations resulting from rotations of the head or body.

The stereocilia in the apical hair bundles of the mechanosensitive hair cells in each of these organs are embedded in an overlying accessory gel-like structure, the otolithic membrane (in the utricle and the saccule) and the cupula (in the semicircular canals). Their apical surfaces are bathed in endolymph: tight junctional complexes between the apices of the hair cells and their adjacent supporting cells separate the endolymph from the perilymph that bathes their basolateral surfaces. Deflection of the stereocilia (caused by displacements of their overlying accessory membranes by fluid movements in the membranous labyrinth) produces either an increased or decreased rate of opening of the mechanotransduction channels at their tips, depending on whether they are deflected towards or away from the tallest row respectively. The change in the membrane potential of the receptor cell is signalled to the brain as a change in the firing frequency of the vestibular nerve afferents (either an increase or a decrease of the basal resting discharge, depending on the direction of stimulation). The signals are compared centrally with visual and somatosensory signals, which also signal the position of the head in space (for a more detailed account, see Furness 2002).

Semicircular canals

Angular acceleration and deceleration of the head cause a counterflow of endolymph in the semicircular canals, which deflects the cupola of each crista and bends the stereociliary/kinociliary bundles. When a steady velocity of head movement is reached, the endolymph rapidly adopts the same velocity as the surrounding structures because of friction with the canal walls, so that the cupula and receptor cells return to their resting state. The three semicircular canals are orientated at right angles to each other, which means that all possible directions of acceleration can be detected. Directional sensitivity to head movement is coded by opposing receptor signals: the left and right semicircular canals of each functional pair (e.g. the left and right superior canals), respond oppositely to any movement of the head that affects them (Fig. 37.13). Some vestibular neurones receive a bilateral input from vestibular receptors, which means that they can compare the discharge rates of right and left canal afferents, a mechanism that increases the sensitivity of the system.

Fig. 37.13 Response of the horizontal semicircular canals to head rotation in the horizontal plane. The firing rates of afferents from the left and right horizontal canals are equivalent at rest (A). However, when the head is turned to the right (B) or to the left (C), receptor depolarization and afferent fibre excitation occurs on the side to which the head turns; there is inhibition on the contralateral side.

Maculae

In the maculae, the weight of the otoconial crystals creates a gravitational pull on the otoconial membrane and thus on the stereociliary bundles of the sensory cells which are inserted into its base. Because of this, they are able to detect the static orientation of the head with respect to gravity. They also detect shifts in position according to the extent to which the stereocilia are deflected from the perpendicular. The two maculae are set at right angles to each other, and the cells of both maculae are orientated functionally in opposite directions across their striolar boundaries. Movement causes depolarization of the hair cells on one side of the striola and hyperpolarization of cells on the other side; because the striola is curved, small groups of hair cells on the macular epithelium each respond to a specific direction of head tilt or linear acceleration (Fig. 37.14). Moreover, because the otoconia have a collective inertia/momentum, linear acceleration and deceleration along the anteroposterior axis can be detected by the lag or overshoot of the otoconial membrane with respect to the epithelial surface, and so the saccular macula is able to signal these changes of velocity.

Fig. 37.14 Head tilt is encoded by a macular map of directional space. These diagrams depict the responses of the utricular maculae to head tilt. Firing rates in the vestibular afferents that innervate receptors on either side of the striola (red and blue lines) are equivalent when the head is upright (A). When the head is tilted to the right (B) or to the left (C), the stereocilia are deflected by displaced otoconia: hair cells on the upward slope side of the striola increase their firing rate, while those on the downward slope decrease their firing rate.

The macular receptors can also be stimulated by low frequency sound which sets up vibratory movements in the otoconial membrane, although this appears to require relatively high sound levels. Efferent synapses on the afferent endings of the type I sensory cells and on the bases of type II cells receive inputs from the brain stem which appear to be inhibitory. They serve to reduce the activity of the afferent fibres either indirectly, in the case of the type I cells, or directly, for the type II cells.

Visual reflexes

The vestibular system plays a major role in the control of visual reflexes, which allow the fixation of gaze on an object in spite of movements of the head, and require the coordinated movements of the eye, neck and upper trunk. Constant adjustments of the visual axes are achieved chiefly through the medial longitudinal fasciculus, which connects the vestibular nuclear complex with neurones in the oculomotor, trochlear and abducens nuclei and with upper spinal motor neurones (Fig. 37.15; see Fig. 39.12), and also by the vestibulospinal tracts.

Fig. 37.15 Some of the fibre components of the medial longitudinal fasciculus.

Abnormal activity of the vestibular input or central connections has various effects on these reflexes, e.g. the production of nystagmus. This can be elicited by the caloric test, a clinical test of vestibular function, by syringing the external auditory meatus with water above or below body temperature, a procedure which appears to stimulate the cristae of the lateral semi-circular canal directly. Spontaneous high activity in the afferent fibres of the vestibular nerve is seen in Ménière’s disease, in which those affected experience a range of disturbances including the sensation of dizziness and nausea, the latter reflecting the vestibular input to the vagal reflex pathway.

Cochlear nerve

Intratemporal cochlear nerve

The cochlear nerve connects the organ of Corti to the cochlear and related nuclei of the brain stem. The cochlear nerve lies inferior to the facial nerve throughout the internal acoustic meatus (see above). It becomes intimately associated with the superior and inferior divisions of the vestibular nerve, which are situated in the posterior compartment of the canal, and leaves the internal acoustic meatus in a common fascicle (Fig. 37.12A).

There are approximately 30–40,000 nerve fibres in the human cochlear nerve (for review, see Nadol 1988). Their fibre diameter distribution is unimodal, and ranges from 1 to 11 μm, with a peak at 4–5 μm. Functionally, the nerve contains both afferent and efferent somatic fibres, together with adrenergic postganglionic sympathetic fibres from the cervical sympathetic system.

Afferent cochlear innervation

The afferent fibres are myelinated axons with bipolar cell bodies that lie in the spiral ganglion in the modiolus (Fig. 37.2B; Fig. 37.16). There are two types of ganglion cell: most (90–95%) are large type I cells, the remainder are smaller type II cells (see reviews by Nadol 1988, Eybalin 1993). Type I cells contain a prominent spherical nucleus, abundant ribosomes and many mitochondria; in many mammals (although possibly not in humans) they are surrounded by myelin sheaths. In contrast, type II cells are smaller, always unmyelinated, and have a lobulated nucleus. The cytoplasm of type II cells is enriched with neurofilaments, but has fewer mitochondria and ribosomes than type I cells.

Fig. 37.16 Transmission electron micrograph showing several type II ganglion cells and nerve fibres in a human spiral ganglion. Note the absence of myelin from the surrounding sheaths of the ganglion cells.

(By courtesy of H Felix, M Gleeson and L-G Johnsson, ENT Department, University of Zurich and GKT School of Medicine, London.)

Each inner hair cell is in synaptic contact with the unbranched peripheral processes of approximately 10 type I ganglion cells. The processes of type II ganglion cells diverge within the organ of Corti and innervate more than 10 outer hair cells. The peripheral and central processes of type I ganglion cells are relatively large in diameter and are myelinated, whereas those of type II are smaller and unmyelinated. The peripheral processes of both types of cell radiate from the modiolus into the osseous spiral lamina, where the type I axons lose their myelin sheaths before entering the organ of Corti through the habenula perforata.

Three distinct groupings of afferent fibres have been identified: inner radial, basilar and outer spiral fibres (Fig. 37.17).

Fig. 37.17 A simplified view of the innervation of the organ of Corti (see text for further details). There is a great contrast between the convergent afferent innervation of the inner hair cells (approximately 10 fibres to each cell) and the divergent supply of the outer hair cells (one afferent fibre to 10 cells).

Inner radial fibres

The inner radial fibre group consists of the majority of afferent fibres. They run directly in a radial direction to the inner hair cells, each of which receives endings from several of these fibres.

Basilar fibres

Basilar fibres are afferent to the outer hair cells and take an independent spiral course, turning towards the cochlear apex near the bases of the inner hair cells. They run for a distance of about five pillar cells before turning radially again and crossing the floor of the tunnel of Corti, often diagonally, to form part of the outer spiral bundle.

Outer spiral bundles

The afferent fibres of the bundles of the outer spiral group course towards the basal part of the cochlea, continually branching off en route to supply several outer hair cells. The outer spiral bundles also contain efferent fibres (see below).

Efferent cochlear fibres

The efferent nerve fibres in the cochlear nerve are derived from the olivocochlear system (see reviews by Warr 1992, Guinan 1996). Within the modiolus, the efferent fibres form the intraganglionic spiral bundle, which may be one or more discrete groups of fibres situated at the periphery of the spiral ganglion (Fig. 37.17). There are two main groups of olivocochlear efferents: lateral and medial. The lateral efferents come from small neurones in and near the lateral superior olivary nucleus and arise mainly, but not exclusively, ipsilaterally. They are organized into inner spiral fibres that run in the inner spiral bundle before terminating on the afferent axons that supply the inner hair cells. The medial efferents originate from larger neurones in the vicinity of the medial superior olivary nucleus, and the majority arise contralaterally. They are myelinated and cross the tunnel of Corti to synapse with the outer hair cells mainly by direct contact with their bases, although a few synapse with the afferent terminals. The efferent innervation of the outer hair cells decreases along the organ of Corti from cochlear base to apex, and from the first (inner) row to the third. The efferents use acetylcholine, γ-aminobutyric acid (GABA), or both as their neurotransmitter. They may also contain other neurotransmitters and neuromodulators.

Activity of the medial efferents inhibits cochlear responses to sound: the strength of the activity grows slowly with increasing sound level. They are believed to modulate the micromechanics of the cochlea by altering the mechanical responses of the outer hair cells thus changing their contribution to frequency selectivity and sensitivity. The lateral efferents related to the inner hair cells also respond to sound. They appear to modify transmission through their postsynaptic action on inner hair cell afferents. The cholinergic fibres may excite the radial fibres, while those containing GABA may inhibit them, although their role is less well understood than that of the medial efferents (see review by Guinan 1996).

Autonomic cochlear innervation

Autonomic nerve endings appear to be entirely sympathetic. Two adrenergic systems have been described within the cochlea: a perivascular plexus derived from the stellate ganglion and a blood vessel-independent system derived from the superior cervical ganglion. Both systems travel with the afferent and efferent cochlear fibres and seem to be restricted to regions away from the organ of Corti. The sympathetic nervous system may cause primary and secondary effects in the cochlea by remotely altering the metabolism of various cell types and by influencing the blood vessels and nerve fibres with which it makes contact.

Anatomy of hearing

Sounds waves entering the external ear are converted into electrical signals in the cochlear nerve by the peripheral auditory system (Fig. 37.18). The axons in the cochlear nerve constitute the auditory component of the vestibulocochlear nerve and terminate in the dorsal and ventral cochlear nuclei: onward connections make up the ascending (central) auditory pathway.

Fig. 37.18 The principal activities of the peripheral auditory apparatus. For clarity, the cochlea is depicted as though it had been uncoiled, but it is normally coiled as in the inset. Different sound frequencies differentially excite different regions of the cochlea, the specific locations being given in kHz from 0.1 to 20 kHz in humans. Note that the frequency map is logarithmic, so that each decade occupies an equivalent distance on the basilar membrane. The components are drawn roughly to scale for the human ear, in which the cochlea is 35 mm in length. The points of maximal stimulation of the basilar membrane by high frequency and low frequency vibrations, together with their transmission pathways through the external and middle ear, are also indicated.

Peripheral auditory system

Vibrations in the air column in the external acoustic meatus cause a comparable set of vibrations in the tympanic membrane and auditory ossicles. The chain of ossicles acts as a lever which increases the force per unit area at the round window by 1.2 times while the reduction in size of the round window compared with the tympanic membrane increases the force per unit area of the oscillating surface a further 17 times. This overcomes the inertia of the cochlear fluids and produces in them pressure waves that are conducted almost instantaneously to all parts of the basilar membrane. The latter varies continuously in width, mass and stiffness from the basal to the apical end of the cochlea. Each part of the basilar membrane vibrates, but only the region tuned to a specific frequency will respond maximally to a pure tone entering the ear. A wave of mechanical motion, the travelling wave, is propagated along the basilar membrane to the position where it responds maximally and then dies away again. With increasing frequency, the locus of maximum amplitude moves progressively from the apical to the basal end of the cochlea. The pattern of vibrations in the basilar membrane thus varies with the intensity and frequency of the acoustic waves reaching the perilymph. Because of the arrangement of the hair cells on the basilar membrane, these oscillations generate a largely transverse shearing force between the outer hair cells and the overlying tectorial membrane (in which the apices of the hair cell stereocilia are embedded). This movement depends on the mechanical properties of the entire organ of Corti, including its cytoskeleton, which stiffens this structure. The inner hair cell stereocilia, which probably do not touch the tectorial membrane although they come very close to it, are likely to be stimulated by local movements of the endolymph. Displacement of the stereociliary bundle of a hair cell activates mechanoelectrical transduction (MET) ion channels near the tips of its stereocilia, and this allows potassium and calcium ions from the endolymph to enter the hair cell (see overviews by Fettiplace 2002, Fettiplace & Hackney 2006). This induces a depolarizing receptor potential and the release of neurotransmitter onto the cochlear afferents at the base of the cell. In this way a specific group of auditory axons is activated at the position of maximal basilar membrane vibration.

The mechanical behaviour of the basilar membrane is responsible for a rather broad discrimination between different frequencies (passive tuning, see overview by Ashmore 2002), but fine frequency discrimination in the cochlea appears to be related to physiological differences between the hair cells. Individual tuning of hair cells may result from differences in shape, stereociliary length, or possibly variations in the molecular composition of sensory membranes, and may have a role in cochlear amplification (active tuning).

The activity of the outer hair cells appears to play an important part in regulating inner hair cell sensitivity at specific frequencies. Outer hair cells can change length when stimulated electrically at frequencies of many thousands of cycles per second. The rapidity of these changes in length indicates a novel type of motile mechanism, which is believed to depend on conformational changes in proteins located in the plasma membrane of the cells (see Fettiplace & Hackney 2006) (Fig. 37.19). When the membrane potential of the outer hair cells changes, they generate forces along their axes. When the mechanoelectrical transduction channels open, they are thought to oppose the viscous forces which tend to damp down the vibration of the cochlear partition, and adjust the mechanics of the organ of Corti on a cycle-by-cycle basis. Alternatively they may alter the mechanics of the partition more slowly under the influence of the efferent pathway.

Fig. 37.19 The putative motors of outer hair cells. Outer hair cells can generate force, mechanically boosting sound-induced vibrations of the hair bundle and augmenting frequency tuning. Two mechanisms have been advanced to explain this cochlear amplifier: the somatic motor and the hair bundle motor. A, In the resting state, Cl⁻ ions are bound to prestin molecules in the lateral membrane of the hair cell. When force is applied to the hair bundle, the cell is depolarized, the Cl⁻ ions dissociate and the prestin changes conformation, reducing its area in the plane of the membrane and shortening the hair cell body (the somatic motor). Adaptation of mechanoelectrical transduction (MET) channels, which are activated by bending of the stereocilia at their tapered base, also causes the hair bundle to produce extra force in the direction of the stimulus (the hair bundle motor). The amplitudes of the hair bundle movements have been exaggerated to illustrate the concept. B, The effects of the somatic motor (blue arrows) on the organ of Corti mechanics, which leads to downward motion of the reticular lamina (the upper surface of the organ of Corti) and a negative deflection of the hair bundle. This is a negative feedback pathway, as a positive deflection of the hair bundle causes outer hair cell depolarization, cell contraction and opposing motion of the bundle. (See Fettiplace & Hackney 2006.)

At a particular frequency, an increase in the intensity of stimulus is signalled by an increase in the rate of discharge in individual cochlear axons. At greater intensities it is signalled by the number of activated cochlear axons (recruitment).

The respective roles of the two groups of hair cells have been much debated, particularly since differences in their innervation and physiological behaviour have become apparent. Because of their rich afferent supply, inner hair cells are believed to be the major source of auditory signals in the cochlear nerve. Some evidence for this view is based on the finding that animals treated with antibiotics that are specifically toxic to outer hair cells are still able to hear, but their sensitivity and frequency discrimination is impaired.

Some electrical responses of the cochlea can be recorded with extracellular electrodes. The most significant is the endolymphatic potential, a steady potential recordable between the cochlear duct and the scala tympani, which is caused by the different ionic compositions of their fluids. As the resting potential of hair cells is approximately 70 mV (negative inside) and the endolymphatic potential is positive in the cochlear duct, the total transmembrane potential across the apices of hair cells is 150 mV. This is a greater resting potential than is found anywhere else in the body, and provides the driving force for mechanotransduction and for the cochlear amplifier.

Under stimulation by sound, a rapid oscillatory cochlear microphonic potential can be recorded. It matches the frequency of the stimulus and movements of the basilar membrane precisely, and appears to depend on fluctuations in the conductance of hair cell membranes, probably of the outer hair cells. At the same time, an extracellular summating potential develops, a steady direct current shift related to the (intracellular) receptor potentials of the hair cells. Cochlear nerve fibres then begin to respond with action potentials which are also recordable from the cochlea. Intracellular recording of auditory responses from inner hair cells has confirmed that these cells resemble other receptors: their steady receptor potentials are related in size to the amplitude of the acoustic stimulus. At the same time, afferent axons are stimulated by synaptic action at the bases of the inner hair cells. They fire more rapidly as the vibration of the basilar membrane increases in amplitude, up to a threshold that depends on the sensitivity of the specific nerve fibre involved. Each inner hair cell is contacted by axons with response thresholds that range from 0 decibels sound pressure level (dBSPL), the approximate threshold of human hearing, to those which respond to intensities in excess of 100 dBSPL; the loudest sound tolerable is around 120 dBSPL. Each axon responds most sensitively to the frequency represented by its particular cochlear location, its characteristic frequency (Fig. 37.18).

Central auditory pathway

The primary afferents of the auditory pathway arise from cell bodies in the spiral ganglion of the cochlea. The axons travel in the vestibulocochlear nerve, which enters the brain stem at the cerebellopontine angle. Afferent fibres bifurcate, and terminate in the dorsal and ventral cochlear nuclei (Fig. 37.20). The dorsal cochlear nucleus projects via the dorsal acoustic stria to the contralateral inferior colliculus. The ventral cochlear nucleus projects via the trapezoid body or the intermediate acoustic stria to relay centres in either the superior olivary complex, the nuclei of the lateral lemniscus, or the inferior colliculus. The superior olivary complex is dominated by the medial superior olivary nucleus which receives direct input from the ventral cochlear nucleus on both sides, and is involved in localization of sound by measuring the time difference between afferent impulses arriving from the two ears.

Fig. 37.20 The main features of the human ascending auditory pathway. A, A series of sections showing that ipsilateral and commissural connections occur at most levels in this system. The major connections are shown by the thicker arrows; thinner arrows denote less heavy projections. B, The main stations of the auditory pathway.

The inferior colliculus consists of a central nucleus and two cortical areas. The dorsal cortex lies dorsomedially, and the external cortex lies ventromedially. Secondary and tertiary fibres ascend in the lateral lemniscus. They converge in the central nucleus, which projects to the ventral division of the medial geniculate body of the thalamus. The external cortex receives both auditory and somatosensory input. It projects to the medial division of the medial geniculate body, and, together with the central nucleus, also projects to olivocochlear cells in the superior olivary complex and to cells in the cochlear nuclei. The dorsal cortex receives an input from the auditory cortex and projects to the dorsal division of the medial geniculate body (see Ch. 21). Connections also run from the nucleus of the lateral lemniscus to the deep part of the superior colliculus, to coordinate auditory and visual responses.

The ascending auditory pathway crosses the midline at several points both below and at the level of the inferior colliculus. However, the input to the central nucleus of the inferior colliculus and higher centres has a clear contralateral dominance: during the initial stages of cortical auditory processing, both hemispheres respond most strongly to the contralateral ear. The medial geniculate body is connected reciprocally to the primary auditory cortex, which lies in the posterior half of the superior temporal gyrus and also dives into the lateral sulcus as the transverse temporal gyri (Heschl’s gyri) (see Ch. 23). Secondary areas of the auditory cortex are located in an adjacent belt region, and other regions of auditory association cortex have been described in a parabelt region beyond the secondary cortex.

The corpus callosum, particularly the posterior third of the body, contains auditory interhemispheric fibres that originate from the primary and second auditory cortices. Asymmetries of minicolumn number in primary and association auditory regions have been correlated with axonal fibre numbers in the subregions of the corpus callosum through which they project (Chance et al 2006).

The presence of tonotopic gradients in the primary auditory cortex is well established in animals and in humans. Hemispheric differences for frequency selectivity (i.e. the ability of the cochlea to separate the acoustic frequencies along its length like an acoustic prism) and tonotopic organization have been reported, for example, the right hemisphere appears to be most responsive to acoustic sound features such as pitch, whereas the left hemisphere seems to be more involved in processing temporal dynamics such as the phonological aspects of speech. Morphological asymmetries favouring the left hemisphere in the planum temporale and Heschl’s gyri have been correlated with left hemispheric dominance for language functions, but a direct link between structure and function has not been clearly established: studies often show relative rather than absolute differences in hemispheric specialization for particular attributes.

The transformation of the physical characteristics of sound into ‘auditory objects’ is thought to occur in the transition from primary to secondary auditory cortex. (For a critical perspective on auditory objects, see Griffiths & Warren 2004.)

Deafness

Hearing impairment is the most common disabling sensory defect in humans. Two causes of deafness are usually distinguished: conductive hearing loss and sensorineural hearing loss.

Conductive hearing loss may result from trauma to the external or middle ears, blockage of the external auditory meatus, or disruption of the tympanic membrane (e.g. by intense sounds or extreme pressure changes) (Ch. 36). It may also result from chronic inflammation of the tympanic membrane (e.g. by a cholesteatoma, which may also damage the ossicles); from an infection of the middle ear (otitis media with effusion), which produces a fluid build-up in the normally air-filled middle ear and so impedes the movements of the ossicles; or from otosclerosis, an inappropriate thickening of bone around the footplate of the stapes.

Sensorineural hearing loss is the most prevalent form of hearing impairment. It refers to loss or damage of the sensory hair cells or their innervation. The sensory cells of the inner ear are particularly vulnerable to mechanical trauma produced by high intensity noise and to changes in their physiological environment caused by infection or hypoxia. Changes in their ionic environment rapidly lead to degenerative processes that result in hair cell loss, often by apoptosis, and produce either hearing loss or vestibular dysfunction. These changes can be induced by drugs such as the aminoglycoside antibiotics, some diuretics, and certain anticancer drugs. A decrease in cochlear sensitivity, presbyacusis, almost invariably occurs with age: hair cells at the high frequency end of the cochlea tend to be lost first. At least 60% of hearing loss may have a genetic basis, a significant proportion may be non-syndromic, and most of these genes are inherited in an autosomal recessive mode.

Ménière’s disease is a distressing disorder of the inner ear characterized by episodes of hearing loss, tinnitus and vertigo. Histological examination of an affected ear reveals endolymphatic hydrops (swelling of the endolymphatic spaces), suggesting poor drainage of the endolymph via the endolymphatic sac.

Surgical approaches to the inner ear

The inner ear may be approached surgically from various directions. The promontory that overlies the basal turn of the cochlea and the oval window may be opened via the middle ear (after elevating the tympanic membrane). The lateral semicircular canal may be opened via the aditus (after widening the bony external acoustic meatus and removing the incus). The arcuate eminence may be opened to give access to the anterior semicircular canal via the floor of the middle cranial fossa. The posterior semicircular canal may be opened deep to the mastoid segment of the facial (Fallopian) canal via the mastoid process (after drilling away the overlying air cells). All of these approaches are usually reserved for destructive operations on the labyrinth to treat intractable vertigo.

The round window niche and its membrane may be approached via a posterior tympanotomy. In this procedure, the mastoid air cells are removed to allow access to the bony triangle bounded above by the fossa of incus, superficially by the chorda tympani, and deeply by the descending portion of the facial nerve. This bone is drilled away carefully to expose the facial recess of the tympanic cavity and the round window niche. Using this access, the stimulating electrode of a multichannel intracochlear implant can be passed into the scala tympani of the cochlea so that it lies against the spiral lamina and can stimulate the adjacent fibres of the cochlear nerve.

The endolymphatic sac may be approached after exenteration of the mastoid air cells by elevating the cortical bone of the anterolateral wall of the posterior cranial fossa, anterior to the sigmoid venous sinus and posterior to the posterior semicircular canal (below a line extended from the axis of the lateral semicircular canal). Access to the sac is required in some operations that aim to control vertigo secondary to Ménière’s disease.

The internal acoustic meatus may be approached, at a cost to hearing, by drilling away the entire bony labyrinth via the posterior cranial fossa (after craniectomy in the occipital region and retraction of the cerebellum), or via the middle cranial fossa (after a temporal craniotomy and retraction of the dura of the middle fossa and the temporal lobe). These approaches are usually used to access tumours of the cerebellopontine angle and internal acoustic meatus.