Situated in the inner ear and consists of a spiral tubular duct embedded in the petrous bone, lined by membranes. Membraneous compartments are filled with fluid and open into the vestibule of the inner ear at the cochlear base. Two membrane-covered openings in the bone at the base of the spiral, the oval and round windows. The oval window membrane is attached to the stapes, through which sound pressure waves enter the cochlear fluids. The round window functions to release the pressure induced by sound stimulation in the incompressible internal fluids preventing rupture of the internal membranes.
Pressure waves caused by sound travel through the cochlea They are analysed by a complex and delicate sensory epithelium situated within the cochlear duct, the organ of Corti.The basic function of the organ of Corti is to transduce and process sound stimuli, converting them into electrical signals in the auditory nerve for transmission to the higher auditory pathway.The stimulus is also analysed for frequency content and amplitude, both properties being encoded in the individual and ensemble discharge patterns of auditory nerve fibres.
Structure of the cochlear duct The cochlear duct is subdivided by two longitudinally running membranes that separate three chambers, the scala tympani, scala media and scala vestibuli. The organ of Corti runs in a spiral along the floor of the scala media, situated on its lower boundary, an acellular layer called the basilar membrane. The scala media is triangular in section, the other boundaries represented by Reissners membrane, which runs obliquely with respect to the basilar membrane from a ridge of tissue, the spiral limbus near the modiolus, to the lateral wall that runs along the inside of the bony wall.
The basilar membrane stretches across the cochlear duct from a bony shelf spiralling around the central bony modiolus , the osseous spiral lamina, to a bony promontory, the spiral prominence, on the inside of the outer wall of the cochlea. Although the cochlear duct widens substantially towards the basal end of the spiral, the width of the basilar membrane decreases, the difference being accounted for by the more rapidly increasing width of the osseous spiral lamina. The organ of Corti extends across the upper surface of the basilar membrane from the spiral limbus situated over the osseous spiral lamina to the Claudius cells that lie between the edge of the sensory region and the outer anchorage of the basilar membrane.
Underneath the basilar membrane is a layer of spindleshaped cells, the tympanic cells, whose long axes are orientated in an apical-basal direction along the cochlea. A branching spiral vessel lies under the basilar membrane.
The longitudinal ridge of the spiral limbus is composed of a layer of epithelial cells, the interdental cells, forming its upper surface and a main body containing blood vessels and connective tissue cells embedded in extracellular matrix. The side of the limbus facing the organ of Corti is concave. The concavity is lined by cells and forms a longitudinal groove, called the inner sulcus, which borders the region of the organ of Corti containing the sensory cells and the supporting cells. An acellular flap, the tectorial membrane forms a thin layer over the weakly convex top of the spiral limbus and projects over the inner sulcus and across the organ of Corti. It widens substantially in cross sectional area as it does so, to a maximum, then tapers again to a thin edge that lies over the outer side of the organ of Corti.
Reissners membrane consists of two layers of cells separated by a basement membrane. The layer facing into the scala tympani is the mesothelial cell layer and consists of cells with an extremely thin cytoplasm and prominent central nucleus. Facing the scala media is the endothelial cell layer, consisting of a greater density of thicker cells covered by a dense mat of microvilli. The cells within each layer are joined by tight junctions which act as an impermeable barrier to ions and small molecules.
The lateral wall consists of the stria vascularis, composed of three layers of cells on the external side of which is a layer of fibrocytes and connective tissue called the spiral ligament. Both regions are supplied with blood vessels. The three layers of the stria vascularis are composed of marginal cells facing the scala media, the intermediate cells and the basal cells. Intermediate cells tend to have highly convoluted membranes. The marginal cells also have tight junctions connecting them together.
The cells of the lateral wall contain a variety of ion pumps, enzymes and transport proteins associated with homeostatic mechanisms for maintaining the ionic composition of the fluids of the cochlea. In fact, the composition of the fluid within the scala media, endolymph, is unusual for an extracellular fluid, containing high potassium but low sodium levels at an unusually high positive electrical potential (+ 80 m V) called the endolymphatic potential (EP). This contrasts with the scala tympani and scala vestibuli, both of which are filled with perilymph that has high sodium content and 0 m V electrical potential. At the apex of the cochlea, these two outer chambers are joined via an aperture called the helicotrema.
Maintenance of the EP is crucial to hearing. As with both the stria vascularis and Reissners membrane, the cells of the organ of Corti facing the scala media are joined by tight junctions. Thus the whole of the scala media is chemically and electrically isolated from the other scalae, the only communication being through ion channels in the sensory cells of the organ of Corti. The EP is involved in driving currents through transduction channels that are fundamental to hair-cell function and is thus a vital component required for producing the high sensitivity to the cochlea.
Cellular architecture and function of the organ of Corti The organ of Corti extends with a repetitively patterned structure along the spiral for approximately 35 mm in humans. However, there are gradual variations longitudinally along the cochlear spiral that systematically change the mechanical characteristics of the organ of Corti-basilar membrane complex.
The sensory region consists of two types of sensory hair cell that are characterized by an apical bundle of hairs called stereocilia. In both cases, the stereo ciliary bundle projects into the overlying endolymph from the apical flattened surface. Inner hair cells usually form a single longitudinal row running along the inner side of the sensory region with respect to the centre of the spiral, whilst outer hair cells form three rows running along the outer side of the epithelium. The number of outer hair cell rows may increase to four or five over short, apparently random, lengths of the organ of Corti, especially in basal regions of the cochlea.
There may also be additional inner hair cells outside the normal single row. These two types of hair cell are separated by two rows of pillar cells forming a triangular arch-like structure in cross section and enclosing the tunnel of Corti running lengthwise The inner pillar row borders the inner hair cell row, and the outer pillar row borders the innermost of the three outer hair cell rows.
The hair cells have specific types of supporting cell associated with each of them. Inner phalangeal cells lying next to the pillar cells and border cells lying next to the first inner sulcal cells completely enclose the inner hair cells. The outer hair cells are enclosed only at their bases and their apices. The base is held in the cup-shaped body of a Deiters cell which extends a thin phalangeal process up to contact the apical part of adjacent outer hair cells. The origin of the process is from the top of the cell body and it projects at an angle towards the reticular lamina.
In the inner two rows of outer hair cells, the phalangeal process passes alongside the neighbouring outer hair cell to join the reticular lamina and form tight junctions between the more apical outer hair cell in the row behind, and its neighbour. Thus, the Deiters cell belonging to the base of one hair cell supports the apices of the next two along the cochlea, but in the row behind. In the case of the third row of outer hair cells, the Deiters cells have the same basic pattern, but the process terminates along the outer edge of the outermost row of outer hair cells thus forming a border to the hair-cell region. The arrangement of the Deiters cells and outer hair cells produces a repetitive triangular structure underneath the reticular lamina which is presumed to provide strong mechanical support for the outer hair cells.
The apical regions of both supporting cells and hair cells reach the upper surface of the organ of Corti. But only in the supporting cells do the basal ends rest on the basilar membrane, those of the hair cells held above the basilar membrane within the supporting cell framework. The upper surfaces of hair cells and their supporting cells, and the pillar cells, are held together by tight junctions. This system of junctions when viewed from above the organ of Corti gives the appearance of a network in which the hair cells and supporting cells are embedded, which has been named the reticular lamina. Beyond the third row of outer hair cells lie Hensens cells which contain large lipid droplets. The outer edge of the organ of Corti curves down from the reticular lamina, lined by the Hensens cells, to a single layer of cuboidal Claudius cells which cover the remainder of the basilar membrane out to the spiral prominence.
As well as the gradual narrowing of the basilar membrane, the length of outer hair cells and their supporting cells gradually decreases towards the basal end. They also tend to be longer in the outermost row and shorter in the innermost row, differences that are more pronounced near the apex of the spiral and diminish towards the base. The reticular lamina, which is oblique with respect to the basilar membrane near the apex, becomes more parallel to it near the base as the length of the outer hair cells decreases. Other systematic changes include a gradual decrease in the length and increase in the number of the stereocilia on both hair cell types towards the base and other ultrastructural features. Hensens cells become smaller and the tectorial membrane changes in cross-sectional area, becoming thicker near the basal end of the cochlea.
The functions of the organ of Corti are to detect sounds and decompose them into their component frequencies, in the process converting the physical vibrations into an electrical response (mechanoelectrical transduction) and then causing neural signals to be transmitted along the auditory nerve and higher auditory pathway for central processing. Sound pressure waves in cochlear fluids set up waves of motion along the basilar membrane (called travelling waves) which peak in different parts of the spiral according to frequency, resulting in mapping of high to low frequency components along its length. The morphological gradients described above contribute to this mapping by gradually varying the mechanical resonance properties of the basilar membrane-organ of Corti complex. The hair cells detect the basilar membrane motion, being stimulated more strongly at the point coinciding with the peak of the travelling wave than elsewhere.
The innervation pattern of the organ of Corti strongly suggests that inner hair cells contribute most to the neural signalling representing sensory transduction and processing by the cochlea. The cochlea is also known to contain an active amplifier that enhances the ability to detect and separate frequencies in sound. The outer hair cells are thought to represent this amplifier, mechanically boosting sound-induced vibrations of the basilar membrane to produce a sharply tuned, highly sensitive displacement pattern along the basilar membrane that is reported by inner hair cells.
Outer hair cells The apical part of the outer hair cell is the sensory end, bearing the stereociliary bundle, whilst the basal end consists of the rounded synaptic pole where the cell connects with afferent and efferent nerve fibres of the auditory nerve. The apical surface is flattened and, when viewed from above, triangular or heart-shaped whilst the cell bodies are cylindrical. The stereocilia are arranged in a very pronounced W- or V- shape. The number of rows can be as much as five. The rows increase in height across the bundle, like a staircase, along a radial axis from the modiolus to the lateral wall. In the case of the outer hair cells, the top of the bundle is in contact with the underside of the tectorial membrane whilst the inner hair stereocilia are probably free standing.
The stereocilia are cylindrical, narrowing sharply to an ankle region where they join the cell and, at least in the intermediate rows, with a bevelled tip. They are angled towards each other so that they converge at their tips. The shorter stereocilia are slightly narrower than the intermediate and taller stereocilia. Stereocilia contain a core of parallel actin filaments cross- linked by the actin associated proteins plastin and espin. In the lower third of the stereocilium, dense material becomes associated with the centre of the core. Along with a group of actin filaments, this extends down as a rootlet which contains tropomyosin into an actin-rich filamentous mat, the cuticular plate, located beneath the apical membrane. The stereocilia also contain several unconventional (nonmuscle) myosins and are enriched in calmodulin and other calcium handling proteins.
The stereocilia are connected together by extracellular filaments. A single filament, the tip link runs from the tip of each stereocilium of the shorter rows to the side of the adjacent stereocilium immediately behind. At the upper attachment of the tip link, there is a distinctive electron dense plaque lying between the membrane and the actin core. At the lower attachment there is dense material over the actin core, separated by a gap from the membrane of the tip. Below the lower attachment is a zone called the contact region where the membranes of the two converging stereocilia approach very closely. Lateral links connect the shafts of adjacent stereocilia, both within and between rows. Where the lateral links connect to the stereocilia, the membrane and the adjacent actin filaments inside show an increased density.
The stereo ciliary membrane contains proteins associated with calcium control (plasma-membrane calcium ATPases) and mechanosensitivity (mechanoelectrical transduction channels). The transduction channels are associated with the regions near the tips of the shorter stereocilia. During transduction, they are gated (opened) mechanically by movements of the hair bundle, which modulate their rate of opening and closing. Deflections of the bundle are driven by the motion of the basilar membrane-organ of Corti complex that causes shearing of the tectorial membrane parallel to the surface of the organ of Corti. This drives the outer hair cell stereociliary bundle backwards and forwards.
Deflections in the direction of increasing stereociliary height depolarize the outer hair cells by causing the channels to open and allow an influx of cations, whilst opposing deflections hyperpolarize the hair cells by closing the channels. The magnitude of the receptor potentials produced by depolarizing deflections is increased by the high positive EP (+ 80 m V) which, together with the resting membrane potential of the hair cell (around -70 mV), results in a much greater potential difference than normally present even in neurones of 150 m V between endolymph and the inside of the hair cell. This large driving force increases the sensitivity of the sensory cells substantially. The tip link represents a gating spring for the transduction channels, a physical mechanism to open them.
The hair bundle may also have an active role in mechanical amplification. The bundles show evidence of force production that produces active motion which adjusts the position of the bundle (adaptation). The rapid force production driven by a calcium-dependent process could enhance the mechanoelectrical transduction response to amplify the very smallest stimuli. Again, the high EP would make this amplification more effective. The unconventional myosins could interact with actin in the stereocilia providing one source of such force production. The cuticular plate contains actin filaments that are more randomly organized into a meshwork than the parallel bundle found in the stereocilium. Nevertheless, there are regional differences and structural features in the cuticular plate that indicate a high level of organization in both the
The cuticular plate is interrupted in the region just behind the stereociliary bundle and adjacent to its centre. Here, a channel of cytoplasm runs through the plate to the apical membrane. This channel contains a basal body. The cuticular plate acts as an anchoring structure for the stereocilia but could also be a site where the mechanical properties of the hair bundle are modified via the rootlets.
In the cell body, there is a concentration of Golgi apparatus, mitochondria and endoplasmic reticulum just beneath the cuticular plate and many outer hair cells contain a large body (Hensens body) composed of multiple concentric layers of membrane, especially in cells near the cochlear apex. These layers appear to be contiguous with a stack of submembraneous cisternae lying just beneath the plasma membrane and extending down the side from near the cuticular plate to the level of the nucleus. The nucleus is spherical and situated in the basal region of the cell. The plasma membrane contains a protein called prestin that changes shape when subjected to a voltage change. Isolated outer hair cells can be made to undergo very fast contraction-elongation cycles by electrical stimulation, a property called electromotility.
Isolated outer hair cells can be made to undergo very fast contraction-elongation cycles by electrical stimulation, a property called electromotility. The cortical lattice is believed to act as a cytoskeletal spring involved in converting the voltage-evoked conformational changes in prestin into the movement of the whole hair cell. This outer hair cell motility is generally regarded as the main source of active cochlear amplification An alternative or additional source of amplification may be the hair bundle. Synaptic specializations associated with afferent and efferent terminals are present in the basal pole.
The outer hair cells show other gradients in ultrastructural morphology along the length of the cochlear spiral and also across the three rows. These gradients are presumed to reflect differences in cell physiology and micro mechanics that contribute to the systematic changes in properties of the organ of Corti- basilar membrane complex.
Inner hair cells The inner hair cells have a flattened or slightly concave apical surface that is ovoid when viewed from the scala media and a flask-shaped cell body with a wide centre tapering basally and apically. The inner hair cell stereo ciliary bundle consists of three to four relatively linear rows of stereocilia, their long axes running parallel with the hair-cell row. However, there is often evidence of a shallow notch approximately halfway along where the rows indent slightly, and the ends of the bundle tend to curve round to some degree towards the modiolus. The effect of these features is to produce a shallow version of the more pronounced W-shape of outer hair cells, although the stereo ciliary arrangement is less precise.
Each row contains stereocilia of generally similar height, though tending to shorten near the ends of the row. The height of the rows increases in a step-wise manner across the bundle from the modiolar to the strial side. The stereocilia themselves are cylindrical actin-containing structures, with bevelled tips in the second tallest row and rounded tips in the others, and narrow ankles. The stereocilia have dense rootlets that penetrate into an apical cuticular plate and thus appear to be anchored within.
Externally, the stereocilia are again connected together by filamentous lateral links, which bind them both sideways and across the rows, and a contact region and tip links. The process of transduction by hair cells is similar to that described for outer hair cells. However, unlike the tallest stereocilia of the outer hair cells, the inner hair cell stereocilia do not appear to insert into the tectorial membrane. Thus, sound-induced motion of the basilar membrane is thought to stimulate the inner hair cell bundle via fluid motion of the endolymph between the tectorial membrane and the hair cell apex.
The cell body contains a region rich in Golgi bodies, endoplasmic reticulum, mitochondria and other evidence of synthetic activity just below the cuticular plate and the neck of the hair cell. The centre of the cell is occupied by a large spherical nucleus, with clusters of mitochondria and endoplasmic reticulum around it. Beneath the lateral plasma membrane in the neck region and extending down to the level of the nucleus is a single layer of cisternal membranes. The space between these membranes and the plasma membrane contains a cortical lattice of actin and spectrin filaments and rows of pillars connecting to the membrane similar to that of outer hair cells. However, there is no evidence of prestin in the membrane of the inner hair cells, and they do not appear to display electro motility like that of outer hair cells.
The basal end of the cell is the synaptic pole where the terminals of the afferent auditory nerve fibres make synaptic contact. This region is filled with vesicles and coated and uncoated membraneous tubules and has synaptic specializations called synaptic ribbons with associated synaptic vesicles. Depolarization of the hair cell is believed to result in calcium-dependent vesicular release of the amino acid glutamate onto the postsynaptic afferent terminal, which contains glutamate receptors. This depolarizes the nerve ending, resulting in the generation of action potentials. Efferent contacts directly onto inner hair cells are rare.
Deiters cells are associated with outer hair cells, and are also called outer phalangeal cells. The latter name arises from the presence of the phalangeal process that extends up from the body of the Deiters cell. The process is supported by one or more microtubular bundles that appear to originate in basal filamentous material and end in the apical junctional complex and associated material. The Deiters cell cup contains dense material directly beneath the outer hair cell base which is rich in actin and appears to be physically strongly attached to the hair-cell base.
The inner phalangeal and border cell lateral membranes are closely associated with that of the inner hair cell on the strial and modiolar sides respectively. Both cells are longer than the hair cell and jointly surround the basal pole before extending down to attach to the basilar membrane next to each other where their nuclei are located. At their tops, they are connected to the hair cell and other supporting cells by junctional complexes. Both cells display finger-like projections that interdigitate with afferent auditory nerve fibres as they approach the base of the inner hair cell. These cells are presumed to have a protective role because they strongly express transporters associated with glia-like uptake of glutamate after its release from the hair cells.
Between the two hair cell types are the two rows of inner and outer pillar cells, or rods of Corti. These cells are supported by a thick microtubular bundle emanating from apical and basal filamentous zones composed of actin and other cytoskeletal proteins. The two pillar cells have radial feet resting on the basilar membrane above which the pillars are angled towards each other, forming the arch over the tunnel of Corti, the rounded head of the outer pillar cell contacting the concave underside of the head of the inner pillar cell in a strong joint held together by junctional complexes. Thus, the inner pillar cell head curves over the top of the outer pillar cell head and forms a rectangular profile obscuring the top of the outer pillar cell when viewed from above the reticular lamina, and lying between the inner hair cells and first row of outer hair cells.
The outer pillar, however, produces a process from the side of the head region, which contains a microtubular bundle, and extends up to the reticular lamina in the direction of the stria vascularis. It surfaces between two adjacent outer hair cells of the first row. Like all other supporting cells in the organ of Corti, the outer pillar cell has an apical surface exposed to the endolymph. The cytoskeletal organization of pillar cells strongly suggests that they have an important role in providing physical support to the organ of Corti.
Acoustic information from the hair cells is transferred by the auditory portion of the VIIIth cranial nerve (the vestibulocochlear nerve) to the ipsilateral cochlear nuclear complex in the brain stem. The auditory nerve is composed of afferent fibres projecting from spiral ganglion neurones, the cell bodies of which reside in the modiolus, just central to the osseus spiral lamina. The spiral ganglion thus follows the course of the organ of Corti inside the modiolus. The neurones are of two types, type I that innervate the inner hair cells and type II that innervate the outer hair cells. The afferent innervation of the two hair cells types differs considerably in both number and distribution of fibres.
The majority of spiral ganglion neurones (up to 95 percent) are type I and innervate the inner hair cells in a convergent manner. Up to 20 type I neurones innervate each inner hair cell via a peripheral process that terminates on the hair-cell base at a ribbon synapse. These synapses consist of post-synaptic density on the afferent terminal and a presynaptic density on the hair cell membrane, attached to which is a dense presynaptic ribbon or bar. A single layer of synaptic vesicles usually clusters around the bar.
The cell body of the type I spiral ganglion neurone, its peripheral process and the central axon, which projects to the cochlear nucleus, are myelinated. The peripheral process becomes unmyelinated in the osseus spiral lamina just before it enters the organ of Corti through a hole (the foramen nervosum) in the upper border of the spiral lamina, the habenula perforata, to approach the inner hair cell. At the hair-cell base, each process widens into a bulb where the synapse forms. The central process enters the modiolus. At the apex of the cochlear spiral, the central processes enter the middle of the modiolus and, progressively down the spiral, more are added successively to the periphery of the nerve. Thus, as the nerve grows to its maximum diameter where it exits through the internal auditory meatus at the base of the spiral, low frequency fibres occupy the centre of the auditory nerve, with fibres of increasingly higher frequency found towards the periphery.
The responses of the type I auditory nerve fibres reflect the input from inner hair cells. When the inner hair cell is depolarized it releases a neurotransmitter, generally believed to be glutamate, from the presynaptic vesicles onto post-synaptic glutamate receptors on the nerve ending which itself then becomes depolarized. Provided the amount of depolarization is sufficient, this triggers action potentials in the nerve fibre.
As noted above, a fibre originating near the base of the cochlea will have its best response to a high frequency, and one originating near the apex to a low frequency. For any tone, the peak of the travelling wave will occur at the point of maximum resonance on the spiral for that frequency. If the tone is loud enough after cochlear amplification, there will be sufficient displacement of the inner hair cell stereocilia to depolarize the hair cell and evoke action potentials in the attached cochlear nerve fibre. The number of action potentials per second increases with increasing sound intensity and for frequencies below 5 kHz, the action potentials can become phase locked (i.e. occur at a particular point in the cycle) and so provide information on the frequency content of the stimulus as well as the intensity.
Each individual nerve fibre is thus defined by its characteristic frequency, which is the frequency for which it has its lowest threshold. For each fibre, the threshold increases (i.e. there is a weaker response) for tones that are higher or lower than the characteristic frequency, reflecting the change in position of the peak of the travelling wave and the narrowness of the peak. The latter depends on the cochlear amplifier. Thus, the sharpness of tuning of the basilar membrane organ of Corti complex is represented by the sharpness of tuning of the inner hair cell and the nerve fibre. This, in turn, ultimately determines the ability of the auditory system, to distinguish between sounds of different frequency. When the amplifier is impaired, for example through loss of outer hair cells, fibres will have higher thresholds and broader tuning, making separation of frequencies (frequency selectivity) poorer and requiring louder sounds to evoke a response.
The type II neurones innervate the outer hair cells in a divergent manner and tend to be smaller than the type I neurones. Their peripheral processes and central axons are unmyelinated. The peripheral processes enter the organ of Corti through the same route as that of the type I processes, and traverse the inner hair cell and inner pillar cell rows, crossing the floor of the tunnel of Corti as basilar fibres. They then travel towards the cochlear base for varying distances as outer spiral fibres, before branching to innervate up to ten outer hair cells each. The relatively small number of fibres to the outer hair cells, which are three times more numerous than inner hair cells, suggests they are not the primary signalling pathway from the cochlea, and indeed type II cells are difficult to record from using microelectrodes, unlike the type I cells.
Tthe auditory nerve alos contains myelinated and unmyelinated efferents that represent descending projections from the brainstem. The majority of the myelinated fibres end on outer hair cells directly, whereas the majority of unmyelinated efferent fibres end on the peripheral processes of afferent auditory nerves just below the inner hair cell.
SUMMARY OF FUNCTIONALPROPERTIES OF THE ORGAN OF CORTI- BASILAR MEMBRANE COMPLEX
The cochlea performs frequency analysis, splitting complex sounds up into component tones and signalling that information to the brain. Thus the cochlea responds to sound stimuli by: − producing travelling waves along the basilar membrane that peak more apically for decreasing sound frequency and whose amplitude reflects the intensity of the sound; − detecting the motion of the basilar membrane through deflection of the stereocilia which produce receptor potentials in the hair cells graded in size with the amplitude of the motion.
− enhancing the motion of the basilar membrane with a biomechanical amplifier residing in the stereo ciliary bundles and/or the outer hair cell lateral wall (somatic motility); enhancement is nonlinear and maximally amplifies motion at the point of maximum response, hence producing high frequency selective peaks in the travelling waves;− detecting the resultant basilar membrane motion through deflection of inner hair cell stereocilia which results in neurotransmitter release and the production of action potentials in the auditory nerve fibres.
− The frequency is encoded by • (1) the place of origin of the nerve fibres (i.e. those connected to the hair cells which are being stimulated most) and • (2) the timing of action potentials (for frequencies below 5 kHz), whilst intensity is coded by the rate of action potential firing.
The main evidence for the duplex activity of the hair cells in the cochlea is: − the differential distribution of innervation with afferent fibres mainly to inner hair cells and efferent fibres mainly to outer hair cells, indicating inner hair cells are the main signalling pathway; − selective loss of outer hair cells produced by amino glycoside antibiotics which causes a reduction in sensitivity and frequency selectivity (broadening of tuning), indicating their primary role in determining these properties compared with inner hair cells; − otoacoustic emissions (sound generated within the cochlea) which is evidence of active amplification;
− evoked motility of the outer hair cells in vitro and in vivo, the latter also capable of generating otoacoustic emissions which suggests they are the site of an active process;− active force production by the outer hair cell stereocilia that has been observed in the adult mammalian cochlea in vitro.
The output of the cochlea travels along auditory nerve fibres a short distance in the cochlear nerve before entering the brainstem. There are several regions that participate in the afferent auditory pathway between the cochlear nerve and auditory cortex. In ascending order, the most important of these are the cochlear nuclear complex, superiory olivary complex, inferior colliculus, medial geniculate nucleus and then auditory cortex (Figure 226.10). The pattern of connections between these nuclei is complex and not fully understood, and only the main nuclei and projections are represented in Figure 226.10. As can be seen, there are commissural connections at various points and multiple collaterals that make the pathway very intricate. What follows is a simplified description that covers the main stages of the auditory pathway and their likely functions.
The cochlear nuclear com plex The cochlear nuclear complex is subdivided into dorsal cochlear nucleus (DCN) and ventral cochlear nuclei, the latter composed of anteroventral cochlear nuclei (AVCN) and posteroventral cochlear nuclei (PVCN).20 These three regions are distinguishable on the basis of their location and cytoarchitecture (the range of cells of different morphology). The central processes of type I spiral ganglion neurones enter the cochlear nuclear complex and immediately bifurcate, sending branches to the DCN or PVCN and the AVCN. Low frequency fibres divide ventrally, and high frequency fibres dorsally so that the cochleotopic map of frequency, represented anatomically by the distribution of fibres in the auditory nerve, is maintained across the cochlear nuclei as a tonotopic map of neurones responding to progressively higher frequency from one side to the other. The auditory nerve afferents in the AVCN terminate on the principal projection neurones of the cochlear nuclear complex, the spherical/bushy cells, so called because they have round cell bodies and bushy dendritic fields (Figure 226. 1 1a-d). The end of most auditory nerve fibres expands into a single very large terminal, the end bulb of Held, which cups around the soma of the spherical cell (Figure 226. 1 1c) (very low frequency neurones, <1 kHz, may branch to form two endbulbs). One or two such terminals are found on each spherical cell. This large excitatory terminal contains large numbers of round neurotransmitter vesicles typical of glutamatergic terminals (Figure 226. 1 1e) and ensures rapid transmission of the signal from the auditory nerve fibre that faithfully preserves the original frequency selectivity and sensitivity of the cochlear response. Accordingly, these cells have electro physiological responses to sound that are called primary-like because they reflect the primary input from Auditry nerve fibres. However, in the DCN or PVCN, several auditory nerve fibres may contact a single multipolar (or stellate) cell, which have more complex responses. This multiple input means these cells
Also in the PVCN are octopus cells, which have an extended dendritic field lying across a number of auditory nerve fibres, so that they receive input representing a range of frequencies. These cells respond rapidly and may be responsible for determining the precise time of arrival of sounds. They also send signals to motor nuclei in the brain stem and midbrain so they may be involved in acoustic startle responses, where loud or unexpected sounds evoke movement. As well as these projection neurones, the cochlear nuclei contain interneurones and receive inputs from higher up the auditory pathway that produces inhibition and generates more complex responses in some neurones. In particular, it has been suggested that these complex responses in the DCN are important in determining what sounds are.
The superior olivary com plex The auditory pathway splits as it leaves the cochlear nuclear complex. The dorsal pathway projects directly to the inferior colliculus, the ventral pathway divides further and projects to both the ipsilateral and contralateral superior olivary complex (Figure 226.10). This makes the superior olivary complex the first part of the ascending auditory pathway where major binaural comparisons can be made. The superior olives receive binaural information from spherical/bushy cells. This arises from collaterals from the output fibres of the cochlear nuclei on the same side that then cross over to the opposite superior olivary complex. This enables the superior olives to function in sound localization. Each superior olivary complex contains an Sshaped lateral olivary nucleus, a disc-shaped medial olivary nucleus (Figure 226. 10) and the medial nucleus of the trapezoid body together with smaller periolivary nuclei. Within the medial olivary nucleus, there are neurones that use the binaural inputs to compare the time of arrival of sounds to each ear. For example, for a sound coming from the left of a persons head, the left ear would receive the sound first, the right ear second because of the difference in distance to the two ears. The further to the left, the greater the difference in time of arrival. From experimental work in owls, computation of the interaural time delay would allow the medial olivary nucleus to localize a sound.21 In fact, a spatial map seems to be present, represented by gradual changes in the responses of the neurones across the anterior-posterior axis of the medial olive to specific interaural time differences. This method of localization would work for discontinuous sounds and for relatively low frequency continuous tones, but breaks down at higher frequencies for continuous tones because individual auditory nerve fibre responses cannot encode accurate timing information above 5 kHz due to limitations on the rate of firing of action potentials.
Sound localization at higher frequencies may be carried out by comparing sound intensities.22 If a sound source is on the left of a persons head, as before, it is closer to the left ear than to the right and so sounds louder. Neurones that detect differences in sound intensity are located in the lateral superior olives. Most of these neurones receive an excitatory input from the ipsilateral cochlear nucleus and an inhibitory one from the contralateral cochlear nucleus. Thus if a sound is of equal intensity in both ears because it originates on the midline, the inhibition and excitation of the binaural neurones between the two nuclei balances out. If, however, a sound is louder in the left ear, the excitation will be stronger for the neurones in the ipsilateral olive and the inhibition weaker enhancing the ipsilateral response, whilst the converse will hold true for the neurones in the contralateral olive, which will have a much weaker response.
Inferior col liculus There are four bumps on the surface of the midbrain, which together form the corpora quadrigemina. These are composed of the two superior and two inferior colliculi. The inferior colliculi receive direct input from the brainstem auditory nuclei via a tract called the lateral lemniscus. Each of the inferior colliculi consists of a central nucleus which receives the major auditory input, and an outer region composed of a dorsal cortex and an external lateral cortex. The external portions of the inferior colliculus receive connections from cerebral cortex and from mutimodal sources respectively. In the inferior colliculi, the two pathways that emerge from the cochlear nuclear complex join together again for further analysis. More complex responses are found in inferior collicular neurones, and further features are extracted and mapped towards understanding what a sound is. The central nucleus is layered into isofrequency bands (Figure 226. 1 2a). Along each band, the cells have flattened dendritic fields and respond best to approximately the same frequency. The higher frequency bands are found towards the midline of the brain, low frequency bands more towards the outside, producing a tonotopic map. Superimposed on each band is another map that relates to intensity. This is best visualized by thinking of each isofrequency band as a disc. The cells in the centre of the disc have low thresholds which means they respond to quiet sounds, whilst moving out to the periphery of the disk there are concentric areas in which the threshold of the neurones increases, hence requiring louder and louder sounds to stimulate them (Figure 226. 12b). There are also neurones that respond to timevarying stimuli,22 such as changes in frequency (frequency modulated - for example, a sound moving towards you or away, such as an aeroplane) or in intensity (amplitude modulated - for example, an air-raid siren) . These responses also seem to be mapped approximately from front to back across each inferior colliculus.
These intersecting maps in the inferior colliculi are thus able to extract complex features of sounds. At the next nucleus in the auditory pathway, cells have been found that respond to particular complex sounds, for example, the mew of a kitten. These maps in the inferior colliculus thus provide a basis for recognizing patterns in sound. There are also neurones in the inferior colliculus that are involved in sound localization; as many as threequarters of the neurones may have binaural responses. These neurones may provide input to the superior colliculus in which there are visual and auditory space maps that can therefore be compared so that sounds can be assigned to specific objects. The inferior colliculus is also involved in auditorymotor responses, for example, controlling middle ear muscles, which can be used to attenuate loud sounds and protect the ear. In addition, there are projections to motor nuclei that contribute to turning the head or moving the eyes in response to sound.
The m ed i a l g e n i c u l ate n u clei and a u d itory co rtex The thalamus contains three regions where auditory influence is known to occur, the medial geniculate body, the posterior nucleus and part of the reticular nucleus of the thalamus. The most important for auditory function are the geniculate nuclei which are bilateral rounded regions lying on the surface of the thalamus. These nuclei have three major divisions each receiving a separate, parallel pathway from the inferior colliculus. The ventral division is organized to no topically into isofrequency layers within which there are ordered maps of neurones responding to different auditory cues and is the tonotopic pathway, receiving its input from the central nucleus of the inferior colliculus. Secondly, the diffuse pathway enters the dorsal division and is not tonotopically organized, arising from the dorsal cortex of the inferior colliculus. The cells here respond to many different frequencies, many only to complex sOUIlds.23 The medial division receives multimodal inputs from external lateral cortex of the inferior colliculus involving several other sensory systems including the auditory system. Neurones within this division thus respond to one or more modality and can be modified by learning.
The main projection to the primary auditory cortex arises from the ventral division of the medial geniculate nucleus and terminates in area Al, corresponding to Brodmanns area 4l in the human brain, within the lateral fissure of the temporal lobe (Figure 226.13a). The dorsal division of the medial geniculate nucleus projects to the non-primary auditory areas around Al, and the medial division projects diffusely to the whole region and to surrounding cortical fields. Al, like the preceding auditory nuclei, is organized into isofrequency layers arranged tonotopically from low frequency in the rostral end to high frequency in the caudal end (Figure 226.1 3b),z4 Most cells within Al respond t o binaural stimulation. There are two main types of response: neurones that summate excitatory responses from both ears and neurones that receive excitatory stimulation from one ear and inhibitory stimulation from the other. Bands of cells displaying excitation-excitation and excitation-inhibition responses run alternately across the isofrequency layers (Figure 226. 1 3c) . The main function of these cells, and of primary auditory cortex in general, appears to be sound localization. Complex responses can be found in neurones in the areas surrounding Al. These include responses to features such as specific delays between significant parts of a complex sound, and the simultaneous occurrence of harmonically related frequencies. These types of feature extraction are likely to be important in the analysis of time-varying acoustical signals such as human speech.23
There are descending projections from each of the stations of the ascending auditory pathway, down as far as the cochlear nuclei and from the superior olivary complex to the cochlea. Some of these descending projections may participate in attention level and anticipation of signals. The olivo cochlear feedback loop is a major descending projection. Following afferent input to the superior olives, fibres completing this loop originate from neurones in or around the superior olivary complex and project back along the auditory nerve into the cochlea. Those originating adjacent to the contralateral medial superior olives, which cross the midline and are myelinated, form the majority and constitute the crossed olivocochlear bundle. They contribute largely to the efferent projection to the outer hair cells (Figure 226.9c), also known as the medial efferent system. The fibres end in relatively large primarily cholinergic nerve terminals next to a subsynaptic cistern in the hair cells, and contain large numbers of vesicles (Figure 226.ge ) . It is believed they function to suppress outer hair cell motility to make the cells less sensitive, providing protection from very loud sounds. A smaller number of unmyelinated efferents originate from the lateral superior olive ipsilaterally and contribute mainly to the efferent projection synapsing with the peripheral processes of type I spiral ganglion neurones beneath the inner hair cells (Figure 226.9f), although a few terminate directly on the hair cells. This has been called the lateral efferent system and may comprise two functional subdivisions, capable of inducing either slow increases or decreases in the magnitude of the response of auditory nerve fibres. Since these fibres originate in the lateral superior olive, which is involved in sound localization, they may be useful in maintaining accurate binaural comparisons during slow changes in interaural sensitivity.25
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