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INNERVATION

VESTIBULOCOCHLEAR NERVE

The vestibulocochlear nerve emerges from the cerebellopontine angle (see Fig. 19.3). It courses through the posterior cranial fossa to enter the petrous temporal bone via the internal acoustic meatus, where it divides into an anterior trunk, the cochlear nerve, and a posterior trunk, the vestibular nerve (Fig. 37.12A; see Fig. 27.8). Both contain the centrally directed axons of bipolar neurones, together with a smaller number of efferent fibres that arise from brain stem neurones and terminate on cochlear and vestibular sensory cells. In humans, the intratemporal portion of the vestibulocochlear nerve consists of two histologically distinct portions: a central glial zone adjacent to the brain stem, and a peripheral or non-glial zone (Bridger & Farkashidy 1980). In the glial zone the axons are supported by central neuroglia, whereas in the non-glial zone they are ensheathed by Schwann cells. The non-glial zone extends into the cerebellopontine angle medial to the internal acoustic meatus in more than 50% of human vestibulocochlear nerves. During development, a gap of several weeks has been reported between the onset of Schwann cell myelination distally and glial myelination proximally: it has been suggested that the gap may coincide with the time of the final maturation of the organ of Corti. (For further details about the development of the human cochlear nerve see Ray et al 2005.)

  

Fig. 37.12  The vestibulocochlear nerve (human). A, Transverse section. The cochlear nerve (comma-shaped profile on the left) abuts the inferior division of the vestibular nerve (right). The singular nerve is a separate fascicle between the superior and inferior divisions of the vestibular nerve. B, A portion of a vestibular ganglion, showing neuronal perikarya, myelinated axons and small blood vessels.
(Toluidine blue stained resin sections by courtesy of H Felix, M Gleeson and L-G Johnsson, ENT Department, University of Zurich and School of Medicine, King’s College London.)

In audiological practice, it is important to distinguish between intratemporal and intracranial lesions. However, this surgical distinction does not correlate with the precise anatomical description of peripheral and central portions of the auditory and vestibular systems. Clinically, the term ‘peripheral auditory lesion’ is used to describe lesions peripheral to the spiral ganglion, and the term ‘peripheral vestibular disturbance’ includes lesions of the vestibular ganglion and the entire vestibular nerve.

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.

Vestibular (Scarpa’s) ganglion

The cell bodies of the neurones in the vestibular ganglion vary considerably in size: their circumferences range from 45 to 160 μm (Felix et al 1987). No topographically ordered distribution relating to size has been found. The cell bodies are notable for their abundant granular endoplasmic reticulum, which in places forms Nissl bodies, and prominent Golgi complexes. They are covered by a thin layer of satellite cells and are often arranged in pairs, closely abutting each other so that only a thin layer of endoneurium separates the adjacent coverings of satellite cells. This arrangement has led to speculation that ganglion cells may affect each other directly by electrotonic spread (ephaptic transmission: see Felix et al 1987).

Two distinct sympathetic components have been identified in the vestibular ganglion: a perivascular adrenergic system derived from the stellate ganglion, and a blood vessel-independent system derived from the superior cervical ganglion.

Intratemporal vestibular nerve

The peripheral processes of the vestibular ganglion cells are aggregated into definable nerves, each with a specific distribution. The main nerve divides at and within the ganglion into superior and inferior divisions, which are connected by an isthmus. The superior division, the larger of the two, passes through the small holes in the superior vestibular area at the fundus of the internal acoustic meatus (Fig. 37.4) and supplies the ampullary crests of the lateral and anterior semicircular canals via the lateral and anterior ampullary nerves, respectively. A secondary branch of the lateral ampullary nerve supplies the macula of the utricle; however, the greater part of the utricular macula is innervated by the utricular nerve, which is a separate branch of the superior division. Another branch of the superior division, Voit’s nerve, supplies part of the saccule.

The inferior division of the vestibular nerve passes through small holes in the inferior vestibular area (Fig. 37.4) to supply the remainder of the saccule and the posterior ampullary crest via saccular and singular branches, respectively; the latter passes through the foramen singulare. Occasionally, a very small supplementary or accessory branch innervates the posterior crest; it is probably a vestigial remnant of the crista neglecta, an additional area of sensory epithelium found in some other mammals but seldom in man.

Afferent and efferent cochlear fibres are also present in the inferior division of the vestibular nerve, but leave at the anastomosis of Oort to join the main cochlear nerve (see review by Warr 1992). Another anastomosis, the vestibulofacial anastomosis, is situated more centrally between the facial and vestibular nerves, and is the point at which fibres originating in the intermediate nerve pass from the vestibular nerve to the main trunk of the facial nerve.

There are approximately 20,000 fibres in the vestibular nerve of which 12,000 travel in the superior division and 8000 travel in the inferior division. The distribution of fibre diameters is bimodal, with peaks at 4 μm and 6.5 μm. The smaller fibres go mainly to the type II hair cells and the larger fibres tend to supply the type I hair cells. In addition to the afferents, efferent and autonomic fibres have been identified. Efferent fibres synapse exclusively with the afferent calyceal terminals around type I cells and usually with the afferent boutons on type II cells, although a few are in direct contact with the cell bodies of type II cells. The autonomic fibres do not contact vestibular sensory cells, but terminate beneath the sensory epithelia.

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.

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