Membranous labyrinth
643
83
reTPaHC
the helicotrema (see Fig. 38.1C). From Rosenthal’s canal, many tiny
canals, the habenula perforata, radiate through the osseous lamina to Facial nerve area
its rim, where they each carry a fascicle of the cochlear nerve through
the foramen nervosum to the organ of Corti. A secondary spiral lamina
Superior
projects inwards from the outer cochlear wall towards the osseous spiral vestibular area
lamina and is attached to the outer edge of the basilar membrane. It is Transverse crest
most prominent in the lower part of the first turn; the gap between the
Inferior
two laminae increases progressively towards the cochlear apex, which
vestibular area
means that the basilar membrane is wider at the apex of the cochlea
than at the base.
Tractus spiralis
Foramen singulare
foraminosus
cut obliquely
MICROSTRUCTURE OF THE BONY LABYRINTH
Fig. 38.3 The fundus of the left internal acoustic meatus, exposed by a
section through the petrous part of the left temporal bone nearly parallel
The wall of the bony labyrinth is lined by fibroblast-like perilymphatic
to the line of its superior border.
cells and extracellular matrix fibres (see Fig. 38.2). The morphology of
the cells varies in different parts of the labyrinth. Where the perilym-
phatic space is narrow, as in the cochlear aqueduct, the cells are reticular
or stellate in form; they give off sheet-like cytoplasmic extensions that
INTERNAL ACOUSTIC MEATUS
cross the extracellular space. Where the space is wider, as in the scalae
vestibuli and tympani of the cochlea and much of the vestibule, the
The internal acoustic meatus (internal acoustic/auditory canal) is sepa-
perilymphatic cells on the periosteum and the external surface of the
rated from the internal ear at its lateral fundus by a vertical plate divided
membranous labyrinth are extremely flat and resemble a squamous
unequally by a transverse (falciform) crest (Fig. 38.3). Five nerves –


epithelium. Elsewhere, on parts of the perilymphatic surface of the
facial, nervus intermedius, cochlear, superior and inferior vestibular –
basilar membrane, the cells are cuboidal.
pass through openings in the vertical plate, above and below the
Recent evidence suggests that micropores or canaliculi (canaliculi
transverse crest. The facial and superior vestibular nerves enter canals
perforantes) (0.2–23.0 µm diameter) are more widely distributed
that are superior to the crest. The facial nerve is anterior to the superior
within the bony surfaces lining the perilymphatic space than was previ-
vestibular nerve, from which it is separated at the lateral end of the
ously suspected; they are numerous in the peripheral and modiolar
meatus by a vertical ridge of bone (Bill’s bar). The nervus intermedius
portions of the osseous spiral lamina and the floor of the scala tympani,
lies between the facial motor root and the superior vestibular nerve, to
but sparse in the osseous wall of the scala vestibuli. The proposal that
which it may be adherent. The superior vestibular area contains open-
these canaliculi normally provide an extensive fluid communication
ings for nerves to the utricle and anterior and lateral semicircular ducts.
channel between the scala tympani and the spiral canal of the cochlea
Below the crest, an anterior cochlear area contains a spiral of small
could have implications not only for novel drug-based cochlear thera-
holes, the tractus spiralis foraminosus, which encircles the central coch-
pies delivered via the scala tympani and the delivery of stem cells or
lear canal. Behind this, the inferior vestibular area contains openings
appropriate cell lines into the deafened cochlea, but also for the design
for saccular nerves, and most posteroinferiorly, a single hole (foramen
of implanted perimodiolar electrode arrays (Shepherd and Colreavy
singulare) admits the nerve to the posterior semicircular duct. It has
2004). (For further reading about the changes in the inner ear that are
been suggested that vascular loops in the internal acoustic meatus
induced by implanted cochlear electrodes, both acute and long-term,
(from the anterior inferior cerebellar artery) might generate pulsatile


see Keifer et al (2006).)
tinnitus.
Composition of inner ear fluids
MEMBRANOUS LABYRINTH
Perilymph resembles cerebrospinal fluid in ionic composition, partic-
ularly in the scala tympani. Its composition differs a little between The membranous labyrinth is separated from the periosteum by a space
the two cochlear scalae: concentrations of potassium, glucose, amino that contains perilymph and a web-like network of fine blood vessels
acids and proteins are greater in the scala vestibuli. This has led to (see Figs 38.1C, 38.2). It can be divided into two major regions: the
the suggestion that perilymph in the scala vestibuli is derived from vestibular apparatus and the cochlear duct.
plasma via the endothelial boundary of the cochlear blood vessels,
whereas the perilymph in the scala tympani contains some cerebros-
VESTIBULAR APPARATUS
pinal 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 The vestibular apparatus consists of three membranous semicircular
contains approximately 5 mM K+, 150 mM Na+, 120 mM Cl– and canals that communicate with the utricle, a membranous sac leading
1.5 mM Ca2+. into a smaller chamber, the saccule, via the utriculosaccular duct. This
The membranous labyrinth is filled with endolymph, a fluid pro- Y-shaped duct has a side branch to the endolymphatic duct, which
duced by the marginal cells of the stria vascularis and the dark cells of passes to the endolymphatic sac, a small but functionally important
the vestibule (see review by Wangemann and Schacht (1996)) (see Fig. expansion situated under the dura of the petrous temporal bone. From
38.2B). Whatever their relative contributions, endolymph probably cir- the saccule, a narrow canal, the ductus reuniens, leads to the base of
culates in the labyrinth; it enters the endolymphatic sac, where it is the cochlear duct. These various ducts and sacs form a closed system


transferred into the adjacent vascular plexus via the specialized epithe- of intercommunicating channels. Endolymph is resorbed into the cere-
lium of the sac. Pinocytotic removal of fluid may also occur in other brospinal fluid from the endolymphatic sac, which therefore provides
labyrinthine regions. the site for the drainage of endolymph for the entire membranous
Endolymph contains greater K+ (150 mM) and Cl– (130 mM) con- labyrinth.
centrations and lower Na+ (2 mM) and Ca2+ (20 µM) concentrations The terminal fibres of the vestibular nerve are connected to the five
than perilymph. The high potassium concentration is important for the specialized sensory epithelia (two maculae and three cristae) in the
function of the mechanosensory hair cells and is maintained by the walls of the membranous labyrinth. Maculae are flat plaques of sensory
actions of the lateral wall, which contains two tissues, namely: the spiral hair cells surrounded by supporting cells, and are found in the utricle
ligament and the stria vascularis. Together, these tissues promote the and saccule. The cristae (crests) are ridges bearing sensory hair cells and
recirculation of potassium from perilymph back to endolymph by supporting cells. They are found in the walls of the ampullae near the
uptake via potassium channels and gap junctional communication. utricular openings of the three semicircular canals, one for each canal.
Gap junctions are formed from connexins; their importance to this
Utricle
process is emphasized by the fact that mutations in connexins are sig-
nificant causes of hearing loss (Duman and Tekin 2013). The vestibular
regions may not have an endolymphatic potential, as their lateral wall The utricle is the larger of the two major vestibular sacs. It is an irregular,
structure is simplified compared to that of the cochlea, although the oblong, dilated sac that occupies the posterosuperior region of the
difference in potassium concentration between endolymph and peril- vestibule (see Fig. 38.1C), and contacts the elliptical recess (where it is
ymph remains important. a blind-ended pouch) and the area inferior to it.


Inner ear
644
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nOITCeS
The macula of the utricle (or utriculus) is a specialized neurosensory midline of the striola (see Fig. 38.5). The macula in each utricle is
epithelium lining the membranous wall, and is the largest of the ves- approximately horizontal when the head is in its normal position.
tibular sensory areas (Fig. 38.4). It is triangular or heart-shaped in Linear acceleration of the head in any horizontal plane will result in
surface view and lies horizontally with its long axis orientated antero- the otolithic membrane lagging behind the movement of the membra-
posteriorly and its sharp angle pointing posteriorly (Fig. 38.5). It is flat nous labyrinth as a result of the inertia produced by its mass. The
except at the anterior edge, where it is gently folded in on itself, and it membrane thus maximally stimulates one group of hair cells by deflect-
measures 2.8 mm long by 2.2 mm wide. The mature form of the macula ing their bundles towards the striola whilst inhibiting others by deflect-
is reached early in development, but in the adult a bulge is often present ing their bundles away from it. Hence each horizontal movement of
on the anterolateral border; there is sometimes an indentation at the the head will produce a specific pattern of firing in utricular afferent
anteromedial border. The epithelial surface is covered by the otolithic nerve fibres.
membrane (statoconial membrane), a gelatinous structure in which
many small crystals, the otoconia (otoliths, statoliths), are embedded. Saccule
A curved ridge, the ‘snowdrift line’, runs along the length of the otolithic
membrane. It corresponds to a narrow crescent of underlying sensory
The saccule (or sacculus) is a slightly elongated, globular sac lying in
epithelium termed the striola, 0.13 mm wide. The density of sensory
the spherical recess near the opening of the scala vestibuli of the cochlea
hair cells in this strip of epithelium is 20% less than in the rest of
(see Fig. 38.1C). The saccular macula is an almost elliptical structure,


the macula. The striola is convex laterally and runs from the medial
2.6 mm long and 1.2 mm at its widest point. Its long axis is orientated
aspect of the anterior margin in a posterior direction towards, but not
anteroposteriorly but, in contrast to the utricular macula, the saccular
reaching, the posterior pole. The part of the macula medial to the striola
macula lies in a vertical plane on the wall of the saccule. Its elliptical
is called the pars interna and is slightly larger than the pars externa,
shape is very slightly distorted by a small anterosuperior bulge. Like the
which is lateral to it. The significance of this area is that the sensory
utricular macula, it is covered by an otolithic (statoconial) membrane
cells are functionally and anatomically polarized with respect to the
and possesses a striola similar to that of the utricle, 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 (see Fig.
38.5). The part of the macula above the striola is termed the pars
Hair cells Supporting cell nuclei 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 orienta-
tion, 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.
Vestibular nerve fibres
Semicircular canals
The lateral, anterior and posterior semicircular ducts follow the course
Fig. 38.4 A section of the utricular macula from a guinea pig, showing the
of their osseous canals. Throughout most of their length they are
relative positions of the hair cells and supporting cell nuclei. Semi-thin
securely attached, by much of their circumference, to the osseous walls.
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 They are approximately one-quarter of the diameter of their osseous


means that well-fixed human tissue is rarely obtained for histology. canals (see Fig. 38.2). The medial ends of the anterior and posterior
Guinea pigs are one of the most frequently used animal models of human canals fuse to form a single common duct, the crus commune, before
hearing and their inner ear ultrastructure is very similar.) (Courtesy of RM entering the utricle. The lateral end of each canal is dilated to form an
Walsh, DN Furness and CM Hackney, The Institute of Science and ampulla, within the ampulla of the osseous canal. The short segment
Technology in Medicine, School of Life Sciences, Keele University.) of duct between the ampullae and utricle is the crus ampullaris.
A B C
Otoliths
Saccule Otolithic
membrane
Kinocilium Stereocilia
Cuticular
plate
Afferent
nerve calyx Type II
hair cell
Utricle
Type I
hair cell Efferent
Synaptic bar nerve
and vesicles ending
Otolithic
membrane Efferent Afferent
nerve ending nerve
ending
Macula
Sensory epithelium, Supporting
hair cells (blue) and cells
hair bundles (black)
Fig. 38.5 A, The 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, A 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.
(B, Courtesy of DN Furness, The Institute of Science and Technology in Medicine, School of Life Sciences, Keele University.)


Membranous labyrinth
645
83
reTPaHC
Fig. 38.7 A scanning
electron micrograph of
a stereociliary bundle
from the utricle (guinea
pig). The stereocilia are
arranged in rows of
increasing height
towards the tallest
Cupula element, the kinocilium.
Deflection in the
direction of the
kinocilium results in
depolarization of the
hair cell. The inset
shows a tip link
connecting a short
stereocilium tip to the
Cilium tall stereocilium side
behind. (Courtesy of DN
Stereocilia
Furness, The Institute of
Science and
Receptor
cells Technology in
Medicine, School of Life
Supporting Sciences, Keele
cell
University.)
Nerve
fibre
Fig. 38.6 A section of an ampullary crest.
The membranous wall of each ampulla contains a transverse eleva- Close to the inner surface of their basal two-thirds, every cell con-
tion (septum transversum), on the central region of which is a saddle- tains numerous synaptic ribbons with associated synaptic vesicles. The
shaped sensory ridge, the ampullary crest, containing hair cells and postsynaptic surface of an afferent nerve ending encloses the greater part
supporting cells. It is broadly concave on its free edge along most of its of the sensory cell body in the form of a cup (chalice or calyx). Efferent
length and has a concave gutter (planum semilunatum) at either end nerve fibres make synapses with the external surface of the calyx, rather
between the ridge and the duct wall. Sectioned across the ridge, the than directly with the sensory cell.
crests of the lateral and anterior semicircular canals have smoothly The kinocilium confers structural polarity on the bundle, which
rounded corners; the posterior crest is more angular. A vertical plate of relates to functional polarity. The stereocilia and kinocilium are all
gelatinous extracellular material, the cupula, is attached along the free interconnected by fine extracellular filaments of various types, called
edge of the crest (Fig. 38.6). It projects far into the lumen of the cross links. One in particular, the tip link, connects the shorter stereo-


ampulla so that it is readily deflected by movements of endolymph cilia in each row with adjacent stereocilia in the taller row next to it
derived from head rotations within the duct, by means of which stimuli (see Fig. 38.7). The tip link is common to all types of hair cell and is
are delivered to the sensory hair cells. The three semicircular canals thus thought to play a central role in transduction; mutations in the proteins
detect angular accelerations during tilting or turning movements of the that comprise the tip link are significant in Usher syndrome, which is
head in all three different planes of three-dimensional space. characterized by auditory and visual abnormalities. Deflection of the
bundle towards the kinocilium results in depolarization of the hair cell
and increases the rate of neurotransmitter release from its base. Deflec-
Microstructure of the vestibular system
tion away from the kinocilium hyperpolarizes the hair cell and reduces
the release of neurotransmitter. How deflections produce these responses
The maculae and crests detect the orientation of the head with respect will be considered in more detail later.
to gravity and changes in head movement by means of the mechano- There is much greater variation in the sizes of type II sensory cells
sensitive hair cells. These hair cells are in synaptic contact with afferent (see Fig. 38.5B,C; Fig. 38.8). Some are up to 45 µm long and almost
and efferent endings of the vestibular nerve on their basolateral aspect. span the entire thickness of the sensory epithelium, whereas others are
The entire epithelium lies on a bed of thick, fibrous connective tissue shorter than type I cells. They are mostly cylindrical, but otherwise
containing myelinated vestibular nerve fibres and blood vessels. The resemble type I cells in their contents and the presence of an apical
axons lose their myelin sheaths as they perforate the basal lamina of kinocilium and stereocilia. However, their kinocilia and stereocilia tend
the sensory epithelium. There are two types of sensory hair cell in the to be shorter and less variable in length. The most striking difference


vestibular system, type I and type II (see Figs 38.5, 38.8). between type I and II cells is their efferent nerve terminals: type II cells
Type I vestibular sensory cells measure 25 µm in length, with a free receive several efferent nerve boutons containing a mixture of small
surface of 6–7 µm in diameter. The basal part of the cell does not reach clear and dense-core vesicles around their bases, and afferent endings
the basal lamina of the epithelium. Each cell is typically bottle-shaped, are small expansions rather than chalices.
with a narrow neck and a rather broad, rounded basal portion contain- Polarization allows the hair cells to have specific orientations that
ing the nucleus (see Fig. 38.5). The apical surface is characterized by optimize their function within each sensory organ. In the maculae, they
30–50 stereocilia (large, regularly arranged, modified microvilli about are arranged symmetrically on either side of the striola. In the utricle,
0.25 µm across) and a single kinocilium (with the typical ‘9 + 2’ the kinocilia are positioned on the side of the sensory cell nearest to
arrangement of microtubules characteristic of true cilia). The kinocil- the striola so that the excitatory direction is towards the midline. In the
ium is considerably longer than the stereocilia, and may attain 40 µm, saccule, the structural and functional polarity is the opposite, i.e. away
whereas the stereocilia are of graded lengths. They are characteristically from it. In the ampullary crests, the cells are orientated with their rows
arranged in regular rows behind the kinocilium in descending order of of stereocilia at right angles to the long axis of the semicircular duct. In
height, the longest being next to the kinocilium (Fig. 38.7). The kino- the lateral crest, the kinocilia are on the side towards the utricle, whereas
cilium emerges basally from a typical basal body, with a centriole in the anterior and posterior crests they are away from it. These different
immediately beneath it. arrangements are important functionally because any given acceleration


Inner ear
646
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nOITCeS
ST Fig. 38.8 Human vestibular hair cells (transmission
electron micrographs). 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. Other abbreviations: SC,
supporting cells. B, Human type II vestibular hair
cell. A bouton-type afferent nerve terminal is in
contact with the basal part. (Courtesy of H Felix,
SC M Gleeson and L-G Johnsson, ENT Department,
University of Zurich and GKT School of Medicine,
London.)
VR
C
A B
of the head maximally depolarizes one group of hair cells and maxi- epithelium. The epithelial lining and subepithelial connective tissue
mally inhibits a complementary set, thus providing a unique represen- become more complex where the duct dilates to form the endolym-
tation of the magnitude and orientation of any movement (for further phatic sac. An intermediate or rugose segment and a distal sac can be
details, see Furness (2002)). distinguished. In the intermediate segment, the epithelium consists of
The type I and II sensory cells are set within a matrix of supporting light and dark cylindrical cells. Light cells are regular in form and have
cells that reach from the base of the epithelium to its surface and form numerous long surface microvilli with endocytic invaginations between
rosettes round the sensory cells, as seen in surface view. Although their them and large clear vesicles in their apical region. In contrast, dark
form is irregular, they can easily be recognized by the position of their cells are wedge-shaped and have a narrow base, few apical microvilli
nuclei, which tend to lie below the level of sensory cell nuclei and just and dense, fibrillar cytoplasm.
above the basal lamina (see Fig. 38.4). The apices of the supporting The endolymphatic sac has important roles in the maintenance of
cells are attached by tight junctions to neighbouring supporting cells vestibular function. Endolymph produced elsewhere in the labyrinth is


and to the hair cells to produce the reticular lamina, a composite layer absorbed in this region, probably mainly by the light cells. Damage to
that forms a plate that is relatively impermeable to cations other than the sac, or blockage of its connection to the rest of the labyrinth, causes
via the mechanosensitive transduction channels of the hair cells. endolymph to accumulate; this produces hydrops, which affects both
The otolithic membrane is a layer of extracellular material divided vestibular and cochlear function. The epithelium is also permeable to
into two strata. The external layer is composed of otoliths or otoconia, leukocytes, including macrophages, which can remove cellular debris
which are barrel-shaped crystals of calcium carbonate with angular from the endolymph, and to various cells of the immune system that
ends, up to 30 µm long, and heterogeneous in distribution. They are contribute antibodies to this fluid.
attached to a more basal gelatinous layer into which the stereocilia and
kinocilia of the sensory cells are inserted (see Fig. 38.5). The gelatinous
material consists largely of glycosaminoglycans associated with fibrous COCHLEAR DUCT
protein.
The cochlear duct is a spiral tube that runs within the bony cochlea (see
Epley’s manœuvre
Figs 38.1C, Fig. 38.9). The osseous spiral lamina projects for part of the
Benign paroxysmal positional vertigo is a condition in which a sensa-
distance between the modiolus and the outer wall of the cochlea and
tion of rotation with associated nystagmus is induced by adopting a
is attached to the inner edge of the basilar membrane. Above it is a
particular position (with the abnormal ear dependent). It is believed
thickened ridge of endosteum called the spiral limbus. The endosteum
that calcium carbonate crystals from the otoliths become freed from the
of the outer wall is thickened to form a spiral cochlear ligament that
otolithic membrane and, in certain positions, drop into the ampulla of
projects inwards as a triangular basilar crest attached to the outer rim
the posterior semicircular canal, possibly becoming adherent to the
of the basilar membrane. Immediately above this is a concavity, the


cupula and rendering it gravity-sensitive. In certain positions, the align-
external spiral sulcus (sulcus spiralis externus), above which the thick,
ment of the axis of the posterior semicircular canal with gravity results
highly vascular periosteum projects as a spiral prominence. Above the
in the displacement of the cupula and the activation of the vestibulo-
prominence is a specialized, thick epithelial layer, the stria vascularis.
ocular reflex, leading to compensatory nystagmoid eye movements in
A second, thinner vestibular membrane, Reissner’s membrane, extends
response to apparent head movements.
from the spiral limbus to the outer wall of the cochlea, where it is
Epley’s canalith repositioning procedure relies on the adoption of a
attached above the stria. Reissner’s membrane consists of two layers of
series of body postures designed to allow the aberrant crystals (or cana-
squamous epithelial cells separated by a basal lamina. The side facing
liths) to float out of the posterior semicircular canal and to stick to the
the scala vestibuli bears flattened perilymphatic cells, with tight junc-
wall of the vestibule. Cure rates in excess of 80% have been recorded
tions between them, creating a diffusion barrier. The endolymphatic
and the procedures have largely superseded surgical procedures designed
side is lined by squamous epithelial cells with many microvilli; these
to denervate the ampulla of the posterior semicircular canal (singular
are also joined by tight junctions and are involved in ion transport. The
neurectomy) or obliterate the canal completely.
canal thus enclosed between the scala tympani and the scala vestibuli
is the cochlear duct (see Fig. 38.9B). It is triangular in cross-section
Endolymphatic duct and sac
throughout the length of the cochlea. The closed upper end, the lagena,
is attached to the cupula. The lower end of the duct turns medially,
The endolymphatic duct runs in the osseous vestibular aqueduct and narrowing into the ductus reuniens, and connects with the saccule (see
becomes dilated distally to form the endolymphatic sac. This is a struc- Fig. 38.1C).


ture of variable size, which may extend through an aperture on the The organ of Corti, the sensory epithelium of the cochlea, sits on
posterior surface of the petrous bone to end between the two layers of the basilar membrane. The apices of the sensory hair cells and the sup-
the dura on the posterior surface of the petrous temporal bone near the porting cells it contains are joined by tight junctions to form the reticu-
sigmoid sinus (see Fig. 38.1C). The surface cells throughout the entire lar lamina. The diffusion barriers that line the cochlear duct ensure that
endolymphatic duct resemble those lining the non-specialized parts of the apices of the sensory hair cells are bathed in endolymph, whereas
the membranous labyrinth and consist of squamous or low cuboidal their lateral and basal regions are bathed in perilymph.


Membranous labyrinth
647
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reTPaHC
A B
Pharyngotympanic
tube
Scala vestibuli
Stria
vascularis
Tympanic
membrane Cochlear duct
External Spiral
Reissner’s membrane spiral prominence
sulcus
Tectorial
Cupula membrane
Second turn Spiral limbus
of cochlea
Osseous
Handle of malleus spiral lamina
Spiral
Modiolus ligament
Basilar
Organ membrane
of Corti
Cochlear nerve
Internal spiral
Vestibular nerve sulcus
Spiral
Vestibule ganglion
Scala tympani
Facial nerve
Stapedius
C
Inner hair cell Interdental Tectorial Inner hair Outer hair
cells membrane cells cells
Spiral Hensen’s Pillar Reticular Stria
Stereocilia limbus stripe cells lamina vascularis
Cuticular plate
Basal body
Afferent endings
Efferent endings
Outer hair cell
Stereocilia
Phalangeal
process
Habenula
perforata
Deiters’ cell Osseous Tunnel Spaces Outer spiral
spiral lamina of Corti of Nuel sulcus
Cochlear Inner spiral
nerve fibres sulcus Basilar Hensen’s Claudius’
membrane cells cells
Fig. 38.9 The cochlea. A, A horizontal section through the left temporal bone showing the position of the cochlea with respect to the tympanic cavity.
B, A section through the second turn of the cochlea seen in A. The modiolus is to the left. C, The structure of the cochlear organ of Corti and stria
vascularis, showing the arrangement of the various types of cell and their overall innervation. The organization of the inner and outer hair cells and their
synaptic connections are also depicted. A and B are drawn from histological sections.
The stria vascularis lies on the outer wall of the cochlear duct, above descending cytoplasmic processes of the marginal dark cells and the
the spiral eminence (see Fig. 38.9B and C). It has a special stratified ascending processes of the intermediate and basal cells envelop the
epithelium containing a dense intraepithelial capillary plexus and three intraepithelial capillaries. The stria vascularis is involved in ion trans-


cell types: superficial marginal, dark or chromophil cells; intermediate port and helps to produce the unusual ionic composition of endo-
light, or chromophobe cells; and basal cells. The endolymphatic surface lymph. It is the source of the large positive endocochlear electrical
consists only of the apices of marginal cells. The intermediate and basal potential, maintenance of which is directly dependent on adequate
cells lie deeper within the stria and send cytoplasmic processes towards oxygenation of the epithelial cells, provided by the intraepithelial capil-
the surface, between the deeper parts of the marginal cells. The long lary plexus.


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The spiral ligament is lateral to the stria vascularis. It is composed
of thick collagenous fibres interspersed with fibrocytic cells of several
different types, and root cells that send large processes into the ligament
from the region of the basilar crest. The fibrocytes and the root cells
express connexins and have gap junctional systems (Jagger et al 2010)
that allow transcellular transport of ions, although the two types of cell
are not directly coupled. It is thought that potassium recycling from
perilymph to endolymph is mediated partly through these gap junc-
tional networks emanating from the Deiters’ cells underlying the
sensory hair cells. Late-onset hearing loss (DFN3) is thought to be the
result of fibrocyte degeneration (Minowa et al 1999).


Inner ear
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nOITCeS
The osseous spiral lamina consists of two plates of bone between There are almost 6000 inner pillar cells. Their bases rest on the
which are canals for the cochlear nerve fibres. On the upper plate, the basilar membrane near the tympanic lip of the internal spiral sulcus,
periosteum is thickened to form the spiral limbus (limbus laminae spi- and their bodies form an angle of approximately 60° with the basilar
ralis) (see Fig. 38.9B). It ends externally in the internal spiral sulcus, membrane. Their heads resemble the proximal end of the ulna, with
which in section is shaped like a C. Its upper part, the overhanging deep concavities for the heads of the outer pillar cells, which they over-
limbic edge, is the vestibular labium, and the lower tapering part is the hang to form the top of the tunnel of Corti. There are almost 4000 outer
tympanic labium, which is perforated by small holes (the habenula pillar cells. They are longer and more oblique than the inner pillar cells,
perforata) for branches of the cochlear nerve. The upper surface of the and form an angle of approximately 40° with the basilar membrane.
vestibular labium is crossed at right angles by furrows, separated by Their heads fit into the concavities on the heads of the inner pillar cells
numerous elevations – the auditory teeth (dentes acustici). The limbus and project externally between the first row of outer hair cells as thin
is covered by a layer that appears superficially to be squamous epithe- processes that contact the processes of the Deiters’ cells. The distances
lium; however, only the cells over the ‘teeth’ are flat, and those in the between the bases of the inner and outer pillar cells increase from the
furrows are flask-shaped interdental cells. The epithelium is continuous cochlear base to its apex, whereas the angles they make with the basilar
with the epithelium in the internal spiral sulcus and on the inferior membrane diminish.
surface of Reissner’s membrane. During development, the interdental Cochlear hair cells are the sensory transducers of the cochlea; col-


cells secrete some of the material that forms the tectorial membrane. The lectively, they detect the amplitude and frequency of the sound waves
limbus contains fibrocytic cells interspersed with extracellular matrix. that enter the cochlea. All cochlear hair cells have a common pattern
of organization. They are elongated cells with a group of modified
Basilar membrane apical microvilli or stereocilia (which contain parallel arrays of actin
filaments), similar to those of vestibular hair cells, and a group of syn-
aptic contacts with cochlear nerve fibres at their rounded bases (Fig.
The basilar membrane stretches from the tympanic lip of the osseous
38.10). The inner hair cells form a single row along the inner edge of
spiral lamina to the basilar crest of the spiral ligament (see Fig. 38.9B
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 basilar 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
A
The organ of Corti consists of a series of epithelial structures that lie on
the zona arcuata of the basilar membrane (see Fig. 38.9B and C). The
more central of these structures are two rows of cells: the internal
(inner) and external (outer) 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 (see Fig. 38.9C). 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 sup-
porting 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 sur-
rounding 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 intercom-
B
municating complex of spaces of Nuel and tunnel of Corti is filled with
perilymph, which diffuses through the matrix of the basilar membrane.
Fig. 38.10 Scanning electron micrographs of the surface of the organ of
The fluid in these spaces is also sometimes called the cortilymph; it is
Corti (guinea pig). A, The reticular lamina. Three rows of V-shaped
possible that minor alterations in perilymphatic composition occur
stereociliary bundles can be seen protruding from the apices of the outer
within it because it is exposed to the activities of synaptic endings and
hair cells. They are separated from the single row of inner hair cells
specialized excitable cells.
(which have relatively linear stereociliary bundles) by the apices of the
Each pillar cell has a base or crus, an elongated scapus (rod) and an


inner pillar cells. B, The stereociliary bundle of one outer hair cell,
upper end or caput (head) (see Fig. 38.9C); each crus and caput are in
showing three rows of stereocilia increasing in height; deflection of the
contact, but the scapi are separated by the tunnel of Corti. Electron stereocilia in the direction of the tallest row results in depolarization of the
microscopy shows many microtubules, 30 nm in diameter, arranged in hair cell. Microvilli can be seen on the surface of Deiters’ cells (front right).
linked parallel bundles of 2000 or more in the scapus, originating in The inset shows a tip link connecting a short stereocilium tip to the side
the crus and diverging above the scapus to terminate in the head region. of the tall stereocilium behind. (Courtesy of DN Furness, The Institute of
The nucleus is situated in the foot-like expansion resting on the basal Science and Technology in Medicine, School of Life Sciences, Keele
lamina. University.)


Membranous labyrinth
649
83
reTPaHC
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 (see Fig. 38.10A).
These two groups have distinctive roles in sound reception; the differ-
ences 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 arrange-
ment of these cells is very precise, and this pattern is closely related to
the sensory performance of the cochlea.
The inner hair cells are pear-shaped and slightly curved; the nar-
rower 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 (see Fig. 38.9C). The inner hair cells are sur-
rounded 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 (see Fig.
38.10A). 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. Fig. 38.11 A scanning electron micrograph of a portion of the organ of
The apex bears 50–60 stereocilia, arranged in several ranks of progres- Corti (guinea pig), dissected to expose the outer row of outer hair cells
sively ascending height, the tallest on the strial side. The tips of the and their attendant Deiters’ cells with narrow phalangeal processes. The
shorter rows are connected diagonally to the sides of the adjacent stereociliary bundles of two rows of outer hair cells are visible above the
taller stereocilia by thin filaments called tip links; each stereocilium is reticular lamina. (Courtesy of DN Furness, The Institute of Science and
also connected to all its neighbours by a variety of lateral links. The Technology in Medicine, School of Life Sciences, Keele University.)


height of a stereociliary row varies along the length of the cochlea,
being tallest at the apex and shortest at its base. The stereociliary
bases insert into a transverse lamina of dense fibrillar material, the The apices of the hair cells and supporting cells that form the reticu-
cuticular plate, which lies immediately beneath the apical surface of lar lamina are linked by tight junctions, adherens junctions and desmo-
each inner hair cell. The cuticular plate includes a small aperture con- somes. This arrangement is significant for two reasons. The reticular
taining a basal body. During development, a kinocilium containing lamina creates a highly impermeable barrier to the passage of ions other
microtubules is anchored here, a condition that persists in vestibular than via the mechanotransducer channels in the stereociliary mem-
hair cells. branes. It also forms a rigid support between the apices of the hair cells,
At its base, each inner hair cell forms 10 or more synaptic contacts coupling them mechanically to the movements of the underlying
with afferent endings, each being marked by a presynaptic structure basilar membrane, which causes lateral shearing movements between
similar to the ribbon synapses of the retina. Occasionally, an efferent the stereocilia and the overlying tectorial membrane. If there is hair cell
synapse makes direct contact with a hair cell base but these are usually loss as a result of trauma such as excessive noise or ototoxic drugs, the
presynaptic to the terminal expansions of afferent endings, rather than supporting cells expand rapidly to fill the gap, disturbing the regular
to the hair cell itself. pattern of the reticular lamina (phalangeal scars) but restoring its
Outer hair cells are long cylindrical cells that are nearly twice as tall function.
as the inner hair cells (see Figs 38.9C, 38.10A). There is a gradation of
Hair cell mechanotransduction
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 Mechanotransduction is central to both hearing and balance, and is a
surrounded by the apical or phalangeal processes of the Deiters’ cells phenomenon that is shared by all the sensory hair cells of vestibular


or, on the internal side of the inner row, by the heads of the outer pillar and auditory epithelia. The basic mechanism is the same in all of the
cells. The stereocilia, which may number up to 100 per cell, are arranged different types of hair cell and is dependent on common features of the
in three rows of graded heights; the tallest is on the outer side. The rows hair bundle: specifically, the ‘staircase’ array of stereocilia and the tip
are arranged in the form of a V or W, depending on cochlear region, links connecting the tips of shorter and adjacent taller stereocilia.
the points of the angles directed externally. The stereocilia are also Deflections of the bundle in an excitatory (depolarizing) direction cause
graded in height according to cochlear region; those of the cochlear tension in the tip link, which acts to open non-specific cationic mecha-
base are shortest. Like those of inner hair cells, the stereocilia possess noelectrical transduction channels. These then allow influx of potas-
tip links (see Fig. 38.10B) and other filamentous connections with their sium and calcium ions, depolarizing the cell and leading to calcium
neighbours, and are inserted at their narrow bases into a cuticular plate. influx through basolateral voltage-gated calcium channels, which causes
The tallest stereocilia are embedded in shallow impressions on the release of neurotransmitter. The hair cells also have basolateral potas-
underside of the tectorial membrane. sium channels that allow efflux of potassium into perilymph. The tip
The rounded nucleus is positioned near the base of the cell. Below link that is central to this process consists of two molecules of cadherin
the nucleus are a few ribbon-like synapses associated with afferent 23 and two molecules of protocadherin 15, forming a strong twisted
endings of the cochlear nerve. The latter are fewer in number and strand (Hackney and Furness 2013). The importance of this structure is
smaller than the cluster of efferent boutons that contact the base of the emphasized by the fact that mutations in these cadherins underlie deaf-
cell. The neurotransmitter at the afferent synapse in both inner and blindness conditions (Usher syndrome).


outer hair cells is glutamate whereas that of the efferent endings is The sensitivity of mechanotransduction is greatly increased by the
acetylcholine, although other neurotransmitters or neuromodulators combination of the hair-cell membrane potential (−70 mV) and the
have been demonstrated. endolymphatic potential (+80 mV), producing an overall driving
Cochlear hair cells respond with phenomenal speed and sensitivity potential of up to 150 mV. It is therefore vital to maintain a high potas-
to sound vibrations that cause submicron deflections of their stereocili- sium concentration, despite its depletion from endolymph during
ary hair bundles. Outer hair cells not only detect these vibrations but detection of acoustic or balance stimuli.
also generate force to increase auditory sensitivity and frequency dis-
Tectorial membrane
crimination. (See Fettiplace and 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. The tectorial membrane overlies the sulcus spiralis internus and organ
Their expanded bases lie on the basilar membrane and their apical ends of Corti, and is a stiff, gelatinous plate (see Fig. 38.9B and C). It contains
partially envelop the bases of the outer hair cells (see Fig. 38.9C; Fig. collagen types II, V and IX, interspersed with glycoproteins (tectorins),
38.11). Each has a finger-like (phalangeal) process that extends diag- which contribute approximately half of the total protein.
onally upwards between the hair cells to the reticular membrane, where In transverse section, the tectorial membrane has a characteristic
it forms a plate-like expansion that fills the gaps between hair cell shape. The underside is nearly flat and the upper surface is convex, and
apices. it is thin on the modiolar side where it is attached to the vestibular
Five or six rows of columnar supporting cells or external limiting labium of the spiral limbus. Its outer part forms a thickened ridge,
cells, such as Hensen’s cells and Claudius’ cells, lie external to the overhanging the edge of the reticular lamina. The lower surface is rela-


Deiters’ cells (see Fig. 38.9C). tively smooth, except where the stereocilia of the outer hair cells are


Inner ear
650
4
nOITCeS
embedded in the membrane, leaving a pattern of W- or V-shaped inden-
tations: 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.
VASCULAR SUPPLY
ARTERIES
The inner ear is principally supplied by the labyrinthine artery. The
stylomastoid branch of either the occipital artery or the posterior aur-
icular artery (see Fig. 29.8) also supplies the semicircular canals.
Labyrinthine artery
A
The labyrinthine artery arises from the basilar artery (see Fig. 19.2), or
sometimes from the anterior inferior cerebellar artery. It divides at the
bottom of the internal acoustic 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.
VEINS
Descriptions of the venous drainage of the inner ear are often conflict-
ing, presumably reflecting both normal variation and the existence of
connections between the veins that provide the potential for collateral
circulation. Veins draining the vestibule and semicircular canals accom-
pany the arteries. They pass towards the utricle to form the vein of the
vestibular aqueduct, which empties into either the sigmoid or inferior
petrosal sinus. The vein of the cochlear aqueduct (inferior cochlear B
vein) usually drains into either the inferior petrosal sinus or the supe-
rior bulb of the internal jugular vein. It is formed by the union of the Fig. 38.12 The vestibulocochlear nerve (human). A, A transverse section.
common modiolar and vestibulocochlear veins and provides almost all The cochlear nerve (the comma-shaped profile on the left) abuts the
of the venous outflow of the cochlea (Wright and Roland 2013). The inferior division of the vestibular nerve (right). The singular nerve is a


common modiolar vein is formed by the union of the anterior and separate fascicle between the superior and inferior divisions of the
posterior spiral veins near the basal cochlear turn; the vestibulocochlear vestibular nerve. B, A portion of a vestibular ganglion, showing neuronal
vein is formed by the confluence of the anterior and posterior vestibular perikarya, myelinated axons and small blood vessels. (Toluidine blue
veins and the vein of the round window. When present, a labyrinthine stained resin sections courtesy of H Felix, M Gleeson and L-G Johnsson,
vein drains the apical and middle cochlear coils into either the posterior ENT Department, University of Zurich and School of Medicine, King’s
College London.)
part of the superior petrosal sinus or the transverse sinus or the inferior
petrosal sinus.
For details of the microvasculature of the cochlea of humans and tion does not correlate with the precise anatomical description of
other mammals, see Axelsson (1988). 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 ves-
INNERVATION tibular disturbance’ includes lesions of the vestibular ganglion and the
entire vestibular nerve.
VESTIBULOCOCHLEAR NERVE
Vestibular nerve
The vestibulocochlear nerve emerges from the cerebellopontine angle
(see Figs 21.4, 21.5). It courses through the posterior cranial fossa in The cell bodies of the bipolar neurones that contribute to the vestibular
close association with the facial nerve, nervus intermedius and labyrin- nerve lie in the vestibular ganglion, which is situated in the trunk of the
thine vessels. Together with these structures, it enters the petrous tem- nerve within the lateral end of the internal acoustic meatus (Fig.
poral bone via the porus acusticus of the internal acoustic meatus, and 38.12B). Their peripheral processes innervate the maculae of the utricle
divides into an anterior trunk, the cochlear nerve, and a posterior trunk, and saccule and the ampullary crests of the semicircular canals (see


the vestibular nerve (Fig. 38.12A; see Fig. 37.7). Both contain the cen- below). Their axons travel to the central nervous system in the vestibular
trally directed axons of bipolar neurones, together with a smaller num- nerve, which enters the brainstem at the cerebellopontine angle and
ber of efferent fibres that arise from brainstem neurones and terminate terminates in the vestibular nuclear complex (see p. 319). Neurones in
on cochlear and vestibular sensory cells. In humans, the intratemporal this complex project to motor nuclei in the brainstem and upper spinal
portion of the vestibulocochlear nerve consists of two histologically cord, and to the cerebellum and thalamus. Thalamic efferent projections
distinct portions: a central glial zone adjacent to the brainstem, and a pass to a cortical vestibular area that is probably located near the intra-
peripheral or non-glial zone (Bridger and Farkashidy 1980). In the glial parietal sulcus in area 2 of the primary somatosensory cortex.
zone, the axons are supported by central neuroglia, whereas in the non-
Vestibular (Scarpa’s) ganglion
glial zone they are ensheathed by Schwann cells. The non-glial zone
sometimes extends into the cerebellopontine angle medial to the inter- The cell bodies of the neurones in the vestibular ganglion vary consider-
nal acoustic meatus in human vestibulocochlear nerves. During devel- ably in size: their circumferences range from 45 to 160 µm (Felix et al
opment, a gap of several weeks has been reported between the onset of 1987). No topographically ordered distribution relating to size has
Schwann cell myelination distally and glial myelination proximally; it been found. The cell bodies are notable for their abundant granular
has been suggested that the gap may coincide with the time of the final endoplasmic reticulum, which in places forms Nissl bodies, and promi-
maturation of the organ of Corti. (For further details about the develop- nent Golgi complexes. They are covered by a thin layer of satellite cells


ment of the human cochlear nerve, see Ray et al (2005).) and are often arranged in pairs, closely abutting each other so that only
In audiological practice, it is important to distinguish between a thin layer of endoneurium separates the adjacent coverings of satellite
intratemporal and intracranial lesions. However, this surgical distinc- cells. This arrangement has led to speculation that ganglion cells may


Innervation
651
83
reTPaHC
affect each other directly by electrotonic spread (ephaptic transmission: of the vestibular nerve afferents (either an increase or a decrease of the
see Felix et al (1987)). basal resting discharge, depending on the direction of stimulation). The
Two distinct sympathetic components have been identified in the signals are compared centrally with visual and somatosensory signals,
vestibular ganglion: a perivascular adrenergic system derived from the which also signal the position of the head in space (for a more detailed
stellate ganglion, and a blood vessel-independent system derived from account, see Furness (2002)).
the superior cervical ganglion.
Semicircular canals
Intratemporal vestibular nerve Angular acceleration and deceleration of the head cause a counterflow
The peripheral processes of the vestibular ganglion cells are aggregated of endolymph in the semicircular canals, which deflects the cupula of
into definable nerves, each with a specific distribution. The main nerve each crista and bends the stereociliary/kinociliary bundles. When a
divides at and within the ganglion into superior and inferior divisions, steady velocity of head movement is reached, the endolymph rapidly
which are connected by an isthmus. The superior division, the larger of adopts the same velocity as the surrounding structures because of fric-
the two, passes through the small holes in the superior vestibular area tion with the canal walls, so that the cupula and receptor cells return
at the fundus of the internal acoustic meatus (see Fig. 38.3) and sup- to their resting state. The three semicircular canals are orientated
plies the ampullary crests of the lateral and anterior semicircular canals approximately at right angles to each other, which means that all pos-
via the lateral and anterior ampullary nerves, respectively. A secondary sible directions of acceleration can be detected. Directional sensitivity
branch of the lateral ampullary nerve supplies the macula of the to head movement is coded by opposing receptor signals: the left and
utricle; however, the greater part of the utricular macula is innervated right semicircular canals of each functional pair (e.g. the left and right
by the utricular nerve, which is a separate branch of the superior divi- anterior canals) respond oppositely to any movement of the head that


sion. Another branch of the superior division, supplies part of the affects them (Fig. 38.13). Some vestibular neurones receive a bilateral
saccule. input from vestibular receptors, which means that they can compare
The inferior division of the vestibular nerve passes through small the discharge rates of right and left canal afferents, a mechanism that
holes in the inferior vestibular area (see Fig. 38.3) to supply the remain- increases the sensitivity of the system.
der of the saccule and the posterior ampullary crest via saccular and
Maculae
singular branches, respectively; the latter passes through the foramen
singulare. Occasionally, a very small supplementary or accessory branch In the maculae, the weight of the otoconial crystals creates a gravita-
innervates the posterior crest; it is probably a vestigial remnant of the tional pull on the otoconial membrane and thus on the stereociliary
crista neglecta, an additional area of sensory epithelium found in some bundles of the sensory cells that are inserted into its base. Because of
other mammals but seldom in humans. this, they are able to detect the static orientation of the head with respect
Afferent and efferent cochlear fibres are also present in the inferior to gravity. They also detect shifts in position according to the extent to
division of the vestibular nerve, but leave at the anastomosis of Oort to which the stereocilia are deflected. The two maculae are set at right
join the main cochlear nerve (see review by Warr (1992)). Another angles to each other, and the cells of both are orientated functionally
anastomosis, the vestibulofacial anastomosis, is situated more centrally in opposite directions across their striolar boundaries. Movement
between the facial and vestibular nerves, and is the point at which fibres causes depolarization of the hair cells on one side of the striola and
originating in the intermediate nerve pass from the vestibular nerve to hyperpolarization of cells on the other side; because the striola is
the main trunk of the facial nerve. curved, small groups of hair cells on the macular epithelium each
There are approximately 20,000 fibres in the vestibular nerve, of respond to a specific direction of head tilt or linear acceleration (Fig.


which 12,000 travel in the superior division and 8000 travel in the 38.14). Moreover, because the otoconia have a collective inertia/
inferior division. The distribution of fibre diameters is bimodal, with momentum, linear acceleration and deceleration along the anteropos-
peaks at 4 µm and 6.5 µm. The smaller fibres go mainly to the type II terior axis can be detected by the lag or overshoot of the otoconial
hair cells and the larger fibres tend to supply the type I hair cells. In membrane with respect to the epithelial surface, and so the saccular
addition to the afferents, efferent and autonomic fibres have been iden- macula is able to signal these changes of velocity.
tified. Efferent fibres synapse exclusively with the afferent calyceal ter- The macular receptors can also be stimulated by low-frequency
minals around type I cells and usually with the afferent boutons on sound, which sets up vibratory movements in the otoconial membrane,
type II cells, although a few are in direct contact with the cell bodies of although this appears to require relatively high sound levels. Efferent
type II cells. The autonomic fibres do not contact vestibular sensory cells synapses on the afferent endings of the type I sensory cells and on the
but terminate beneath the sensory epithelia. bases of type II cells receive inputs from the brainstem that appear to
be inhibitory. They serve to reduce the activity of the afferent fibres
Anatomy of balance and posture either indirectly, in the case of the type I cells, or directly, for the type
II cells.
The vestibular labyrinths on each side of the head are arranged sym- Visual reflexes
metrically with respect to each other. Vestibular sensory pathways are
The vestibular system plays a major role in the control of visual reflexes,
concerned with perception of the position of the head in space and
which allow the fixation of gaze on an object in spite of movements of
movement of the head; they also establish important connections for
the head, and require the coordinated movements of the eye, neck and
reflex movements that govern the equilibrium of the body and the fixity
upper trunk. Constant adjustments of the visual axes are achieved
of gaze.
chiefly through the medial longitudinal fasciculus, which connects the


The vestibular system consists of two otolithic organs – the utricle
vestibular nuclear complex with neurones in the oculomotor, trochlear
and the saccule – and three semicircular canals. The otolithic organs
and abducens nuclei and with upper spinal motor neurones (Fig.
detect linear acceleration due to gravitational pull (gravito-inertial
38.15; see Fig. 41.12), and also by the vestibulospinal tracts. See page
acceleration) and the direction of other linear accelerations such as the
347 and Fig. 22.24 for further information about the vestibulo-ocular
up-and-down movements of the head that occur in running. They also
reflex.
respond when the head is tilted relative to gravity: so-called pitch
Abnormal activity of the vestibular input or central connections has
(forward and backward tilting) and roll (side-to-side tilting) move-
various effects on these reflexes, e.g. the production of nystagmus. This
ments. The semicircular canals detect angular accelerations resulting
can be elicited by the caloric test, a clinical test of vestibular function,
from rotations of the head or body.
by syringing the external acoustic meatus with water above or below
The stereocilia in the apical hair bundles of the mechanosensitive
body temperature, a procedure that appears to stimulate the cristae of
hair cells in each of these organs are embedded in an overlying acces-
the lateral semicircular canal directly. Spontaneous high activity in the
sory gel-like structure, the otolithic membrane (in the utricle and the
afferent fibres of the vestibular nerve is seen in Ménière’s disease, in
saccule) and the cupula (in the semicircular canals). Their apical sur-
which those affected experience a range of disturbances, including the
faces are bathed in endolymph; tight junctional complexes between the
sensation of dizziness and nausea, the latter reflecting the vestibular
apices of the hair cells and their adjacent supporting cells separate the
input to the vagal reflex pathway.
endolymph from the perilymph that bathes their basolateral surfaces.
As noted earlier, deflection of the stereocilia (caused by displacements
Cochlear nerve
of their overlying accessory membranes by fluid movements in the


membranous labyrinth) produces either an increased or a decreased
Intratemporal cochlear nerve
rate of opening of the mechanotransduction channels at their tips,
depending on whether they are deflected towards or away from the The cochlear nerve connects the organ of Corti to the cochlear nuclei
tallest row, respectively. The change in the membrane potential of the and related nuclei of the brainstem. The cochlear nerve lies inferior to
receptor cell is signalled to the brain as a change in the firing frequency the facial nerve throughout the internal acoustic meatus (see above). It


Inner ear
652
4
nOITCeS
A At rest Rest with adrenergic postganglionic sympathetic fibres from the cervical
sympathetic system.
afferent cochlear innervation
Utricle Lateral The afferent fibres are myelinated axons with bipolar cell bodies that
L R semicircular canal
lie in the spiral ganglion in the modiolus (see Fig. 38.9B; Fig. 38.16).
Right ampulla There are two types of ganglion cell: most (90–95%) are large type I
cells, while the remainder are smaller type II cells (see review by Nadol
Left (1988)). Type I cells contain a prominent spherical nucleus, abundant
ampulla ribosomes and many mitochondria; in many mammals (although pos-
Cupula sibly not in humans), they are surrounded by myelin sheaths. In con-
Receptor trast, type II cells are smaller and always unmyelinated, and have a
cell lobulated nucleus. The cytoplasm of type II cells is enriched with
neuro filaments but has fewer mitochondria and ribosomes than type I
cells.
Basilar fibres Basilar fibres are the peripheral processes of type II
ganglion cells and are afferent to the outer hair cells. They take an
Left and right
lateral canal independent spiral course, turning towards the cochlear apex near the
90 spikes/second afferent discharge rate 90 spikes/second 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.
B Right turn Right turn
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)).
Endolymph
Within the modiolus, the efferent fibres form the intraganglionic spiral
flow in
canal bundle, which may be one or more discrete groups of fibres situated at


the periphery of the spiral ganglion (Fig. 38.17). There are two main
groups of olivocochlear efferents: lateral and medial. The lateral effer-
ents 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
Inhibition Excitation 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 contralat-
10 spikes/second 180 spikes/second erally. 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
C Left turn Left turn
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 neuro-
transmitter. 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
make contacts most commonly with the inner radial afferent fibres
rather than the inner hair cell base (see Fig. 38.9C) and appear to
modify transmission through the 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
Excitation Inhibition
efferents (see review by Guinan (1996)).
autonomic cochlear innervation
180 spikes/second 10 spikes/second
Autonomic nerve endings appear to be entirely sympathetic. Two adren-


Fig. 38.13 The response of the lateral semicircular canals to head rotation ergic systems have been described within the cochlea: a perivascular
in the horizontal plane. The firing rates of afferents from the left and right plexus derived from the stellate ganglion and a blood vessel-independent
lateral canals are equivalent at rest (A). However, when the head is turned
system derived from the superior cervical ganglion. Both systems travel
to the right (B) or to the left (C), receptor depolarization and afferent fibre
with the afferent and efferent cochlear fibres and seem to be restricted
excitation occurs on the side to which the head turns; there is inhibition
to regions away from the organ of Corti. The sympathetic nervous
on the contralateral side.
system may cause primary and secondary effects in the cochlea by
remotely altering the metabolism of various cell types and by influen-
cing the blood vessels and nerve fibres with which it makes contact.
becomes intimately associated with the superior and inferior divisions
of the vestibular nerve, which are situated in the posterior compartment
Anatomy of hearing
of the canal, and leaves the internal acoustic meatus in a common
fascicle (see Fig. 38.12A).
There are 30–40,000 nerve fibres in the human cochlear nerve (for Sound waves entering the external ear are converted into electrical
review, see Nadol (1988)). Their fibre diameter distribution is unimo- signals in the cochlear nerve by the peripheral auditory system
dal, and ranges from 1 to 11 µm, with a peak at 4–5 µm. Functionally, (Fig. 38.18). The axons in the cochlear nerve constitute the auditory
the nerve contains both afferent and efferent somatic fibres, together component of the vestibulocochlear nerve and terminate in the dorsal


Innervation
653
83
reTPaHC
A At rest B Right tilt C Left tilt
Ocular
counter roll
Utricular
maculae
Otoconia Striola Receptor Otolith
cell membrane
Kinocilia
50 spikes/second 50 spikes/second 90 spikes/second 10 spikes/second 10 spikes/second 90 spikes/second
Discharge rate of afferents Discharge rate of afferents Discharge rate of afferents
on left and right sides on left and right sides on left and right sides
Fig. 38.14 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 green 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.
and ventral cochlear nuclei (see p. 319); onward connections make up tion channels near the tips of its stereocilia, and this allows potassium
the ascending (central) auditory pathway. and calcium ions from the endolymph to enter the hair cell (see earlier
and overview by Fettiplace and Hackney (2006)). This induces a depo-
Peripheral auditory system larizing receptor potential and the release of neurotransmitter on to the
Vibrations in the air column in the external acoustic meatus cause a cochlear afferents at the base of the cell. In this way, a specific group of
comparable set of vibrations in the tympanic membrane and auditory auditory axons is activated at the position of maximal basilar mem-
ossicles. The chain of ossicles acts as a lever that increases the force per brane vibration.
unit area at the round window by 1.2 times while the reduction in size The mechanical behaviour of the basilar membrane is responsible
of the round window compared with the tympanic membrane increases for a broad discrimination between different frequencies (passive


the force per unit area of the oscillating surface a further 17 times. This tuning; see overview by Ashmore (2002)), but fine frequency discrimi-
overcomes the inertia of the cochlear fluids and produces in them pres- nation in the cochlea appears to be related to physiological differences
sure waves that are conducted almost instantaneously to all parts of the between the hair cells. Individual tuning of hair cells may result from
basilar membrane. The latter varies continuously in width, mass and differences in shape, stereociliary length, or possibly variations in the
stiffness from the basal to the apical end of the cochlea. Each part of molecular composition of sensory membranes, and may have a role in
the basilar membrane vibrates, but only the region tuned to a specific cochlear amplification (active tuning).
frequency will respond maximally to a pure tone entering the ear. A The activity of the outer hair cells appears to play an important
wave of mechanical motion, the travelling wave, is propagated along part in regulating inner hair-cell sensitivity at specific frequencies.
the basilar membrane to the position where it responds maximally and Outer hair cells can change length when stimulated electrically at
then dies away again. With increasing frequency, the locus of maximum frequencies of many thousands of cycles per second. The rapidity of
amplitude moves progressively from the apical to the basal end of the these changes in length indicates a novel type of motile mechanism,
cochlea. The pattern of vibrations in the basilar membrane thus varies which is believed to depend on conformational changes in prestin, a
with the intensity and frequency of the acoustic waves reaching the protein that displays piezoelectricity and is located in the plasma
perilymph. Because of the arrangement of the hair cells on the basilar membrane of the cells (Fettiplace and Hackney 2006, Hudspeth 2014)
membrane, these oscillations generate a largely transverse shearing (Fig. 38.19). When the membrane potential of the outer hair cells
force between the outer hair cells and the overlying tectorial membrane changes, they generate forces along their axes. When the mechanoelec-
(in which the apices of the hair cell stereocilia are embedded). This trical transduction channels open, they are thought to oppose the
movement depends on the mechanical properties of the entire organ viscous forces that tend to damp down the vibration of the cochlear


of Corti, including its cytoskeleton, which stiffens this structure. The partition, and adjust the mechanics of the organ of Corti on a cycle-by-
inner hair cell stereocilia, which probably do not touch the tectorial cycle basis. Alternatively, they may alter the mechanics of the partition
membrane, although they come very close to it, are likely to be stimu- more slowly under the influence of the efferent pathway. At a particu-
lated by local movements of the endolymph. Displacement of the lar frequency, an increase in the intensity of stimulus is signalled by an
stereociliary bundle of a hair cell activates mechanoelectrical transduc- increase in the rate of discharge in individual cochlear axons. At greater


Inner ear
654
4
nOITCeS
Interstitial nucleus
Posterior commissure
Oculomotor nucleus
Trochlear nucleus
Abducens nucleus
Nucleus of lateral lemniscus
Lingula
Fastigial nucleus
Uvula
Nodule
Flocculus
Vestibular nuclei
Median raphe
Vestibulospinal tract
Spinal accessory nucleus
Fig. 38.16 A transmission electron micrograph showing several type II
ganglion cells and axons in a human spiral ganglion. Note the absence of
myelin from the surrounding sheaths of the ganglion cells. (Courtesy of H
Felix, M Gleeson and L-G Johnsson, ENT Department, University of
Zurich and GKT School of Medicine, London.)
Medial longitudinal fasciculus
Fig. 38.15 Some of the fibre components of the medial longitudinal
fasciculus.
Outer hair cells Fig. 38.17 A simplified view of the innervation of the
Inner hair cells organ of Corti. Note the contrast between the convergent
afferent innervation of the inner hair cells (approximately
Spiral ganglion 10 fibres to each cell) and the divergent supply of the
outer hair cells (1 afferent fibre to 10 cells).
Type I Type II Intraganglionic Inner radial Inner spiral Spiral tunnel Tunnel radial Outer spiral
ganglion cell ganglion cell spiral bundle fibres fibres bundle fibres fibres


Innervation
655
83
reTPaHC
EXTERNAL EAR MIDDLE EAR INNER EAR
Sound collection and amplification; Amplification of signal (force Mechanical and neural filtering and
source location per unit area); impedance analysis of signals by organ of Corti;
matching between air and stimulus transduction by sensory
water vibrations; neural cells; action potential initiation
reflex and mechanical damping at synapses between cochlear nerve
of excessive vibration; pressure fibres and sensory cells; central
equalizing through control by centrifugal fibres
pharyngotympanic tube
Cochlear nerve
0.2
2
20
Basilar membrane
Cochlea
2
0.5
0.1 5
0.2
20
High-frequency vibrations 10
Low-frequency vibrations
Fig. 38.18 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.
intensities, it is signalled by the number of activated cochlear axons receptors; their steady receptor potentials are related in size to the
(recruitment). amplitude of the acoustic stimulus. At the same time, afferent axons are
The respective roles of the two groups of hair cells have been much stimulated by synaptic action at the bases of the inner hair cells. They
debated, particularly since differences in their innervation and physio- fire more rapidly as the vibration of the basilar membrane increases in
logical behaviour have become apparent. Because of their rich afferent amplitude, up to a threshold that depends on the sensitivity of the
supply, inner hair cells are believed to be the major source of auditory specific nerve fibre involved. Each inner hair cell is contacted by axons


signals in the cochlear nerve. Some evidence for this view is based on with response thresholds that range from 0 decibels sound pressure
the finding that animals treated with antibiotics that are specifically level (dBSPL), the approximate threshold of human hearing, to those
toxic to outer hair cells are still able to hear, but their sensitivity and that respond to intensities in excess of 100 dBSPL; the loudest sound
frequency discrimination is impaired. tolerable is around 120 dBSPL. Each axon responds most sensitively to
Some electrical responses of the cochlea can be recorded with extra- the frequency represented by its particular cochlear location, its charac-
cellular electrodes. The most significant is the endolymphatic potential, teristic frequency (see Fig. 38.18).
a steady potential recordable between the cochlear duct and the scala
Central auditory pathway
tympani, which is caused by the different ionic compositions of their
fluids. As the resting potential of hair cells is approximately 70 mV The primary afferents of the auditory pathway arise from cell bodies in
(negative inside) and the endolymphatic potential is positive in the the spiral ganglion of the cochlea. The axons travel in the vestibulococh-
cochlear duct, the total transmembrane potential across the apices of lear nerve, which enters the brainstem at the cerebellopontine angle.
hair cells is 150 mV. This is a greater resting potential than is found Afferent fibres bifurcate, and terminate in the dorsal and ventral coch-
anywhere else in the body, and provides the driving force for mechano- lear nuclei (Fig. 38.20, see p. 319). The dorsal cochlear nucleus projects
transduction and for the cochlear amplifier. via the dorsal acoustic stria to the contralateral inferior colliculus. The
Under stimulation by sound, a rapid oscillatory cochlear micro- ventral cochlear nucleus projects via the trapezoid body or the inter-
phonic potential can be recorded. It matches the frequency of the mediate acoustic stria to relay centres in either the superior olivary
stimulus and movements of the basilar membrane precisely, and complex, the nuclei of the lateral lemniscus, or the inferior colliculus.


appears to depend on fluctuations in the conductance of hair cell mem- The superior olivary complex is dominated by the medial superior
branes, probably of the outer hair cells. At the same time, an extracel- olivary nucleus, which receives direct input from the ventral cochlear
lular summating potential develops, a steady direct current shift related nucleus on both sides, and is involved in localization of sound by
to the (intracellular) receptor potentials of the hair cells. Cochlear nerve measuring the time difference between afferent impulses arriving from
fibres then begin to respond with action potentials that are also record- the two ears.
able from the cochlea. Intracellular recording of auditory responses The inferior colliculus consists of a central nucleus and two cortical
from inner hair cells has confirmed that these cells resemble other areas. The dorsal cortex lies dorsomedially, and the external cortex lies


Inner ear
656
4
nOITCeS
A B
Hair bundle motor Tectorial membrane
Hair bundle
Force applied
to hair bundle
Outer hair cell
Cl−
Cl− Cl− Cl− Basilar membrane
Cl−
Hinge point
Cl− Cl−
Cl−
Cl−
Cl−
Cl− Cl−
Cl− S mom ota ot ric
Cl− Cl−
Cl− Cl−
Cl−
Cl−
Cl− Cl− Cl− Hair cell
contraction
Cl− Cl−
Prestin
Fig. 38.19 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 and Hackney (2006)).
A
B
Primary
Superior auditory
colliculus cortex
Inferior Medial
colliculus geniculate
Corpus callosum body
Auditory cortex
Nuclei of
the lateral
lemniscus
Superior Medial geniculate body
colliculus
Dorsal cortex Superior
Dorsal and Inferior
Central cortex olivary
intermediate colliculus


External cortex complex
acoustic stria
Dorsal Commissure of Probst
cochlear Lateral lemniscus
nucleus (tract and nuclei) Cochlear
nuclear
Inferior Superior olivary complex: complex
cerebellar Lateral and
peduncle Medial olivary nuclei
Ventral Cochlear nerve
cochlear
Spiral
nucleus
ganglion
Projection via trapezoid body
Major connections Cochlear
hair cells
Less heavy projections
Fig. 38.20 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.


657
83
reTPaHC
Key references
ventromedially. Secondary and tertiary fibres ascend in the lateral lem- Deafness
niscus. They converge in the central nucleus, which projects to the
ventral division of the medial geniculate body of the thalamus. The
Hearing impairment is the most common disabling sensory defect in
external cortex receives both auditory and somatosensory input. It
humans. Two causes of deafness are usually distinguished: conductive
projects to the medial division of the medial geniculate body and,
hearing loss and sensorineural hearing loss.
together with the central nucleus, also projects to olivocochlear cells in
Conductive hearing loss may result from trauma to the external or
the superior olivary complex and to cells in the cochlear nuclei. The
middle ears, blockage of the external acoustic meatus, or disruption of
dorsal cortex receives input from the auditory cortex and projects to the
the tympanic membrane (e.g. by intense sounds or extreme pressure
dorsal division of the medial geniculate body. Connections also run
changes). It may also result from acute or chronic infection associated
from the nucleus of the lateral lemniscus to the deep part of the superior
with a perforation of the tympanic membrane, as in acute otitis media.
colliculus, to coordinate auditory and visual responses.
The ossicular chain may be disrupted by trauma, avascular necrosis of
The ascending auditory pathway crosses the midline at several points
the long process of the incus following a middle ear infection or chole-
both below and at the level of the inferior colliculus. However, the input
steatoma. The most common cause of a conductive hearing loss is a
to the central nucleus of the inferior colliculus and higher centres has
middle ear effusion (glue ear), in which fluid that has replaced the
a clear contralateral dominance; during the initial stages of cortical
normally air-filled middle ear impedes the movements of the tympanic
auditory processing, both hemispheres respond most strongly to the
membrane. Other less common causes include tympanosclerosis, in
contralateral ear. The medial geniculate body is connected reciprocally
which calcium plaques are formed in the tympanic membrane and
to the primary auditory cortex, which lies in the posterior half of


middle ear mucosa that stiffen the tympanic membrane and limit the
the superior temporal gyrus and also dives into the lateral sulcus as the
mobility of the ossicular chain. Similarly, new bone laid down around
transverse temporal gyri (Heschl’s gyri). Secondary areas of the auditory
the oval window and footplate of the stapes, otosclerosis, impedes
cortex are located in an adjacent belt region, and other regions of audi-
stapes mobility and imparts a conductive hearing loss.
tory association cortex have been described in a parabelt region beyond
Sensorineural hearing loss is the most prevalent form of hearing
the secondary cortex.
impairment. It refers commonly to loss or damage of the sensory hair
The corpus callosum, particularly the posterior third of the body, cells or their innervation, but can also result from lateral wall degenera-
contains auditory interhemispheric fibres that originate from the
tion and loss of the endolymphatic potential. The hair cells are particu-
primary and second auditory cortices. Asymmetries of minicolumn
larly vulnerable to mechanical trauma produced by high-intensity noise
number in primary and association auditory regions have been cor-
and to changes in their physiological environment caused by infection
related with axonal fibre numbers in the subregions of the corpus cal-
or hypoxia. These can lead to rapid degenerative processes that result
losum through which they project (Chance et al 2006).
in hair cell loss, often by apoptosis, and produce either hearing loss or
The presence of tonotopic gradients in the primary auditory cortex
vestibular dysfunction. These changes can be induced by drugs such as
is well established in animals and in humans. Hemispheric differences
the aminoglycoside antibiotics, some diuretics and certain anticancer
for frequency selectivity (i.e. the ability of the cochlea to separate the
drugs. A decrease in cochlear sensitivity, presbyacusis, almost invariably
acoustic frequencies along its length like an acoustic prism) and tono-
occurs with age; hair cells at the high frequency end of the cochlea tend
topic organization have been reported, e.g. the right hemisphere appears
to be lost first. At least 60% of hearing loss may have a genetic basis, a
to be most responsive to acoustic sound features such as pitch, whereas


significant proportion may be non-syndromic, and most of these genes
the left hemisphere seems to be more involved in processing temporal
are inherited in an autosomal recessive mode (see Ch. 39).
dynamics such as the phonological aspects of speech. Morphological
Ménière’s disease is a distressing disorder of the inner ear character-
asymmetries favouring the left hemisphere in the planum temporale
ized by episodes of hearing loss, tinnitus and vertigo. Histological
and Heschl’s gyri have been correlated with left hemispheric dominance
examination of an affected ear reveals endolymphatic hydrops (swelling
for language functions but a direct link between structure and function
of the endolymphatic spaces), suggesting poor drainage of the endo-
has not been clearly established; studies often show relative rather
lymph via the endolymphatic sac.
than absolute differences in hemispheric specialization for particular
attributes.
The transformation of the physical characteristics of sound into Surgical approaches to the inner ear
‘auditory objects’ is thought to occur in the transition from primary to
secondary auditory cortex. (For a critical perspective on auditory objects,
Available with the Gray’s Anatomy e-book
see Griffiths and Warren (2004).)
KEY REFERENCES
Duman D, Tekin M 2013 Autosomal recessive nonsyndromic deafness Guinan J Jr 1996 Physiology of olivocochlear efferents. In: Dallos P, Popper
genes: a review. Front Biosci 17:2213–36. AN, Fay RR (eds) The Cochlea. New York: Springer; pp. 435–502.
A review that summarizes genes and mutations reported in families with A comprehensive description of the efferent innervation of the cochlea and
ARNSHL. Mutations in GJB2, encoding connexin 26, make this gene the its function.
most common cause of hearing loss in many populations. Other relatively
Wangemann P, Schacht J 1996 Homeostatic mechanisms in the cochlea.
common deafness genes include SLC26A4, MYO15A, OTOF, TMC1,
In: Dallos P, Popper AN, Fay RR (eds) The Cochlea. New York: Springer;
CDH23 and TMPRSS3.
pp. 130–5.


Fettiplace R, Hackney CM 2006 The sensory and motor roles of auditory A review of inner ear fluids (perilymph and endolymph), how they are
hair cells. Nat Rev Neurosci 7:19–29. produced and their possible functional significance.
A description of proteins involved in the sensory and motor functions of
auditory hair cells, with evidence for each force generator.


Inner ear
657.e1
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reTPaHC
The inner ear may be approached surgically in a number ways. Many
drugs, e.g. gentamicin and steroids, are now injected through the intact
tympanic membrane, where they diffuse through the round window
membrane to access the inner ear. The promontory that overlies the
basal turn of the cochlea and the oval window may be opened through
a tympanotomy (per-meatal elevation of the tympanic membrane). The
semicircular canals are generally accessed by a transmastoid approach.
In days gone by, such an approach would be used to fenestrate the
lateral canal in the management of otosclerosis. The advent of stapes
prostheses and stapedectomy rendered this operation obsolete.
However, the transmastoid approach is the workhorse for much middle
and inner ear surgery because it gives a wide exposure of all the vital
structures. For patients with profound hearing loss, enlarging the
mastoid aditus inferiorly (posterior tympanotomy) to produce a tri-
angle bounded above by the fossa of incus, superficially by the chorda
tympani and deeply by the descending portion of the facial nerve, gives
access to the round window so that it can be instrumented and a multi-
channel cochlear electrode inserted 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. This is then connected to the
cochlear implant device, which is housed in an extension of the mastoid
cavity. Posterior canal obliteration for cupulolithiasis not responsive to
Epley’s manœuvre is also undertaken through the mastoid. The endo-
lymphatic sac may be approached 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). Decom-
pression of the sac or a shunt procedure is favoured by some for the
management of Ménière’s disease. Superior canal dehiscences, as seen
in Minor’s syndrome, are repaired either through the mastoid or by
using a middle fossa approach. In patients with intractable Ménière’s


disease, the whole of the labyrinth can be removed through the mastoid.
Vestibular nerve section and the removal of vestibular schwannomas
are generally undertaken through retrosigmoid, retrolabyrinthine,
translabyrinthine or middle fossa approaches. The choice of approach
is dictated or influenced by the size of the tumour, desire or need to
preserve hearing, and the surgeon’s choice. In recent years, the develop-
ment of auditory brainstem implants has brought hope to those with
profound hearing loss associated with neurofibromatosis type 2 or
cochlear nerve aplasia. These devices are similar to a cochlear implant
but the electrodes are placed directly into the cochlear nucleus within
the brainstem through a retrosigmoid approach.


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curvature (R) in cat, guinea pig and man. J Morphol 115: 1–15. Nadol JB 1988 Comparative anatomy of the cochlea and auditory nerve in
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A review that summarizes genes and mutations reported in families with lear nerve. Hearing Res 202:74–86.
ARNSHL. Mutations in GJB2, encoding connexin 26, make this gene the Shepherd RK, Colreavy MP 2004 Surface microstructure of the perilym-
most common cause of hearing loss in many populations. Other relatively phatic space. Implications for cochlear implants and cell- or drug-based
common deafness genes include SLC26A4, MYO15A, OTOF, TMC1, therapies. Arch Otolaryngol Head Neck Surg 130:518–23.
CDH23 and TMPRSS3.
Spoor F, Zonneveld F 1998 Comparative review of the human bony laby-
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hair cells. Nat Rev Neurosci 7:19–29. pp. 130–5.
A description of proteins involved in the sensory and motor functions of A review of inner ear fluids (perilymph and endolymph), how they are
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