The vestibular system

1. BY
Dr RAMITHA T S
1ST YEAR PG
DEPARTMENT OF ENT
HIMS HASSAN

2. 1. INTRODUCTION
2. EMBRYOLOGY OF INNER EAR
3. ANATOMY –in brief
4. THE VESTIBULAR SYSTEM
5. THE COCHLEA

3.  The inner ear collects, packages and delivers sensory information
relating to hearing via the cochlea, and balance via the vestibular
system.
 Responsible for mechanoelectrical transduction(METs), the
conversion (transduction) of movements – initiated by sound waves
in the cochlea or by changes in the position of the head in space in
the vestibular system – into electrical signals the auditory or
vestibular nerves brain.

5.  The development of the inner ear begins at 4th week and is completed at
25 weeks of gestational age.
 At the seven-somite stage (22 days)- ectoderm overlying the future
inner ear at the level of the first occipital somite thickens otic
placode.
 Otic placode invaginates into the mesenchyme to form an otic pit.
 At 30 days, the otic pit becomes pinched off-forms the otic vesicle/
otocyst.
 Within 1-2 days, its medial portion- the endolymphatic diverticulum,
becomes distinguishable from the lateral utriculosaccular chamber.

8.  This chamber differentiates into an utricular chamber- utriculus
and the semicircular ducts, and a saccular chamber- the sacculus
and the cochlea.
 At 35 days, the centers of diverticula fuse together and break
down,to form three semicircular ducts.
 The superior semicircular duct forms first, at 6 weeks, followed by
posterior and lateral ducts.
 The saccular chamber differentiates by expansion and coiling of the
cochlear duct,which separates from the sacculus by a narrowing at
its dorsal end to form the ductus reuniens.

10.  In the 3rd week, a common macula, or specialized neuroepithelium
appears; its upper part-utricula macula and crista ampullaris of the
superior and lateral semicircular ducts, and its lower part- saccular
macula and crista ampullaris of the posterior semicircular duct.
 At 9 weeks, the hair cells in the vestibular end organs are well
differentiated, they exhibit typical synapses with nerve endings.
 The maculae reach adult form at 14-16 weeks; the cristae at 23
weeks; and the organ of Corti at 25 weeks.

11.  The inner ear consists of a
 Membranous (endolymphatic) labyrinth
 Osseous (bony) labyrinth
 The membranous labyrinth consists of the utricle, saccule, semicircular
ducts, cochlear duct (scala media), and endolymphatic duct and sac. All
of these structures contain endolymph.
 The osseous labyrinth is the bony shell that surrounds the membranous
labyrinth.

13. 1. Three semi-circular ducts.
 Lateral(horizontal), posterior and superior(anterior)
 canals make an angle of 90 degrees with each other.
 This angle is known as solid angle.
2. Utricle
 Three canals open into utricle by five openings.
 Crus commune
3. Saccule
 It is connected to utricle through a utriculo-saccular duct.
 This duct forms endolymphatic duct ends blindly endolymphatic sac.
 Sac is situated in the extradural space .
 Responsible for absoption of endolymph.

14.  The vertical canals is 45 degrees to the sagittal plane.
 The horizontal canal is tilted upwards 30 degrees anteriorly
from the horizontal plane.

15. 4. Membranous cochlea
 k/a scala media or cochlear duct.
 It is a coiled tube – 2 and ½ to 2 and ¾
turns around a bony axis -MODIOLUS
 Connected to the saccule by ductus
reuniens..
 Sensory organ of hearing is
“organ of corti” in the scala
media.
 Sensory organ of balance are
“cristae and macula”.

16. Bony Labyrinth (Endolymphatic) Membraneous
labyrinth
Cochlea Cochlear duct
Vestibule Utricle and saccule
Vestibular aqueduct Endolymphatic duct/sac
Semicircular canals Semicircular ducts

17.  The scala vestibuli and scala tympani are perilymph-containing
structures within the cochlea that parallel the endolymph-containing
cochlear duct (scala media) .
 The scala vestibuli meets scala tympani at the apex of scala media
helicotrema.
 Bony labyrinth communicates laterally with middle ear via oval window
and round window.
 Medially with the cranium via cochlear aqueduct and IAC.

19.  The vestibular apparatus is enclosed in the petrous portion of the
temporal bone.
 Vestibular end organs include
 Three semicircular canals,
 Two maculae,
 Horizontal plane (the utriculus)
 Vertical plane (the sacculus)

20.  The vestibule is between the internal auditory
meatus anteromedially and the middle ear cavity
laterally.
 Entrance to the mastoid antrum is lateral to the
horizontal semicircular canal.
 The cochlea sits anterior to the vestibule and is
connected by the narrow ductus reuniens.
 Posterior and lateral to the vestibule are the mastoid
air cells.
 Medial is the posterior cranial fossa, into which the
endolymphatic duct and sac extend beneath the
dura.

21.  The seventh and eighth cranial nerves emerge from the brainstem
laterally at the cerebellopontine angle.
 Enters the vestibule and cochlea internal auditory meatus located
midway between the cochlea and vestibule.
 The facial nerve lies anterior and dorsal to the vestibulocochlear nerve.
 When past the vestibule, the facial nerve makes a 90-degree turn
inferiorly to exit the temporal bone through the stylomastoid foramen.
 The vestibulocochlear nerve splits into a vestibular division, turns
posteriorly to supply the vestibule, and a cochlear division, turns
anteriorly to supply the cochlea.

23.  The vestibular (Scarpa) ganglion sits at the bottom of the internal
auditory meatus.
 It has two parts, the superior vestibular ganglion and the inferior
vestibular ganglion.
 Large ganglion cells in the superior and inferior ganglia provide
afferent innervation to the central regions of the cristae and
maculae.
 Small ganglion cells innervate the peripheral regions of these end
organs.
 Voit’s anastomosis-the superior vestibular nerve to the
anterosuperior part of the sacculus.
 Vestibulocochlear (Oort’s) anastomosis-from the inferior vestibular
nerve to the cochlear nerve and carries cochlear efferents.

25. The hair bundle and cuticular plate
 The hair bundle is composed of
 rows of stereocilia that increase in height in one direction
across the apical surface of the hair cell.
 a single kinocilium located behind the row of longest
stereocilia, but which is absent in mature hair cells in the
cochlea.
 Stereocilia are giant microvilli, plasma
membrane-bound projections from the apical
surface of the hair cell, enclose cytoskeletal
protein, actin.
 The kinocilium is composed of microtubules
which are not motile.

26.  In cochlear hair cells, the kinocilium is
present as the basal body in the apical
cytoplasm at one side of the stereociliary
bundle.
 The position of the kinocilium (or the basal
body) and the longest row of stereocilia
define the polarity of the asymmetric hair
bundle.
 Deflection of the stereocilia towards the
longest ones opens MET channels, K+
enters and the hair cell becomes
depolarized.
 Deflection in the opposite direction closes
the transducer channels and the hair cell
becomes hyperpolarized.

27.  The parallel actin filaments in the stereocilia
are closely packed and are cross-linked by
proteins such as espin, fimbrin, fascin and
plastin 1.
 This imposes rigidity on the shaft of the
stereocilium, which tapers at its proximal
end, where it is embedded into the cuticular
plate.
 When pushed at its tip, the stereocilium
pivots at the taper like a stiff rod.

28.  Mutations in actin-crosslinker proteins
 ‘jerker’ mouse strain- mutation of cross-linking protein espin, in which
stereocilia are thinner than normal.
 Mice with such mutations have erratic and/or circling behaviours.
 Loss of plastin 1 - progressive hearing loss and balance dysfunction
and progressive thinning of stereocilia.

29.  Actin filaments descend from the
stereocilium into the cuticular plate as a
rootlet.
 The actin bundling protein TRIOBP plays a
key role in the formation and maintenance of
the rootlet.
 The cuticular plate is a rigid platform
formed of a meshwork of actin filaments and
other proteins in the apical cytoplasm of the
hair cell.
 Spectrin, another actin cross-linking protein
- elastic, deformation-resisting properties,
and tropomyosin a protein that binds around
actin and stiffens it.

31.  Myosin family of motor proteins (types 1c, 6, 7a and 15 – all
unconventional, non muscle isotypes) are also localized in the
cuticular plate region and the stereocilia.
 In the mouse mutant where myosin 6 is defective (Snell’s waltzer),
stereocilia are fused and lengthened.
 In mice where myosin 15 is mutated (shaker-2) stereocilia are
reduced in height.
 Mutations in the myosin 7a gene are responsible for Usher
syndrome type 1B.
 shows groups of stereocilia are separated from each other at the hair cell apex and the
kinocilium is misplaced, suggesting an effect on maintenance of orientation and inter
stereociliary stabilization.

32.  The stereocilia in an individual hair bundle are connected
by fibrillar extracellular cross-links -the ‘tip link’ .
 It runs from the top of one stereocilium to the shaft of an
adjacent longer one along the line of morphological polarity
.
 It is the gating element that controls the opening of the
MET channel.
 It is composed of two coiled filamentous
proteins:cadherin 23 (cdh23) and protocadherin 15
(pcdh15).
 Mutations in cdh23 gene- Usher syndrome type 1D in
humans and the ‘waltzer’ mouse strain.
 Mutations in pcdh15 are associated with Usher syndrome
type 1F and the ‘Ames waltzer’ mouse.

33.  Proteins involved in maintaining tip-link tension include myosin 7a,
Sans and harmonin.
 Usher type 1B in humans and the shaker 1 mouse strain in the case of mutation of myosin 7a
 Usher type 1C from defects in harmonin
 Usher type 1G from mutations in Sans.
 ‘Lateral links’ connect the shaft of one stereocilium to all its
neighbours.
 Lateral links hold the bundle together, stabilizing it and aiding the
mechanical coupling of deflections of the stereocilia, such that they
move as a single unit.
 Ankle links connect stereocilia at their proximal ends.
 Top-connectors link stereocilia just below the level of the tip links.

34. THE BASOLATERAL
PLASMA MEMBRANE
Characterized by the presence of ion channels essential for shaping the response
of the cells to mechanical excitation.
hair bundle deflection
hair cell is depolarized by the MET current
outwardly rectifying K+ channels open
outward flow of K+
repolarizes the hair cell membrane
voltage-gated Ca2+ channels open- influx of Ca2+
release of neurotransmitter at the synapses on to the primary afferent nerve
endings

35.  Supporting cells extend from the basement membrane to the apical surface,
and their nuclei are found just above the basement membrane and below
the hair cells.
 They contain well-developed Golgi complexes, mitochondria, and occasional
lipid droplets.
 The upper part contains large numbers of round or ovoid granules,
responsible for the formation of the cupula and otolithic membrane.

36.  The maculae of the utricle and saccule
are flat sheets of epithelium oriented at
right angles to each other, the utricle in
the anterior–posterior plane, the
saccule in the superior–inferior.
 The utricular macula is U-shaped and
the saccular macula is S-shaped.
 The cristae ampullares of the
semicircular canals are saddle-shaped
epithelial mounds contained within the
ampullae.

37.  The utricular and saccular maculae are overlaid by an ‘otoconial
membrane’ which consists of a large number of otoconia (Greek: ‘ear dust’).
 They are crystalline particles composed of calcium carbonate surrounding
a proteinaceous core that sit on a honeycomb-like perforated sheet of non-
collagenous fibrillar extracellular matrix.
 This matrix is composed of proteins - otogelin, α- and β-tectorins and
ceacam 16 (carcinoembryonic antigenrelated cell adhesion molecule 16).
 Hair bundles align with the perforations with the longest stereocilia, in
contact with the edge of the hole.
 The utricular and saccular maculae detect translational motion of the
head in the horizontal and vertical planes.

40.  In the semicircular canals, the
extracellular structure overlying
crista is called cupula.
 The cupula is a gelatinous mass
that consists of
mucopolysaccharides within a
keratin meshwork, which extends
from the surface of the cristae to
the roof and lateral walls of the
membranous labyrinth to form
a fluid-tight partition.
 The principal proteinaceous
component is otogelin.
 The cristae detect rotational
acceleration of the head.

43.  In the cristae the orientation of the hair bundles, which defines
morphological and functional polarity, is the same for all the hair cells in
an individual crista.
 In the cristae of the horizontal and superior canals the shortest stereocilia
of all hair cells are towards the utricle.
 In the cristae of the posterior canal the longest stereocilia (and kinocilium)
are on the side facing the utricle.
 Rotation in a particular direction produce excitatory deflections of all
stereociliary bundles of the cristae in one ear and inhibitory deflections of
the hair bundles of the cristae in the same semicircular canal in the
opposite ear, providing information on the direction of rotation.

45.  In the utricular macula, the shortest stereocilia face the centre -
excitation of the hair cells occurs with deflections of the stereocilia
towards the periphery of the macula.
 In the saccular macula the longest stereocilia, and excitatory hair
bundle deflection, are towards the centre.
 The line of hair bundle polarity reversal in a macula is located in the
middle of the ‘striola’, a delineated strip ,within the body of the
macula and shaped to follow its contour.

47.  Type 1 hair cells are flask-shaped with a single large afferent nerve
ending, a calyx, enclosing the entire basolateral surface of the cell.
 The endings of efferent nerves contact the afferent calyx but not the
hair cell itself.
 Morphologicallly, the stereocilia of type 1 cells are thicker (i.e. they
contain more actin filaments) than type 2 hair cells.
 The calcyeal nerve terminals around type 1 hair cells express high
levels of a calcium-binding protein, calretinin, which is not expressed in
the afferent endings that synapse with type 2 hair cells.
 Type 1 cells predominate across the striolar region of the maculae and
at the crest of the cristae.

48.  Type 2 hair cells are cylindrical with small bouton-like afferent
endings that form synapses towards the basal end of the cell.
 In type 2 hair cells the efferent endings contact the hair cell
directly.
 The nuclei of type 2 hair cells are located higher in the cell than
those of type 1s.
 Type 2 hair cells are confined to the extrastriolar regions of the
maculae and the skirts of the cristae.

49.  The ion-transporting epithelium of the vestibular system is the
region of ‘dark’ cells.
 Confined to the utricle and semicircular canals; there are no dark
cells in the saccule.
 The dark cell is a specialized cuboidal epithelial cell, in which the
basolateral membrane is infolded to create a large surface area.
These infoldings house numerous large mitochondria.
 Dark cells are crucial to maintaining the ionic concentration of
endolymph.
 The basolateral surfaces of the dark cells are exposed directly to
perilymph.

50.  Arises from a small group of 200 neurons lateral to the abducens
nucleus and the genu of the facial nerve, the group e neurons.
 These neurons project ipsilaterally and contralaterally.
 The contralateral pathway crosses at the level of the facial genu, joins
the ipsilateral pathway; both pass ventral to the vestibular nucleus.
 Cochlear efferents joins, originating from the superior olivary complex
(olivocochlear bundle).
 All of the efferents enter the vestibular nerve, fibers branch profusely
to innervate the entire sensory epithelium .

54.  The main blood supply to the vestibular end organs is the internal
auditory (labyrinthine) artery branch of (45%) anterior cerebellar
artery, superior cerebellar artery, or basilar artery.
 The labyrinthine artery divides into two branches: the anterior
vestibular artery and the common cochlear artery.
 The anterior vestibular artery supply to most of the utriculus and the
superior and horizontal ampullae, and small portion of the sacculus.
 The common cochlear artery forms two divisions, the proper cochlear
(or spiral modiolar) artery and the vestibulocochlear artery.
 The posterior vestibular artery supplies posterior ampulla, the major
part of the sacculus, and parts of the body of the utriculus and
horizontal and superior ampullae .