Inner ear

1. Anatomy of inner ear
(vestibular apparatus)

2. Vestibular apparatus is the sensory organ for detection
of sensation of equilibrium
Within petrous(“ rock like”) part of temporal bone,
which guards sensory organs of the inner ear.
Arcuate eminence- important landmark seen on superior
view( corresponds to superior semi circular canal),in
middle cranial fossa approach

3. INNER EAR DEVELOPMENT
4 -8 wks

4. Otic placode -plaque like thickening of the surface ectoderm dorsal to 1st
branchial groove , at the end of 3rd wk – forms bipolar neurons of cochlear
and vestibular ganglion
Auditory pit- invagination into underlying mesenchyme
Otic cyst or auditory vesicle- utricular and saccular components of the
membraous labyrinth
Each vesicle divide into-
Pars superior (dorsal component)-(phylogenetically older)- scc and utricle –
more resistant to developmental malformations
Pars inferior(ventral component)-(phylofenetcally newer)- saccule and
cochlear duct
Otic capsule – mesenchyme tissue surrounding the otocyst forms otic capsule

5. The inner ear is composed of:
• a. Bony labyrinth:(Hard bony outer shell)
1. Cochlea
2. Vestibule( spherical recess lodges saccule , elliptical recess for
utricle)
3. Semicircular canals: Anterior (or superior), Lateral (or Horizontal) &
Posterior semicircular canals
• b. Membranous labyrinth (which is found inside the
bony Labyrinth):
1. Cochlear duct
2. Utricle and saccule
3. 3 Semicircular canals
4. Endolymphatic duct and sac

6. The vestibular apparatus
consists of
o 3 semicircular canals that are sensitive to angular
accelerations (head rotations)
o Otolith organs "utricle & saccule" that are sensitive to
linear accelerations.

7. Bony labyrinth
• Houses the sensory organs and soft tissue structures of
the inner ear
• Consists of
-cochlea
-3 semicircular canals
-vestibule
• Each bone has 3 layers
inner endosteal layer
middle enchondral and intrachondrial bone
outer periosteal layer

8. Semi circular canals
• Lateral (horizontal)
• Superior (anterior vertical)
• Posterior( posterior vertical)
Each canal has ampullated limb
(2mm ) and non ampullated limb (1mm)
Crus commune – non ampullated limbs of posterior and
superior canals fuse
Opens into the vestibule
Solid angle – angle formed by 3 SCC

9. Dehiscent superior
semicircular canal
Thinning or frank
dehiscence of bone
of superior scc
Sound or pressure
induced vertigo
(tullio
phenomenon)

10. Vestibule
Central chamber of bony labyrinth
4mm diameter
Medial wall – spherical recess for saccule
-elliptical recess for utricle
- cochlear recess for cochlear
duct

11. Mike’s dot
• Macula cribrosa
superior
• Passage of superior
vestibular nerve fibres
to the cristae ampullaris of
lateral scc
• Corresponds to extreme
lateral aspect of the IAC
• Important landmark in trans
labyrinthine surgery

12. Fissures in bony labyrinth
3 in number
Fissula antre fenestrum-evagination of perilymphatic space anterosuperior to
oval window
Fossula post fenestrum-evagination of perilymphatic space posterior to oval
window
Hyrtle’s fissure ( tympano meningeal hiatus)
-remnant of embryonic development
-route for csf leakage into middle ear
2 microfissures of temporl bone
1-extends between round window niche and ampulla of posterior scc
2-runs superoiferior to oval window

13. Membranous(endolymphatic) labyrinth
Houses within bony labyrinth
Consists of
• Cochlear duct( scala
media)
• 3 semicircular canal ducts
and their cristae
ampullaris
• Otolith organs (utricle and
saccule)
• Endolymphatic duct and
sac

14. Semicircular canal ducts
• Lateral or horizontal, Anterior or superior and Posterior.
• The semicircular canals are connected to the
utricle at their bases.
• Oriented at right angles to one another.
• Near one end of each canal is widening known as
ampulla, which contains crista ampullaris
and cupula

15. Crista ampullaris
Saddle shaped sensory organ of semicircular duct, contains
sensory hair cells of vestibular system
Crista consists of gelatinous mass, the cupula
Ciliae of hair cells are embedded in the cupula
Cupula act as membranous diaphragm stretching from crista
to opposite wall of canal
Cristae detects rotational acceleration of head

16. Crista ampullaris:
• Each semicircular canal works in concert with a partner located on
the other side of the head, which has its hair cells aligned
oppositely.
• There are three such pairs: the two pairs of horizontal canals,
and the anterior canal on each side working with the posterior
canal on the other side.
• Head rotation deforms the cupula in opposing directions for the
two partners, resulting in opposite changes in their firing rates.

17. The otolith organs:
Utricle & Saccule
• The sensory organs of the utricle and saccule are
the maculae.
Each macula consists of hair cells and supporting cells.
• The ciliary bundles of the hair cells project into the
overlying gelatinous matrix known as the otolith
membrane, which is studded with otoconia
• Otoliths –crystalline particles composed of caco3
surrounding proteinaceous core.( composed of protein
unique to inner ear -otogelin, alpha and beta
tectorins & caecam 16)

18. Macula
• It detect and respond to the position of the head with respect
to linear acceleration and pull of gravity.
• Each macula contains thousands of hair cells (type1 &
type2) that synapse with sensory endings of vestibular nerve
• Each hair cell has 5-10 small cilia called stereocilia plus one
large cilium called kinocilium.
• The kinocilium is always located in one side, and the
stereocilia gradually become shorter.
• Minute filaments connect the Tip of each sterocilium to the
next longer cilia and finally to kinocilium .

19. Macula
• When stereocilia bend to the direction of kinocilium, it results in
opening of K+ channels at the tip of the stereocilia, allowing K+
ions to enter and depolarize the hair cell.
• In respone to depolarization voltage-gated Ca+2 channels are
activated allowing for Ca+2 influx and the subsequent liberation
of transmitters to produce an action potential.

20. Macula
Conversely, bending of
stereocilia in the opposite
direction (backward to the
kinocilium) reduces the
tension on attachments
and this closes the ion
channels causing receptor
hyperpolarization and
inhibition of the cell.

21. Macula
• When the head is upright, the hairs project upward into the
gelatinous material.
• When the head bends forward, backward, or to one side, the
hair cells are stimulated as the gelatinous material of the
maculae sag in response to gravity causing the hair to bend.
• Stimulated hair cells signal nerve fibers resulting in impulses
traveling to the CNS through the vestibular branch of the
vestibulocochlear nerve and informing the brain of the
head’s new position.
• Brain responds by sending motor impulses to skeletal muscles
to contract/relax to maintain balance.

22. Macula
• In the utricle- macula lies
in the horizontal plane of
the inferior surface of
Utricle- so it determines
the orientation of the head in
upright position,
- senses motion in the
horizontal plane
• In the saccule, macula
lies in the vertical plane
and senses motion in the
sagittal plane (eg, up-
down movement)

23. BPPV
Otoliths may dislodge from maculae of utricle and settle within
endolymph of one or more of the 3 semi circular canals ,
most commonly posterior scc
When the position of head changes these displaced otoconia
moves within the canal and sends false signal to the brain , causing
vertigo
Particle repositioning maneuvers

24. Endolymphatic sac and duct
Membranous structure in the inner ear arise from posterior wall
of the saccule.
Formed by union of 2 ducts, from saccule and utricle
Passes through the vestibular aqueduct and terminal part is dilated
to form endolymphatic sac
Endolymphatic sac lies wihin epidural space of posterior cranial fossa
Papillary cystadenomatous tumours can arise from the endolymphatic
sac( can occur bilaterally in Von Hippel Lindau disease ) appear as
retrolabyrinthine destructive masses

25. Endolymphatic sac and duct

26. The vestibular fluids:
• Perilymph
✔Perilymphatic space- btw bony and membranous labyrinth
✔It is similar to the extracellular fluid.
✔(low potassium, high sodium).
✔Formed by blood filtrate from capillaries of spiral ligament
and direct continuation of csf
✔Absorbed in the loose connective tissue of the modiolum

27. ..Vestibular fluids
• Endolymph
✔In the membranous labyrinth.
✔It is similar to the intracellular fluid
✔(high potassium, low sodium).
✔Secreted by stria vascularis of cochlea, planum semilunatum and dark cells of
vestibule
✔Absorbed by endolymphatic sac
✔2 types of endolymphatic circulation- longitudial pattern
-radial flow pattern

28. MENIERE’S DISEASE (endolymphatic hydrops)
Alteration in absorption or secretion of endolymph

29. Endolymphatic sac –
surgical anatomy

30. Trautmann’s triangle
Important landmark to
reach posterior cranial
fossa from mastoid.
Solid angle

31. Internal auditory canal
8mm long 3.4mm diameter
within petrous part of temporal bone
Between posterior cranial fossa and inner ear
Medial end- porus acousticus internus in the posterior cranial fossa
Lateral end – abuts the bony labyrinth at its fundus
Provide passage for CN 7 , CN 8 & Labyrinthine artery
Contains vestibular ganglion

32. Internal auditory canal
At the fundus the vestibular,
facial and cochlear nerves are
in a constant relationship that
is determined by
Horizontal – falciform crust
Vertical- bill’s bar
Bill’s bar is useful landmark in
translabyrinthine surgery of
cerebello-pontine angle as it
seperates superior vestibular
nerve from anteriorly located
facial nerve

33. Vestibular nerves
Superior - superior and lateral scc, utricular macula, superior
portion of saccular macula
Inferior – inferior saccular macula, ampulla of posterior scc
(posterior ampullary branch / singular nerve)

34. Vestibular schwannoma
80% OF CPA tumours, 10% of brain tumours
Originate from the schwann cells of the vestibular nerve
at the transition zone of central and peripheral myelin
(obsteiner redlich zone)- present in medial part of IAC
Radiotherapy – gamma knife
Surgery-approaches
• Middle cranial fossa
• Retrosigmoid
• Translabyrinthine

35. Scarpas’s ganglion
Ganglion of the vestibular nerve
Located inside the IAC
Contains cell bodies of bipolar neurons whose peripheral processes
form synaptic contact with hair cells of vestibular sensory organs
Vestibular nerve carries the impulses to the vestibular nuclei in the
medulla and to vestibulo-
cerebellum

37. Venous drainage
Veins accompanying the
labyrinthine artery ,drain the
inner ear , pass out through the IAC
and drain into the transverse or
inferior petrosal sinus.