Anatomy of the inner ear, vestibule, cochlea, Cochlear fluid, Hair cells, Auditory pathway, auditory neural pathway

1. ANATOMY OF INNER EAR
DR. SHILPA M J

2. Development of inner ear
• Develops from ectoderm in region
of hindbrain
• Thickening of ectoderm called auditory
placode (day 22-23)
• Invaginated to form otic pit
• Separates from surface ectoderm
to form otic vesicle

4. MEMBRANOUS LABYRINTH DEVELOPMENT

5. Development of inner ear
 22nd day – otic placode is seen.
 5th week – Auricle starts forming.
 6th week – The cochlea and semicircular canals starts
forming.
 8th week – The cochlea and semicircular canal assume their
definitive external form.
 10th week – Scala vestibuli and scala tympani appear.
 7th month – External acoustic meatus gets canalized.
 The ear is most sensitive to teratogens during the 4th and 9th
weeks, and can be affected upto the 12th week.

6. ANATOMY OF INNER EAR
• Location – Temporal bone
• LABYRINTH
• Parts - Bony labyrinth
- Membranous labyrinth

7. Osseous labyrinth
• Consists of cavities in petrous part of Temporal bone
• 3 main parts-
 Vestibule
 3 bony SCC
 Bony cochlea

9. Vestibule
• Small ovoid bony chamber – 5mm.
• Lies between medial wall of middle ear & outer part of
IAC
• Fenestra ovale or oval window - lies in lateral wall of
vestibule, separated from ME by foot plate of stapes &
annular ligament
• Aqueduct of vestibule lies in the posterior part of medial
wall of vestibule - a small canal which passes backwards
to the posterior surface of petrous bone, where it opens
under the surface of dura.

10. MEDIAL WALL OF VESTIBULE

11.  Posterior wall of vestibule contains 5 openings of SCC
 Anterior wall contains an elliptical opening into the scala
vestibuli of cochlea
 At the ant part of medial wall lies a depression called
spherical recess - perforated by several minute holes for
passage of filaments of acoustic nerve to saccule

12. • Behind it lies an oblique ridge, crista vestibuli, the ant
end of which is named pyramid of vestibule.
• This ridge bifurcates below to enclose a small
depression, cochlear recess, which carries cochlear nerve
fibres to base of cochlea.
• Above and behind the crest lies elliptical recess which
contains utricle.

13. Bony SCC
 3 bony SCC-Superior, Posterior, Lateral.
 Open to post part of vestibule by 5 apertures
 Superior and post SCC join posteriorly to form
crus commune.

14. Semicircular canals
 Superior SCC – 15 – 20mm
 Posterior SCC – 18 – 22mm
 Lateral SCC – 12- 15mm

15. Bony cochlea
• Coils for 23/4th turns, at distance
of 35 mm around modiolus
• Perforated by numerous
apertures for cochlear nerve
• Basal coil of cochlea forms a
bony bulge on medial wall of
middle ear called promontory

18. Modiolus
• Central bony axis
• Forms inner wall of bony canal of cochlea, which
winds round it.
• Thick at base, but rapidly tapers towards its apex.
• Traversed by several canals which transmit filaments
to Cochlear N.

20. Osseous spiral lamina
• Projects from modiolus into bony canal &
forms a sheet of bone which winds spirally
round the modiolus like ‘the thread of a
screw’.
• Spiral canal of the modiolus- spiral
ganglion
• Divides bony cochlea incompletely & gives
attachment to the basilar membrane

22. Chemistry of cochlear fluids
 Perilymph – Similar in composition to CSF
 High Na+, Low K+.
 Endolymph – Similar in compostion to ICF
 Low Na+ ,High K+
 High level of K+ is pumped into
endolymph from blood by the activity of
Stria Vascularis

24. Membranous labyrinth
• Contains a series of communicating sacs and
ducts, within the bony labyrinth.
• Divided into –
 Vestibular labyrinth (pars superior)
 Endolymphatic sac and duct
 Cochlea (pars inferior)

25. Membranous labyrinth
• Consists of-
 Saccule and utricle in bony vestibule
 3 membranous semicircular ducts
 Ductus cochlearis (scala media) in bony
cochlea
• Contains endolymph

27. Saccule and Utricle
• Utricle- lies in depression on upper wall of vestibule
• Saccule- lies in depression below & in front of utricle
• Communicates indirectly by means of slender
membranous tube called endolymphatic duct

29. Endolymphatic duct
• Location - bony aqueduct
of vestibule.
• 2 branches - utricular and
saccular duct which
separate to open
respectively into utricle and
saccule.
• Utricular duct ,on leaving
the utricle, curves at an
acute angle to form
utriculo-endolymphatic
valve.

30. 3 membranous SC ducts
• At right angles to each other.
• Open to post part of utricle by 5 separate openings.
• Dilates into ampulla at one or the other end.

31. Vestibular receptor organs
 Crista-special sensory epithelium found in
each ampulla
-supplied by br. of vestibular division of 8th CN
 Macula-specialized neurosensory epithelium
in utricle(in horizontal plane) and saccule(in
vertical plane)

33. Epithelium of these receptor organs
contains-
 Sensory cells containing hairs on their free surface
 Supporting cells
 Gelatinous substance lying on hairs and composed of
mucopolysaccharides secreted by supporting cells

34.  Two types of vestibular cell
 Type 1 cells - rounded, flask shaped and surrounded
by a nerve chalice
 Type 2 cells - cylindrical and no chalice.
 Project into gelatinous substance
 Cupula - dome shaped in ampulla.
 In maculae flattened- contains otoconia - called
Statoconial membrane.

36. • Crista in each ampulla sense angular acceleration
• Maculae in saccule & utricle senses linear acceleration
• Head acceleration in angular fashion stimulates SCCs
• Head acceleration in linear fashion stimulates maculae
in utricle & saccule

37. Scala media
• Cochlear duct subdivided by two longitudinally running
membranes.
• Three chambers – scala vestibuli, scala media and scala
tympani
• Scala media - Roughly triangular in shape in axial section
o Basillar membrane, stria vascularis and Reissner’s membrane

39. • Basilar membrane
-forms base of triangle
- stretches from free border of osseous spiral lamina,
via tympanic lip of the spiral limbus to the spiral
ligament
- spiral ligament supports cells of outer sulcus

40. • Continuous with these cells are cells of stria vascularis
-forms the 2nd side of triangle.
 Three layers of cells –marginal cells, intermediate
cells and basal cells
• 3rd side of triangle formed by Reissner’s membrane
• Two layers of cells separated by basement
membrane
• Inner endothelial cells and outer mesothelial cells.
• Tight junctions

42. • Scala media ends as a blind sac & separates bony canal
of cochlea into 2 passages
• Scala vestibuli- upper chamber of perilymph closed by
footplate of stapes; communicates functionally with
middle ear through oval window

44. • Scala tympani-lower chamber closed by secondary
tympanic membrane; communicates functionally with
middle ear through round window
• 2 chambers communicate with one another at apex of
modiolus through helicotrema
• Scala tympani is also in connection with the
subarachnoid space by way of cochlear aqueduct in
cochlear canaliculus.

45. Cochlear receptor organ
ORGAN OF CORTI
• Sense organ of hearing & is set upon
basilar membrane throughout whole length
of membranous cochlea
• 3 main structures
-sensory cells with hairs
-supporting cells
-gelatinous tectorial membrane

46.  Two rows of rod-like bodies, the inner and outer rods or
pillars of Corti.
 The two rows incline toward each other and come into
contact above, enclosing between them and the basilar
membrane a triangular tunnel, the tunnel of Corti
 Two types of hair cell- inner side of the inner rod-inner
hair cells in single layer.
 Outer side of outer rod-3-4 layers of outer hair cells.

47. • Supported externally by
• Cells of Deiter - cup shaped bodies support the hair
cells
• Cells of Hensen- line the outer edge of outer hair cells
• Cells of Claudius- lines the remainder of the basillar
membrane

49. Inner hair cells Outer hair cells
Total no. 4500 12,500
Rows 1 3-4
Shape Flask shaped Cylindrical
Nerve supply Primarily afferent and
very few efferent
Mainly efferent and
very few afferent
Development Earlier Late
Function Transmit auditory
stimuli
Modulate function of
inner hair cells
Vulnerability More resistant Easily damaged by
ototoxic drugs and
high intensity noise
Differences between inner and outer hair cells

50. Arterial blood supply

51. Venous drainage

52. Auditory Neural Pathway
Hair cells are innervated by
dendrites of bipolar cells of
spiral ganglion. Axons of these
cells form cochlear division of
CN VIII
End in Dorsal and ventral
cochlear nuclei on
each side of medulla
Superior olivary nucleus
Nucleus of lateral lemniscus
Inferior colliculus
Medial geniculate body
Auditory cortex