ultra stucture cochlea

1. DR. RAJU MANDAL
JUNIOR RESIDENT
DEPT. OF E.N.T
M.C.H

2. WHAT IS COCHLEA ?
 The inner ear is contained in
the petrous apex of the
temporal bone, and is encased
in a bony structure called the
osseous or bony labyrinth
 The labyrinth consists of three
continuous sections:
1. Vestibule-
2. Cochlea-
3. Semicircular canals-

3. WHAT IS COCHLEA ?
 Snail shaped coiled
tube(cochlos is greek for
“snail”)
 2.5 to 2.75 turns round a
central axix called modiolus.
 30 mm long

4. PARTS OF COCHLEA
A. OSSEUS LABYRINTH.
B. MEMBRANOUS
LABIRYNTH-o
It has 3 canals>
1. Scala media(cochlear duct)
2. Scala vestibuli
3. Scala tympani

5. Scala media- separated froms
scala vestibuli by Reissner’s
membrane & from scala
tympani by basilar membrane.
contains endolymph.
Scala vestibule -
connected to middle ear via oval
window.
contains perilymph
Scala tympani-connected
to middle ear by
round window
contains perilymph

6.  THREE FUNCTIONAL UNITS OF COCHLEA-
ORGAN OF CORTI-
STRIA VASCULARIS-
SPIRAL GANGLION-

7. ORGAN OF CORTI
 It acts as ‘sensor `(auditory
receptor organ)of cochlea.=
inner hair cell
outer hair cell
supporting cell
basilar membrne
tectotial membrane

8. BASILAR MEMBRANE
Travelling wave moves from base to apex
along the basilar memb
For a pure tone stimulus,the
travelling wave reaches a maximum at
characteristic place along basilar memb. &
then decays.
Location of maximum depends on
frequency of stimulus(TONOTOPIC
ORGANIZATION OF COCHLEA)

9. Characteristic frequency at specific location depends on molecular
structure/passive system & active system.
Anatomical change from base to
apex includes increase width of
basilar memb. & size of O.H.C
Base is tuned for frequency as
high as 2oKHz & apex for as low
as 20 Hz
Sensitivity to noise/ototoxic
insult is more at base

10. Passive cochlear
mechanism
Active cochlear mechanism
 MECHANO-ELECTRICAL
Of I.H.C
 Energy cosnuming(prestin)
 Of O.H.C
 Susceptible to hypoxia or
noise induced insults.
 Activation of it causes altered
cochlear output

11. INNER HAIR CELL
 STUCTURES-
 3500 flask-shaped inner hair
cell lined up in single row.
 have hair bundles of highly organised actin filled
3-4 stereocilia
 Sterocilia graded in height-most lateral row
being tallest & most medial row being shortest.
 Sterocilia has dense rootlet that penetrate into
apical cuticular plate.
 Mechano-electric al transduction apparatus
present at tip of sterocilium which contains
mechanically gated cation channel.

12. FUNCTIONS-A.
MECHANO-ELECTRIC TRANSDUCTION>
movement of stapes > displacement of cochlear fluid in scala
vestibuli> incompressibility of perilymph causes movement of
basilar memb.
mechanical deflection of hair cell’s stereocilliay bundle towards
tallest row of stereocilia
Increase mechanical tension in transduction apparatus
Confrontational change in the transduction potein
Increase channel open & depolarzation of cell

14. B. ADAPTATION
 Rapid closure of
transduction protein by
ca⁺⁺ ion binding
“Fast” adatation
 Sliding of myosin based
motor which associated
with transduction
protein
“Slow” adaptation

15. OUTER HAIR CELLS
STUCTURES-
 Three rows of outer hair cell
 “v” shaped arrangement
 Cylindrical shape
 Contain steriocilia bundle like
inner hair cell but it touches
tectorial membrane.

16.  FUNCTIONS-AMPLIFICATION
> necessary for detection of sound at
low sound pressure. It has two mechanism=
A. Electromotility-electrical
stimulus when depolarize ,then OHC contract
& elongates (by Prestin motor protein) when hyperpolarised
OHC exert mechanical force that feeds back
into movement of basilar membrane further moves
basilar membrane moves sterocilia in excitatory
direction Amplification.
B. Active hair bundle movement-

17. SUPPORTING CELLS
1. Deiters cell-phalangeal
supporting cell of OHC
2. Hensen’s cel(outer marginal
cell)
3. Claudius cell
4. Border cell of held

18. TECTORIAL MEMBRANE
• Extracellular structure that overlies both IHC & OHC
• But only tallest sterocilia of OHC embedded into it
FUNCTION:
Previously –thought as simple liver which helps in moves
up & down of basilar memb.
Recently _thought as resonant gel & helps to increase
frequency selectivity of cochlea.

19. STRIA VASCULARIS
 Highly vascularised, multi-layered
tissue that is a part of
lateral wall of the scala media
3 layers-
1. Marginal (tight junction)
2. Intermediate
3. Basal layer(tight junction)
Extracellular space b/w marginal
& basal layer- intrastrial
compartment

20. COCHLEAR HOMEOSTASIS

21. COCHLEAR HOMEOSTASIS
K> enter hair cell by
M.E.T Ch >released
through basolateral surface
of H.C into extracellular
perilymph>
Type 2 & 1 fibrocyte of
spiral ligament takes up K
By KCN QN/E1 marginal
cell release K in Scala
media.
By NKCCL Ch ,K enter basal
layer of Stia vascularis.
ByKCNJ10 ch released to
intastrial space .From which
it actively pumped and
contraported to marginal
layer

22. FUNCTION
Due to such cochlear
homeostasis the endocochlear
potential of endolymph
formed(+85 mv)
With resting membrane
potencial of-45 mv creates 130
mv of K gradientwhich is one
of the main driving force of K
entry into cell.

23. DISEASE DUE TO ALTERED
COCHLEAR HOMEOSTASIS

24. SPIRAL GANGLION &
INNERVATION
It located in Rosenthal’s
canal within modulus of cochlea.
CONTAINS
Afferent fibres-upto cochler
nucleus in brain stem-
A. Type 1 ganglion neuron-
(95%)-thick ,myelinated,
innervate only single I.H.C
B. type2 (5%)ganglionic
neuron-thin ,unmyelinated,
hae multiple branches to
multiple O.H.C.
 Efferent fibres-
From superior olivary nucleus
via brainstem
to both I.H.C & O.H.C
allow central nervous
system to modulate the
operations of cochlea.

25. CENTRAL CONNECTIONS OF
AUDITORY SYSTEM
 Sound driven activity enters
in brain by way of auditory
nerve, it is transformed by no
less than 12 types of
projection in 7 major
processing centers before
converging in the auditory
thalamus.

26. COCHLEAR NUCLEUS
 Prominent bulge in lateral surface of
brainstem
 PARTS-
• Dorsalless prominent
potential site for generation
tinnitus
tonotopically organised with low
frequency ventrolateraly & high
frequency dorsomedially.
project directly to I.C.

27.  VENTRAL...>
a) Antero-ventral>
Initial processor of auditory
nerve inforation.
Bushy cell- send large callibre axon to
b/l olivary complex & helps in sound
localisation.
b) Post-ventral> Stellete neuron & Octopus &
fusiform cell- send fine multiple axon to Inf.
colliculus & help in encoding frequency ,
spectral & sound intensity.

28. SUPERIOR OLIVAY COMPLEX
• In caudal aspect of pons.
• It is first central auditory center to receive binaural
innervation.
• Two part-MSO & LSO
• Sound localisation by inter-aural time delay by MSO
& inter-aural intensity
difference by LSO.
• Also helps in sound detection & compound sound
processing.

29. Projections of S.O.C
ASCENDING DESCENDING
lateral lemniscus
nucleus of lateral
lemnicus
inferior
colliculus
OLIVO-COCHLEAR
BUNDLE
influence cochlear
sensitivity & tunning
through modulation of
OHC
Influence aspect of
hearing in noise.

30. LATERAL LEMNISCUS
• By which medullary & pontine
auditory nerve fibre reach to
inferior colliculus.
• Two part- ventral & dorsal
• Helps in sound localisation &
processing.
• Component of acoustic startle
reflex pathway( with ventral
cochlear nucleus )> form 3 to 5
wave in ABR.
receive differential
innervation from
ipsilateral & contralateral
cochlear nucleus & S.O.C
subdivisions.

31. INFERIOR COLLICULUS
• Present in midbrain
• Helps in sound localisation, frequency determination,
integration of auditory with non-auditory system.
• 3 parts-
1. Central nucleus-projections come from directly/
indirectly , mon/binaural
2. Cortex- Projections to primary & secondary
auditory cortex through M.G.B.
3. Paracentral nucleus

32. Central nucleus is layered into
isofrequency bands
Along each band, the cells have
same dendritic fields &
respond best to approximately
same frequency
Higher frequency in midline &
lower frequency towards
outside
Produce tonotopic map

33. MEDIAL GENICULATE BODY
 Present in thalamus
 3 parts-
A. Ventral-(3 subnuclei)-
projection to auditory
cortex.Helps in
tonotopic organisation.
B. Dorsal-(1o sub nuclei)
C. Medial-
Have both auditory & non-auditory
connections.plays
role in arousal & attending
in auditory stimuli.

34. AUDITORY CORTEX
 Deep within sylvian fissure of superior surface of of the
temporal lobe.
 Consists of multi-layered tonotopically organised
region
1. Primary(A1)-brodman area 41
2. secondary(A2)-broad area 42
3. Anterior auditory field area(AAF/A)
4. Ventral auditory field area(V)
5. Posterior auditory field area(P)

35.  Numerous cortical association areas surround the primary
auditory cortex-
1. Wernicke's area (left side)/ area 22-neural substrate for
receptive language
2. Just posterior to area 22, in the inferior parietal lobe, are
the angular gyrus and supramarginal gyrus (areas 39 and
40)-integrate auditory, somatosensory, and visual
information
3. Broca's area/ area 44 and 45-expressive language, and the
perception of musical syntax.

36.  Functions-
1. Discrimation of sound source
2. Localisation of sound
3. Recognition of voice
4. Auditory memory