Functional aspects of cochlea

anatomy and physiology of cochlea

1. FUNCTIONAL ASPECTS OF
COCHLEA
Presenter:Dr. SoumyaSingh
16/07/16

2. OVERVIEW
•EMBRYOLOGY OF INNER EAR
•ANATOMY OF INNER EAR
•PHYSIOLOGY
•THEORIES OF HEARING

3. Development of inner ear
•Inner ear has 2 parts
•Outer Bony labyrinth-mesoderm
•Inner Membranous labyrinth-ectoderm
•Development starts by 3 week ( 22-23 days)

4. •Initially membranous labyrinth
develops, followed by bony
labyrinth.
•In hind brain at the level of 4
th
ventricle Ectodermal thickening
(in 4mm embryo stage)
Oticplacode(thickening);4mm
Oticpit
Otocyst/oticvesicle
(endolymphaticduct from dorsal
surface-4
th
week);6mm
Membranous labyrinth
(by5-25
th
week of GA)
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6. BONY LABRYNTH
•Develops from the peripheral condensation of the
Mesenchyme enclosing the otocyst
•becomes chondrified to form otic capsule
Ossification begins in around 16th week .
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7. Inner ear is called labyrinth-
complexity of its shape.
•It lies in the petrous part of
the temporal bone.
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8. ANATOMY OF INNER EAR
•The osseous labyrinth consists
of three parts:
1.vestibule
2.Semicircular canals,
3. cochlea.
•lined by periosteum.
•Filled with a clear fluid,
theperilymph,in which the
membranous labyrinth is
situated.
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9. 1.The Vestibule (vestibulum)
•A small ovoid bony
chamber between the
medial wall of the middle
ear and the outer part of
the IAM.
•Lies in the central part of
the bony labyrinth.
•Ovoid in shape but
flattened transversely.
•Measures ~5mm x 5mm x
3mm
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10. 2. The Bony Semicircular Canals
•3 in number{superior , posterior andlateral}.
•situated above and behind the vestibule.
•unequal length , describes the 2/3rd of a circle.
•0.8 mm in diameter.
•Dilated ends-ampulla(double the diameter of the tube).
•They open into the vestibule byfive orifices, one of the
apertures being common to two of the canals-crus
commune.
•Angle b/w 3 SCC is solid angle.
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12. 3.The Cochlea
•Auditory organ
•Enclosed in the temporal bone
•Snail shaped osseous structure
•Derived from greek word
cochlos-snail.
•Coiled to about 2 2/3 turns
around a central axis modiolus.

13. •Modiolus–
•conical central axis / pillar of the cochlea.
•base is broad, and appears at the bottom of the internal acoustic
meatus.
•perforated by numerous orifices, which transmit filaments of the
cochlear division of the acoustic nerve.
•The nerves for the first and a half turn , pass through the
foramina of the tractus spiralis foraminosus.
•Those for the apical turn ,pass through the foramen centrale.
13

15. •basecorresponds with the bottom of the IAM, perforated by
numerous apertures for the passage of the cochlear division of the
acoustic nerve.
•apex(cupula) :directed forwards and lateral, slightly inclined
downward, toward the wall of the tympanic cavity
•Ht: 5 mm. from
base to apex,
•Breadth: 9 mm
across the base
Length of tube :
30mm
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16. •The cochlea partially divided into an upper
duct -scala vestibuli
•lower duct -scala tympani, by a thin bony
shelf called osseous spiral lamina, arising
from the modiolus of the cochlea.
•This division is completed by the membranous
cochlear duct or scala media.
•floor -Basilar membrane (seperates from scala
tympani)
•roof -Vestibular membrane/Reissner’s
membrane(scala vestibuli)

18. •Scala tympani and scala vestibuli are connected through
helicotrema.
•Scala vestibuli-connected to middle ear via oval window.
•Scala tympani-connected to middle ear via round window
•Scala media-end blindly.

19. The Membranous Labyrinth
(labyrinthus membranaceus)
•lodged within the bony cavity,
separated from the bony walls
by perilymph.
•Consists of
1.Cochlear duct( scalamedia)
2.3 semicircular ducts (with
their cristaeampullaris)
3.Membranous vestibule
(Utricle,saccule,Endolymphatic
duct and sac).
•Filled with endolymph.

20. FLUID SPACES OF COCHLEA
1. Perilymphaticspace : scalavestibuli& scala
tympani
this space surrounds membranous labyrinth drains
into CSF –by way of cochlear aqueduct.
2.Endolymphatic space : scalamedia
this space –continuing throughout membranous
labyrinth continues a endolymphaticduct –
endolymphaticsac.

22. ENDOLYMPH
•formed by stria vascularis
•stria vascularis contains increased
concentration of Na+/K+ ATPase,
adenyl cyclase & carbonic
anhydrase
•these enzymes causes active
pumping of ions and transport of
fluid into endolymph
•precursor for endolymph is
perilymph, rather than blood
•absorbed in endolymphatic sac
•high K+ conc & low Na+ conc
•Electric potential +50 to +120 mV

23. PERILYMPH
•Perilymph of S.Vestibuli originates from plasma
•Perilymph of S.Tympani originates from plasma &
CSF
•Increased Na+ conc & decreased K+ conc
•S. Vestibuli contains more K+ than S. Tympani
•Electrical potential
S. Vestibuli +5mV
S. Tympani +7mV

24. PERILYMPH
•Resembles extracellular
fluid.
•Rich in Na+ ions.
•SOURCE:2 theories
1)filtrate of blood serum
from capillaries of spiral
ligament.
2)CSF reaching labyrinth
via aqueduct of cochlea.
ENDOLYMPH:
•Resembles intracellular
fluid.
•Rich in K+ ions.
•SOURCE:
1) Striavascularis
2) Dark cells of utricle &
ampullated ends of
semicircular canals.
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26. Physiology of cochlea
•3 functional units:
1.Organ of corti-sensorof cochlea
(mechanoelectricaltransduction)
2.Stria vascularis-cochlea’s “battery”
(endocochlearpotential)
3.The spiral ganglion –”electrical wires”
Transports eletricalsignals from cochlea to CNS.

27. 1.ORGAN OF CORTI-OC
•Named after alfonsogiacomogasparecorti.
•OC separated from perilymphaticspace by
reticular lamina.
•fluid in OC is called cortiymph.
transducer surface of hair cells are bathed in
endolymph
converts the mechanical vibrations of the basilar
membrane into neural impulses

28. •The fibers of the auditory nerves travel from the organ of
Corti through a system of small perforations in the spiral
lamina collectively called habenula perforata.
•From habenula perforata, nerve fibers travel through a
channel in the center of the modiolus (Rosenthal's
canal), exit the base of the cochlea, and join vestibular
nerve fibers to form the vestibulocochlear nerve.
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29. Cells in organ of corti
•1.OHC
•2.IHC
•3.Outer pillar cells
•4.Inner pillar cells
•5.Inner marginal cell(IMC)
•6.Hensen’s cell(OMC)
•7.Claudius cell
•8.Boettcher cell
•9.Dieter cell

30. Cells in Organ of Corti
1. IHC
2. OHC
3. tunnel of Corti
4. BM
5. RM
6. TM
7. Deiterscells
8. Space of Nuel
8.Cells of Hensen
9.Inner Sulcus

31. 2 types of hair cells in the organ of Corti:
the inner hair cells (IHCs)
the outer hair cells (OHCs).
Each hair cell has a number of small hair-like projections
called stereociliafrom the top of the cell.
These group of highly organized actin-filled stereocilia is called
a stereocilia bundle.
They are graded in height most lateral row –tallest
Most medial row being the shortest.
Each type of hair cell in the ear is connected to the nervous
system by both afferent (ascending) and efferent (descending)
nerve endings.
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32. i = IHC
p = Pillar
o= OHC
d = Deiter
The stereocilia bundle of each hair cell is organized in several rows
forming either a ““W”” or ““V”” pattern for OHCs
shallow ““U”” pattern for IHCs

33. Outer Hair Cells (OHC)
three rows-13,500outer hair cells.
Though OHC > IHC, they receive only about
5%of the innervation of theafferentnerve
fibers from VIII nerve.
The OHC cilia are in a “W” shape.

34. Inner Hair Cells (IHC)
one row of -3,500inner hair cells.
receive about 95%of the innervation from
the afferentnerve fibers from VIII nerve.

35. characteristic Outer hair cell Inner hair cell
Number 12,000 3500
Location Farther from modiolusnearer
No. of rows 3-4 1
Shape of hair cells Cylindrical Flask shape
no.of rows of cilia 6-7 per cell 2-4 rows per cell
Steriocilia arrangementW or vshape Shallow Ushape
Length of steriocilia Long & thin Short& fat
Motility Motile nonmotile
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36. Outerhair cells Inner hair cells
Nervesupply Primarily efferent Mainly afferent
Development Develop late Develop earlier
Function Modulate function of
inner hair cells.
Transmit auditory stimulus
Vulnerability Easily damaged. More resistant.
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37. •The groups of IHCs and OHCs are separated by two
pillar cells of Corti, which structurally support the organ
of Corti.
•These cells are attached at their tips and more widely
separated at the base, forming a triangular shape called
the tunnel of Corti.
•The tunnel is filled with the cortilymphfluid that has
similar properties to the perilymph fluid found in the
bony labyrinth.
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39. •The tops of the hair cells and
supporting cells of the organ of
Corti are tightly connected
together at their tips to form a
continuous layer called the
reticular lamina.
•The reticular lamina isolates all
of the organ of Corti from the
endolymph of the scala media
•except for stereocilia which
project through the reticular
lamina into the endolymph.
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40. •The OHCs are held in position
by the outer pillar cell on one
side and by Deiters cells on the
other side.
•Each Deiters cell holds an OHC
at the bottom and through long
projections called phalangeal
processesfrom above.
•The middle part of an OHC is not
firmly supported and is
surrounded by a perilymph-
filled space called the space of
Nuel.
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41. Next to the Deiters, groups of supporting cells, Hensencells, Claudius cells,
outer spiral sulcus cells, Boettcher cells.
Lateral to these support cells is the Striavascularis 41

42. 2.STRIA VASCULARIS
•highly vascular organ attached to the lateral wall of the
scalamedia.
•3 functions
Produces endolymph, thus maintaining the endocochlearpotential
cochlear homeastasisand ion transport( k+).
The striavascularisis composed of three cell types:
1.Marginal cells, which line the endolymphaticcanal
2.Intermediate cells, which are rich in the pigment
melatonin
3.Basal cells.

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44. Stria vascularis and k+ circulation

46. 3.Spiral ganglion
•Located in rosenthals canal within the modiolus of
cochlea.
•Contains cell bodies of afferent neurons
•95% afferent(type 1 ganglion ) neurons-thick and
myelinated-exclusively innervate IHC
•A dozen type 1 ganglion innervate IHC-converging
innervation pattern)
•5% afferent(type 2 ganglion)neuron-thin and
unmyelinated-innervate OHC
•They divide into multiple branches and innervate
OHC(diverging innervation pattern)

47. •Afferent system carries all auditory information to brain stem.
•Efferent fibers originate in the brain stem from neurons located in
superior olivary complex.
•These send information to cochlea by synapsing with OHC
directly, and with afferent fibers beneath the IHC.

48. Cochlear Functions
•Mechanoelectrical Transduction-Converting
acoustic mechanical energy into electro-
chemical energy.
•Frequency Analysis-Breaking sound up into its
component frequencies

49. Tonotopic organisation of cochlea
•Sound strikes the ear drum vibration is transmitted
to the inner ear by 3 ossicles
•Movement in the stapes which displaces the
perilymph in SV.
•Incompressibility of perilymph causes pressure
gradient in SV & ST-movement of BM and OC
•This displacement is conceptualised as travelling
wave which moves from the base to apex along BM.
•Reaches its max (depending on the frequency of
stimulus) at a characteristic place along BM then
decays.
•Base of cochlea-20khz apex-20hz

50. Psychology 355 50
The Inner
Ear
BM wider at
apex, stiffness
decreases from
base to apex.
Tonotopic
gradient is thus
maintained .

51. •Other factors responsible for tonotopicgradient apart from
BM
1.change in height of hair cell
2.change in length of cellular structures (stereocilliary
bundles)

52. Travelling wave
•Von Bekesy noted that the motion of the basilar
membrane was in the form of a travelling wave.
•The wave oscillates at the frequency of stimulation, but it
is not a sinusoidal wave.

53. Travelling wave characteristics
•Always starts at the base of
the cochlea and
moves toward the apex
•Its amplitude changes as it
traverses the length of the
cochlea
•The position along the
basilar membrane at which
its amplitude is highest
depends on
the frequency of the
stimulus
•All of these characteristics
depend on the change in
stiffness along the length of
the basilar membrane.
•Vibration then dies out
rapidly

54. Travelling wave envelope
•The curve that shows the amplitude of the travelling wave at each
point along the basilar membrane is called its envelope.
•The envelope has a positive and a negative side, but we generally
only talk about the positive half (because that’s when the stereocilia
get pushed over in the right direction to get a neural response).
•The peak of the envelope is at the place where the travelling wave is
biggest.

55. Mechanoeletrical transduction
•Inner Hair Cellsare the true sensory
transducers, converting motion of
stereocilia into neurotransmitter release.
MechanicalElectro-chemical
•Outer Hair Cellshave both forward and
reverse transduction--
Mechanical Electro-chemical
MechanicalElectro-chemical

56. “TIP LINKS”
•Tip Links connect tip of
shorter stereocilia to
the side of a
stereocilium in the next
taller row
•Bending toward taller
rows pulls tip links
•Bending toward
shorter rows relaxes tip
links
•CADHERIN 23 & PROTOCADHERIN 15
components of tip links
•MYOSIN 1c -adaptation process.

57. •THE HAIR CELLS MUST BE
ARRANGED IN SUCH THAT
THEY CAN PICK UP RADIAL
SHEAR.
•SO THE OUTER HAIR CELL
STEREOCILIA ARE CLOSELY
PACKED & HAVE A W OR V
SHAPED ARRANGEMENT
SUCH THAT THE TIP LINKS
RUN RADIALLY.
•STEREOCILIA OF INNER HAIR
CELLARE LOOSELY PACKED &
THE TIP LINKS ARE IN A
STRAIGHT LINE.

58. HAIR CELL TRANSDUCTION
•DISPLACEMENT OF PERILYMPH BY MOVEMENTS OF STAPES CAUSES
VIBRATION OF THE BMIN A DIRECTION PERPENDICULAR TO ITS AXIS .
TECTORIAL MEMBRANE MOTION .
•ORGAN OF CORTI MOVES UPWARD & DOWNWARD .
•MECHANICAL DEFLECTION OF THE STEREOCILIA
(MECHANOELECTRICAL TRANSDUCTION APPARATUS CONTAINING
MECHANICALLY GATED CATIONS CHANNELS AND TIP LINKs)
INCREASE OF MECHANICAL TENSION –CHANGE IN TRANSDUCTION CHANNEL
PROTEIN CONFORMATION -OPENING OF THE CATION CHANNELS

59. 59

61. •K+, Ca+ENTER THE HAIR CELL NEAR TIP
•GENERATION OF RECEPTOR POTENTIAL.
•Local increase of Ca+ affects myosin motor -slippage of
transduction apparatus-decrease in the mechanical tension in tip
links. Further opening of channels
•DEPOLARISATION OF HAIR CELL
•WHEN STEREOCILIA ARE DEFLECTED TOWARDS SHORTER
STEREOCILIA---TRANSDUCTION CHANNELS CLOSE
•HYPERPOLARISATION

62. Resting (or Membrane) Potentials
•Inner Hair Cell = -45 mV
•Outer Hair Cell = -70 mV

63. OHC AND AMPLIFICATION
•Responsible for amplification and sharp
tunning
•Amplification occurs by
•1. Somatic electromotility-
mediated by PRESTIN
•2. hair bundle movements

64. •SOMATIC ELECTROMOTILITY:
•OHC can change their length by 3-5% in
response to stimulus
•When depolarised-contract
•When hyperpolarised –elongate
•hence exerting a extra mechanical force that
feeds back into BM movements (caused by
travelling wave).
•Mediated by PRESTIN-located at lateral
membrane of OHC
•Hyperpolarisation: anion binds to prestin-increase surface area-cell
elongation
•Depolarisation:dissciation of anion –decrease surface area-cell contraction

65. Tectorial Membrane
•Extracellular structure overlies both IHC &OHC
•Only the tallest stereocillia of OHC are embedded in
it, stereocilia of inner hair cells are not embedded
they fit loosely into groove in the HENSEN’S STRIPE
of TM.
•they are driven by viscous drag of endolymph
•TM attached on its inner edge to spiral limbus
•Loosely connected to supporting hensens cells by
TRABECULAE.
•Enhances the frequency selectivity of cochlea by
unknown mechanisms.

66. Tectorial Membrane

67. Electrical Potentials
DC vs. AC
–Direct Current (DC) = stimulus doesn’t
change with time, constant; i.e. battery
–Alternating Current (AC) = always changing
over time, looks like a sine wave

68. Potentials:
•Resting Potentials: voltages which exist
without external stimulation
e.g., Endolymphatic Potential,
Cell Membrane Potential
•Stimulus-Related Potentials: voltages
occurring in response to
stimulus(electrical,mechanical)
We’ll talk about 3 of these from the cochlea

69. Endocochlear Potential (EP)
–Békésy discovered EP by putting the
electrode in the scala media and discovered
a +80 mV potential with respect to a neutral
point on the body
–Tasaki discovered EP was due to the Stria
Vascularis
Two DC Potentials (IP)

70. The endocochlear potential (EP) is the positive
voltage of 80 -100 mV seen in the endolymphatic
space in the scala media of the cochlea

71. -80 mV
Reticular Lamina
+80 mV
Intracellular Potential (IP) or organ of corti
potential (resting potential)
–Recorded -80 mV inside cells of organ of corti

72. GROSS ELECTRICAL RESPONSES OF THE
COCHLEA
•3 TYPES
1.COCHLEAR
MICROPHONIC
AC response this follows the
waveform of stimulus . it is
the produced mainly by the
outer hair cells .

73. 2.SUMMATING POTENTIAL: (DC
shift)this is produced by the
OHC& a part by IHC. it can be
positive or negative depending on
the stimulus .
3.NEURAL POTENTIALS :at the
beginning & at the end which is a
series of negative deflection . it is
produced by action potential
arising from auditory nerve
fibres.

75. THEORIES OF HEARING
•PlacetheoryofHelmholtz.
•TelephonetheoryofRutherford.
•VolleytheoryofWever.
•TravelingwavetheoryofBekesy.

76. PLACE THEORY
•According to Helmholtzbasilar membrane has
different segments that resonated to different
frequencies.The selectivity of cochlea and pitch
discrimination is based on the place of displacement
of BM
•Basilar membrane act as a series of tuned
rasonators.
•Each resonant of basilar membrane which is paticular
to its own place.
•High frequency waves exite the basal region and low
frequency the apical region.

77. TELEPHONIC THEORY
•Rutherfordproposedthattheentirecochlearespondsasa
wholetoallfrequenciesinsteadofbeingactivatedonaplace
byplacebasis.Herethesoundofallfrequenciesare
transmittedasinatelephonecableandfrequencyanalysisis
performedatahigherlevel(brain).
•Damagetocertainportionsofcochleacancausepreferential
lossofhearingcertainfrequenciesi.e.likedamagetothe
basalturnofcochleacausinginabilitytohearhighfrequency
sounds.Thiscannotbeexplainedbytelephonetheory.

78. Volley theory of Wever
Combine both place and telephone theories:
•High frequencies (5000Hz) are perceived as per
place theory (in basal turn)
•Low frequencies (1000Hz) stimulate nerve action
potentials at a rate equal to the stimulus
frequency.
•Intermediate frequencies are represented in the
auditory nerve by asynchronous discharge in
groups of neuron, whose combined activity
represents the frequency to stimulus.

79. TRAVELLING WAVE THEORY
•ProposedbyBekesy
•Thistheoryproposesfrequencycodingtotakeplaceatthe
levelofcochlea.
•Highfrequencyarerepresentedtowardsthebasewhilelower
frequenciesareclosertoapex.

80. Travelling wave theory
•The movements of the
footplate of the stapes set up
a series of traveling waves in
the perilymph of the scala
vestibuli
•High-pitched sounds generate
waves that reach maximum
height near the base of the
cochlea; low-pitched sounds
generate waves that peak near
the apex
•The basilar membrane is not
under tension, and it also is
readily depressed into the
scala tympani by the peaks of
waves in the scala vestibuli

81. helicotrema
oval window vibrates most to
high frequencies
(around 10 kHz) vibrates most to
middle
frequencies
(around 1 kHz) vibrates most to
low frequencies
(down to around
27 Hz) LESS STIFF
STIFF
HIGH
FREQUENCIES
LOW FREQUENCIES

82. Thank you