Physiology & Theories of hearing

1. Auditory Physiology
of Inner ear
&
Theories of Hearing
Dr.Vinod M.K.
GMC,Amritsar

2. THREE FUNCTIONAL UNITS
WITHIN COCHLEA
• ORGAN OF CORTI- the sensor
• STRIA VASCULARIS- the battery
generating endocochlear potential
• SPIRAL GANGLION- electrical wires
connecting cochlea to CNS

4. – Movement of basilar membrane
– Shear force between tectorial membrane &
hair cells
– Cochlear microphonics
– Nerve impulses
Transduction of mechanical
energy to electrical impulses

8. Cochlear hair cells

10. • The tops of the hair cells in
the organ of Corti are held
rigid by the reticular lamina,
and the hairs of the outer
hair cells are embedded in
the tectorial membrane
• The hairs of the inner hair
cells are not attached to the
tectorial membrane, but they
are apparently bent by fluid
moving between the tectorial
membrane and the
underlying hair cells.

11. • Upward movement of the basilar
fiber rocks the reticular lamina
upward and inward toward the
modiolus.Then, when the basilar
membrane moves downward,the
reticular lamina rocks downward
and outward.
•
• The inward and outward motion
causes the hairs on the hair cells
to shear back and forth against
the tectorial membrane.Thus, the
hair cells are excited whenever
the basilar membrane vibrates

13. Tip links
• the tops of the
shorter stereocilia
are attached by
thin filaments to
the back sides of
their adjacent
longer stereocilia
• .

14. MODEL OF STEREOCILIARY
MECHANOELECTRICAL TRANSDUCTION
APPARATUS

15. Mutation in human genes encoding
CADHERIN 23 or PROTOCADHERIN 15
causes USHER SYNDROME
-congenital hearing loss
-progressive loss of vision due to retinitis
pigmentosa

16. Depolarization/activation
• when the cilia are bent in
the direction of the longer
ones, the tips of the
smaller stereocilia are
tugged outward.This
causes a mechanical
transduction that opens
200 to 300 cation-
conducting channels,
allowing rapid movement
of potassium ions from the
surrounding scala media
fluid into the stereocilia,
which causes
depolarization of the hair
cell membrane

17. • The influx of potassium
inside the cell causes
activation of calcium
channels
• This calcium drags the
neurotransmitter filled
vesicle to fuse with cell
membrane at base of
cell.
• Neurotransmitter
(glutamate)releases and
excites the dendrites of
afferent nerve fibres.

20. Transducer Mechanism

22. Endocochlear potential
• An electrical potential of
about +80 millivolts
exists all the time
between endolymph and
perilymph, with positivity
inside the scala media
and negativity outside.
• This is called the
endocochlear potential,
and it is generated by
continual secretion of
positive potassium ions
into the scala media by
the stria vascularis

24. OUTER HAIR CELLS &
AMPLIFICATION
• Outer hair cells play a key role in amplification
of basilar membrane motion.
• One mechanism is somatic electromotility
• Upward stimulation of basilar membrane moves
stereocilia in excitatory direction and
depolarizes outer hair cells,which in turn
shorten,further pulling basilar membrane
upward and amplifies its motion.
• This provides an extra gain of 40-50dB to the
system

25. STRIA VASCULARIS
• Generates endocochlear potential and
maintains unique ion composition of
endolymph
• Comprised of 3 cell types
-marginal
-intermediate
-basal cells

26. K+ FLOW THROUGH ORGAN OF
CORTI & STRIA VASCULARIS

27. • Mutation of human genes KCNE1 &
KCNQ1 cause JERWELL and LANGE-
NIELSEN syndrome(hearing loss and
cardiac arrhythmia)
• Mutation of BARTIN gene or mutation of
ClC-Ka and ClC-Kb subunits of basolateral
chloride channel leads to BARTER
SYNDROME type 4(deafness and renal
salt wasting)

28. SPIRAL GANGLION
• Located in Rosenthal canal within
modiolus of cochlea.
• 95% of afferent fibres are thick and
myelinated and exclusively innervate inner
hair cells.
• Remaining fibres are thin and
unmyelinated,contact outer hair cells.
• Efferent fibres originate in brain stem from
superior olivary complex,allows CNS to
modulate operations of cochlea

29. Auditory pathway
• Eighth (Auditory) nerve
• Cochlear nucleus
• Olivary nucleus (superior)
• Lateral lemniscus
• Inferior colliculus
• Medial geniculate body
• Auditory cortex

32. Theories of hearing
Place / Resonance Theory (Helmholtz, 1857)
Perception of pitch depends on selective
vibration of specific place on basilar membrane.
Telephone Theory (Rutherford, 1886)
Entire basilar membrane vibrates. Pitch related
to rate of firing of individual auditory nerve
fibers.

33. Theories of hearing
Volley Theory (Wever, 1949)
> 5 KHz: Place theory; <400 Hz: Telephone theory
400 – 5000 Hz: Volley theory
Groups of fibres fire asynchronously (volley
mechanism). Required frequency signal is
presented to C.N.S. by sequential firing in groups
of 2 - 5 fibers as each fiber has limitation of 1 Khz.

35. Sound stimulus produces a wave-like vibration of
basilar membrane starting from basal turn towards
apex of cochlea . It increases in amplitude as it moves
until it reaches a maximum & dies off. Sound
frequency is determined by point of maximum
amplitude. High frequency sounds cause wave with
maximum amplitude near to basal turn of cochlea.
Low frequency sound waves have their maximum
amplitude near cochlear apex.
Bekesy’s travelling wave theory

39. Georg von Bekesy
Won Nobel
prize for
his
traveling
wave
theory in
1961

40. Bekesy’s travelling wave theory

42. Thank You