Physiology of Hearing
Physiology of Hearing
Physiology of Hearing
OF
HEARING
J.P. SOUAID, M.D., C.M., FRCS(C)
QUEENSWAY-CARLETON HOSPITAL
THE OTTAWA HOSPITAL
DEPARTMENT OF OTOLARYNGOLOGY
April 10, 2014
J. G. Marsan MD FRCSC
Physics of Sound
DEFINITION
Mechanical radiant energy
Transmitted by longitudinal pressure waves
In a material medium
CHARACTERISTICS
Frequency pitch
Amplitude - loudness
Mismatch of Impedance
Air
Water
Sound pressure
energy
0.1% Energy
absorbed
99.9% Energy
reflected
OBJECTIVES
EXTERNAL EAR
MIDDLE EAR
MIDDLE EAR
Serves as a transformer, acting to
increase the sound energy
transmitted to the cochlear fluids.
Middle-ear transformer effects:
A, The ratio of the areas of the
tympanic membrane and the
stapes footplate (A1/A2) results in
a pressure increase at the oval
window. (17:1) (25 dB)
B, The lever effect caused by the
unequal displacements of the
malleus and incus (L1/L2) about the
incudostapedial joint also
results in a pressure increase.
(1.3x) (2.5 dB)
C, Curved membrane effect.
Certain
areas of TM vibrate
more than others.
D, Phase difference between oval
and round windows (small effect)
TOTAL GAIN: 27.5 dB
17:
Hydraulic
effec
1
Lever effect
1.3x
22:1=(17x1.3):1
Combined effect
Next Step
Mechanical energy
Mechanical energy
COCHLEAR PHYSIOLOGY
Divided into :
1) Scala vestibuli and tympani:
Contains perilymph
ECF like : Na- 140. K- 4-10
Production of perilymph:
Unknown
Ultrafiltrate of blood?
From CSF?
COCHLEAR PHYSIOLOGY
2) Scala media:
Contains endolymph
ICF like: K- 144 meq, Na- 15-25meq
Bounded by :
Reissners membrane
Basilar membrane
Osseous spiral lamina
Lateral wall
COCHLEAR PHYSIOLOGY
Stria vascularis:
Lateral wall
Highly vascularized
Cells contain NA-K-ATPase to produce
endocochlear potential about +80mV
in the scala media
Decreases slightly from base to apex
ORGAN OF CORTI
Characteristic
Inner H.C.
Outer H.C.
Shape
Flask
Cylindrical
Number
3500
12000
Few
Many
Arrangement
Attachement to
tectorial membrane
None or loosely
Intracellular electric
potential
-40mV
-70mV
Stereocilia
COCHLEAR MECHANICS
1. Motion is a traveling wave moving
longitudinally from the base to the apex
of the cochlea.
2. Each point along the cochlear partition
vibrates at a frequency equal to that of the
stimulus. Tonotopic organization:
This means that specific areas of the basilar
membrane respond to specific frequencies
3. High frequency- base of cochlea
4. Low frequency- apex of cochlea
COCHLEAR TONOTOPIC
ORGANIZATION
Auditory CNS
Cochlea
Cochlear Nerve
Cochlear Nucleus (CN)
Superior Olivary
Complex (SOC)
Lateral Lemniscus (B)
Inferior Colliculus (IC)
Medial Geniculate Body
(MGB)
Trapezoid body
Auditory Cortex
(Temporal lobe,
Brodman area 41)
Extensive crossover
Tonotopicity of Cortex also.
MNEMONIC: N.N.S.L.I.M.
AUDIOMETRY
BONE CONDUCTION
TESTING
NORMAL AUDIOGRAM
CONDUCTIVE HEARING
LOSS
SENSORINEURAL HEARING
LOSS
TYMPANOMETRY
TYMPANOMETRY
TYMPANOGRAMS
TYMPANOGRAMS
Otoacoustic Emissions,
OAEs
Mechanism of Otoacoustic
Emissions
Types of OAEs
Otoacoustic
Ultrastructure
Inner HC
Outer HC
Position of nucleus
Center
Base
Cytoplasmic
organelles
Scattered
Adjacent to cell
membrane
Presynaptic
specializations
Large
Small or absent
Glycogen content
Low
High
Relation to
supporting cells
Completely
surrounded
Afferent
innervation
Inner HC
Outer HC
Ganglion cells
Number of
ganglion cells
TYPE 1
27000
TYPE 2
2100
Hair cell to
gangion cell ratio
1.8:1
5.7:1
Source
Lateral superior
olivary complex
Medial superior
olivary complex
Postsynaptic
target
Afferent dendrites
Efferent innerv.