Download Chapter 8

Chapter 8
Hearing
The Auditory Systems
Reference
P142 - 151
P442-454
P651 - 662
Content
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Properties of Sound
Role of Middle Ear in Sound Transmission
Function of Organ of Corti
Homeostatic Imbalances of hearing.
Part 1. Properties of Sound
Sound travels in waves as does light
• 1. Pitch: determined by “frequency,” the number of
cycles per second of a sound wave, measured in hertz
(Hz)
• 2. Loudness: determined by “amplitude” (height) of
the sound wave, measured in decibels (dB)
• 3. Timbre: determined by “complexity and shape” of
the sound wave, gives each sound its unique quality
Loudness of Sound
• 0 dB = hearing threshold
• 50 dB = normal conversation
• 90 dB = danger zone
• 120 dB = Rock concert
• 130 dB = Pain threshold
Part 2 Role of Middle Ear in Sound
Transmission
Mechanisms Involved in Transformer
Process
 Size
difference between Tympanic
Membrane and Stapes Footplate
 Lever
action
First Component of Middle Ear
Transformer Action

Size Difference
– Tympanic membrane
 .59 cm2
– Stapes footplate
2
 .032 cm
– Pressure formula
 Pressure = force/area

Impact on sound
transmission
Pressure gain: 0.59/0.032 = 18.4 (times)
Transformer Action of Middle Ear
Lever Action
Fulcrum Effect
pressure gain: 1.3 times
TRANSFORMER ACTION
AMOUNT OF AMPLIFICATION
Pressure Gain
18.4
1.3
23.9
Contribution from:
TM (Tympanic Membrane) to stapes footplate
Lever action
Total pressure gain
(18.6 x 1.3)
Part 3 Function of Organ of
Corti
a
structure rests atop the basilar membrane
along its length
 contains approx. 16,000 cochlear hair cells
1. How to discriminate the frequency of
the sound? --Traveling Wave Theory
Vibration of Basilar Membrane and the
Traveling Wave Theory
• Sound wave entering at the oval window is
to cause the basilar membrane at the base
of the cochlea to vibrate
• different frequencies cause vibrations at
different locations (places) along basilar
membrane
• higher frequencies at base, lower
frequencies at top
2. 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
Cochlea
 Perilymph similar
in composition to extracellular
fluid.
 High in Na+ and low in K+.
 Endolymph found in the scala media.
 Similar to intracellular fluid. High in K+
and low in Na+
Two DC Potentials (EP)

Endocochlear Potential (EP)
– +80 mV potential with respect to a neutral
point on the body
– due to the Stria Vascularis
+80 mV
Reticular Lamina
-80 mV
Two DC Potentials (IP)
Intracellular Potential (IP)
or organ of corti
potential (resting potential)
–Recorded -80 mV inside cells of organ of corti
Hair Cell in the Organ of Corti
When the basilar
membrane moves,
a shearing action
between the
tectorial
membrane and the
organ of Corti
causes hair cells to
bend
There are little mechanical gates on each
hair cell that open when they are bent.
K+ comes into the hair cell and
depolarizes the hair cell.
The concentration of K+ in the endolymph
is very high so when it comes into the hair
the positive ions come to the cell causing
a depolarization.
Two AC Potentials

Cochlear Microphonic Potential
– Reproduces frequency and waveform of a
sinusoid perfectly
– Generated from hair cell

Action Potential (AP)
– Electrical activity from the VIII Nerve
– Can be measured from anywhere in the cochlea
or in the auditory nerve
Part 4 Homeostatic Imbalances of
hearing.
• Deafness.
– Conduction deafness • possible causes include: perforated eardrum,
inflammation, otosclerosis
– Sensineural deafness - nerve damage
• Tinnitus - ringing in the ear
• Meniere's syndrome - attacks of dizziness,
nausea, caused by excess endolymph in the
media canal
Nerve and Conduction Deafness