Content
• 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
Mechanisms Involved in Transformer Mechanisms Involved in Transformer ProcessProcess
Size difference between Tympanic Tympanic MembraneMembrane and Stapes Footplate
Lever action
First Component of Middle Ear First Component of Middle Ear Transformer ActionTransformer Action
Size Difference– Tympanic membrane
.59 cm2
– Stapes footplate .032 cm2
– Pressure formula Pressure = force/area
Impact on sound transmission
Pressure gain: 0.59/0.032 = 18.4 (times)
Transformer Action of Middle EarTransformer Action of Middle EarLever ActionLever Action
Fulcrum Effect pressure gain: 1.3 times
TRANSFORMER ACTION TRANSFORMER ACTION AMOUNT OFAMOUNT OF AMPLIFICATIONAMPLIFICATION
Pressure Gain Contribution from:
18.4 TM (Tympanic Membrane)Tympanic Membrane) to stapes footplate
1.3 Lever action
23.9 Total pressure gain
(18.6 x 1.3)
Part 3 Function of Organ of Part 3 Function of Organ of CortiCorti
a structure rests atop the basilar membrane along its length
contains approx. 16,000 cochlear hair cells
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 Potentials2. 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
CochleaCochlea
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)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
Intracellular Potential (IP) or organ of corti potential (resting potential)
–Recorded -80 mV inside cells of organ of corti
Two DC Potentials (IP)Two DC Potentials (IP)
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
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.
There are little mechanical gates on each hair cell that open when they are bent.
Two AC PotentialsTwo 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