1. Hearing The Auditory Systems
Chapter 8
Hearing
The Auditory Systems

2. Reference P P P

3. Content Properties of Sound Role of Middle Ear in Sound Transmission
Function of Organ of Corti
Homeostatic Imbalances of hearing.

4. 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

5. Loudness of Sound 0 dB = hearing threshold 50 dB = normal conversation
90 dB = danger zone
120 dB = Rock concert
130 dB = Pain threshold

8. Part 2 Role of Middle Ear in Sound Transmission

9. Mechanisms Involved in Transformer Process
Size difference between Tympanic Membrane and Stapes Footplate
Lever action

10. First Component of Middle Ear Transformer 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)

11. Transformer Action of Middle Ear Lever Action
Fulcrum Effect pressure gain: 1.3 times

12. TRANSFORMER ACTION AMOUNT OF AMPLIFICATION
Pressure Gain Contribution from:
18.4 TM (Tympanic Membrane) to stapes footplate
1.3 Lever action
Total pressure gain
(18.6 x 1.3)

13. Part 3 Function of Organ of Corti
a structure rests atop the basilar membrane along its length
contains approx. 16,000 cochlear hair cells

16. 1. How to discriminate the frequency of the sound? ---
Traveling Wave Theory

17. 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

20. 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

21. 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+

22. Two DC Potentials (EP) Endocochlear Potential (EP)
+80 mV potential with respect to a neutral point on the body
due to the Stria Vascularis

23. +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

24. 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

25. 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.

28. Two AC Potentials Cochlear Microphonic Potential Action Potential (AP)
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

29. 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

30. Nerve and Conduction Deafness