Chapter 8

Hearing

The Auditory Systems

Reference

P142 - 151 P442-454 P651 - 662

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

Part 2 Role of Middle Ear in Sound Transmission

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

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 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

Nerve and Conduction Deafness