Hearing Science: Cochlear Physiology

What are the three modes of cochlear physiology?

hydraulic

mechanical

chemo-electrical

What is the first step of the cochlea?

begins with mechanical stimulation

-stapedial footplate at oval window

What is the second step of the cochlea?

mechanical energy transduced into hydraulic motion of perilymph

-made possible by the polarization effect

What is the third step of the cochlea?

hydraulic action in bony labyrinth mechanically causes motion of endolymoh

-stimulates the Corti's Organ

-soft tissue displaced by wave motion

What is the fourth step of the cochlea?

motion of sensory cells creates chemo-electrical response

Georg von Békésy

Theory of the Traveling Wave

-first to study the function of the auditory organ experimentally

What four observations was Von Békésy's Theory of the Traveling Wave based on?

1. Deflection of the basilar membrane

2. Height of displacement/deflection

3. Point of maximal displacement is frequency specific

4. Frequency specificity is a passive feature

Deflection of the Basilar Membrane

as the stapes stimulates the perilymph at the oval window, the BM pushes downward by pressure of the hydraulic wave

-hydraulic motion travels from base to apex

-one-way direction of the traveling wave

Height of Displacement/Deflection

height of displacement gradually increases as the wave travels to the apex. This is due to the close match between the frequency of the wave's vibrations and the resonance characteristics of the basilar membrane

-the traveling wave reaches maximum amplitude, then quickly diminishes

Point of Maximal Displacement is Frequency Specific

it occurs at the place where the frequency of the wave matches the resonant characteristics of the basilar membrane (BM)

-recall that the BM gets wider and loses stiffness from base to apex. BM "tuned" to vibrate best at specific frequencies.

-highest frequencies at basal end

-lowest frequencies at apical end

-basilar membrane acting as series of passband filters

Frequency Specificity is a Passive Feature

due to gradient change in stiffness of the basilar membrane from base to apex

The Traveling Wave in the Cochlea

mechanical vibrations of the ossicular chain and transformed into hydromechanical energy in the cochlea

-"hydro" = the fluid in the cochlea

-"mechanical" = the motion of the structures within the cochlea

Is the maximal displacement of the basilar membrane frequency specific?

yes

When are low frequency sounds detected?

later than high frequency sounds

What is the function of the cochlea?

when the stapes moves back and forth, all locations along the basilar membrane receive the same vibrational input at the same time due to the incompressible nature of the fluid-filled cavity

Why does the traveling wave in the cochlea always move from base to apex?

it is in response to an applied force, the narrower and stiffer base will always vibrate before the wider and less stiff apex

-there is less inertia (less mass) to overcome at the base

What happens to the amplitude (amount of displacement) of the traveling wave from the base to the apex?

the traveling wave is relatively low at the base and then grows to its peak at the stimulus frequency location, then decays rapidly

What happens when the stereocilia bend towards the tallest row (away from the modiolus)?

triggers the excitatory phase of the hair cells

When does the excitatory phase of the hair cells occur?

when the BM moves towards the scala vestibuli

What happens to the hydromechanical energy?

it is transduced into neural energy as the signal moves into the afferent nerve fibers

Two Models of the Traveling Wave

linear and 3D model

Linear Model of the Traveling Wave

direction: from base to apex

-linear model shows the direction and amplitude of the traveling wave

-the amplitude is the distance between the following trough and the crest of the wave

-signal is processed at a place where the wave reaches the peak amplitude

-the part of the BM with resonant characteristics consistent with the frequency content of the sound input

What did Von Bekesy do?

demonstrated that the place of maximum displacement is frequency dependent

If the sound has a high frequency content...

the wave will not travel very far

If the sound has a low frequency content...

it will travel further

Why does the traveling wave not go beyond the location of "best resonance" (for moderate level sounds)?

due to the increasing resistance caused by an increasing impedence mismatch