his 125 electrical potentials, hair cells, and the eighth nerve

As we previously learned electricity is

the hearing “language” for the brain.

We will review the most accepted

theory regarding the creation and

transmission of this “language” to the

brain.

ELECTRICAL POTENTIALS, HAIR CELLS &

EIGHTH NERVE

Electrical Events Within the Cochlea

The displacement of hair cells produces responses within these cells which cause a “transmitter release” (electric shock) from the base of the hair cell.

This “transmitter release” ultimately generates electrical nerve impulses into the fibers of the eighth nerve.


EIGHTH NERVE


A positive electrical potential has been found within the endolymph fluid of the cochlear duct (scala media).

This positive potential is maintained by the Stria Vascularis which is also responsible for the maintenance of the chemical composition of the endolymph fluid.


EIGHTH NERVE


The inside composition of outer hair cells have been found to have a negative electrical potential.

This electrical difference potential between the inner structure of the outer hair cells (negative) and the endolymph fluid (positive) is very large for a biologic system.


EIGHTH NERVE


When an acoustic stimulus is delivered to

the cochlea, this electrical positive—

negative balance is disturbed.

When this acoustic disturbance occurs,

two responses within the cochlea become

apparent.


EIGHTH NERVE


The two responses within the cochlea are:

1. The frequency of the acoustic stimulus

is reproduced by the cochlea (cochlear

microphone).

2. A sizable shift from the baseline

resting electrical potential is produced

(a summating electrical potential).


EIGHTH NERVE


When both of these potentials (the

frequency and the electrical potential)

reach a maximum “best” frequency, which

also corresponds to the maximum

displacement peak of the travelling wave,

the acoustic stimulus will transform into

an electrical stimulus for the eighth nerve.


EIGHTH NERVE


Let’ review Northern, chapter two, page

#24 figure 2-10.

As you will find the cochlear microphone

frequency response corresponds quite

well with the summating potential created

by the basilar membrane travelling wave

displacement.


EIGHTH NERVE

Hair Cells and the Cochlear Microphone

There is a shearing action which occurs

when the tectorial membrane and the

basilar membrane move and the hair cells

begin to move/shear. This creates the

opportunity for electrical impulses to be

generated within the cochlea.


EIGHTH NERVE


This shearing occurs due to the different

hinge points of each membrane and the

traveling wave movement which initiates

the mechanical articulation at these pivot

points.


EIGHTH NERVE

Note how

embedded

OHCs actually

pull tectorial

membrane

down

Outer Hair Cells: The Active

Cochlear Mechanism


This mechanical action results in the

stereocilia on top of the hair cells to

bend—thus, creating a certain amount of

mechanical gain due to the shearing force

between the two membranes.


EIGHTH NERVE


The outermost row of outer hair cells are

attached to the tectorial membrane.

The other rows drag across the tectorial

membrane and are influenced more by

the eddy movement of the endolymph

fluid than by the shearing action.


EIGHTH NERVE


This shearing force plus the viscous

streaming of endolymph is thought to be

the initial disturbance of the stereocilia

that generates a receptor current which

flows through the rest of the hair cell

body.


EIGHTH NERVE

Cochlear Electrical Potentials

The identification of positive and

negative electrical potentials were

clearly defined by Davis in 1960.

He placed the cochlear electrical

potentials into four classes.


EIGHTH NERVE


The four potentials are: 1. DC (direct current) resting potential with no

acoustic stimulation. 2. CM (cochlear microphonics) which are

alternating current in response to acoustic stimulation.

3. SP (summating potential) which is direct current but only appears with acoustic stimulation.

4. AP the (action potential) of the VIIIth nerve fibers.


EIGHTH NERVE


The cochlea is controlled by two “bio” batteries. The first battery is the hair cells (negative) and the second is the endolymph (maintained by the stria vascularis) positive.

The “variable resistor” (gain knob) are the stereocilia located on top of each hair cell.

This variable resistance changes as the stereocilia move, bend, and swirl.


EIGHTH NERVE

Hair Cell Movement & Electrical Potentials

PLEASE NOTE: When the blood supply to

the stria vascularis or the basilar

membrane is modified or compromised,

the electrical current generated from the

bio-batteries may deteriorate thus,

creating hearing loss.


EIGHTH NERVE


When the hair cells move to and fro, this creates alternating current and the cochlear microphone (CM) is created.

When the hairs cells all move in the same direction, a summating gain potential (SP) is created. How much movement and how many hair cells move, determine the amount of gain.


EIGHTH NERVE


This active gain/amplification process is

located between the basilar membrane

and the eighth nerve and also produces

additional frequency sharpening.

It is only present in life, as it requires bio-

energy to function.


EIGHTH NERVE

Cochlea Performance Summary

The cochlea is:

1. A sixty decibel WDRC amplifier (due primarily to outer hair cell movement).

2. A mechanical frequency analyzer (basilar membrane).

3. A cochlear microphone w/gain control (outer hair cell movement).


EIGHTH NERVE


The type of electrical current received

(alternating or direct) and, the amount of

electrical current received by the eighth

nerve create/generate the appropriate

afferent information for the central

pathways to disburse/process.


EIGHTH NERVE

his 125 electrical potentials, hair cells, and the eighth nerve

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