1. ELECTRICAL POTENTIALS, HAIR CELLS &
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.

2. 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.

3. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Electrical Events Within the Cochlea
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.

4. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Electrical Events Within the Cochlea
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.

5. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Electrical Events Within the Cochlea
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.

6. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Electrical Events Within the Cochlea
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).

7. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Electrical Events Within the Cochlea
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.

8. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Electrical Events Within the Cochlea
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.

9. ELECTRICAL POTENTIALS, HAIR CELLS &
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.

10. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cells and the Cochlear Microphone
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.

11. Outer Hair Cells: The Active
Cochlear Mechanism
Note how
embedded
OHCs actually
pull tectorial
membrane
down

12. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cells and the Cochlear Microphone
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.

13. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cells and the Cochlear Microphone
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.

14. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cells and the Cochlear Microphone
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.

15. ELECTRICAL POTENTIALS, HAIR CELLS &
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.

16. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
 Cochlear Electrical Potentials
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.

17. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Cochlear Electrical Potentials
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.

18. ELECTRICAL POTENTIALS, HAIR CELLS &
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.

19. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cell Movement & Electrical Potentials
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.

20. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cell Movement & Electrical Potentials
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.

21. ELECTRICAL POTENTIALS, HAIR CELLS &
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).

22. ELECTRICAL POTENTIALS, HAIR CELLS &
EIGHTH NERVE
Hair Cell Movement & Electrical Potentials
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.