The document discusses how sound waves cause shearing between the basilar membrane and tectorial membrane in the cochlea. This shearing movement bends the stereocilia on top of hair cells, generating receptor currents. The currents create four types of electrical potentials: resting DC potential; alternating CM potentials from acoustic stimulation; direct SP currents also from sound; and action potentials in the VIII nerve. The movement of hair cells and different electrical potentials in the fluids of the cochlea stimulate the VIII nerve to transmit afferent signals about sound to the brain. Damage to hair cells or their blood supply can cause hearing loss by modifying the electrical currents.

1. Cochlear Microphonics & Hair Cells
 Hair Cell Shearing
There is a shearing action which occurs
when the tectorial membrane and the
basilar membrane move and hair cells
begin to move/shear. This creates the
opportunity for electrical impulses to be
generated within the cochlea.

2. Cochlear Microphonics & Hair Cells
 Hair Cell Shearing
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. (Ref. Zemlin pg. #488)

3. Cochlear Microphonics & Hair Cells
 Hair Cell Shearing
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.

4. Cochlear Microphonics & Hair Cells
 Hair Cell Shearing
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.

5. Cochlear Microphonics & Hair Cells
 Hair Cell Shearing
The 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.

6. Cochlear Microphonics & Hair Cells
 Hair Cell Shearing
We have learned that the perilymph and
endolymph fluids have different
consistencies. This difference creates
different electrical potentials—which have
either a positive or negative potential.

7. Cochlear Microphonics & Hair Cells
 Cochlear Electrical Potentials
This identification of positive and negative
electrical potentials were more clearly
defined by Davis in 1960. He placed the
cochlear electrical potentials into four
classes.

8. Cochlear Microphonics & Hair Cells
 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 current) which is direct current
but only appears with acoustic stimulation.
4. AP the (action potential) of the VIIIth nerve
fibers.

9. Cochlear Microphonics & Hair Cells
 Cochlear Electrical Potentials
The cochlea is controlled by two “bio”
batteries. The first battery is the hair cells
and second is the stria vascularis.
The “variable resistor” (gain knob) are the
stereocilia on top of the hair cells.
The variable resistance changes as the
stereocilia move, bend, and swirl.

10. Cochlear Microphonics & Hair Cells
 Hair Cell Movement & Electrical Potentials
When the hair cells move to and fro, this
creates alternating current and the
cochlear microphone is created.
When the hairs cells all move in same
direction, a summating gain potential is
created. How much and how many move
determines the amount of gain.

11. Cochlear Microphonics & Hair Cells
 Hair Cell Movement & Electrical Potentials
The type of current received (alternating or
direct) and the amount of current received
stimulate the eighth nerve and
create/generate the appropriate afferent
information toward the central pathways.

12. Cochlear Microphonics & Hair Cells
 Hair Cell Movement & Electrical Potentials
When the blood supply to the stria
vascularis or the basilar membrane is
compromised, the electrical current from
the bio-batteries is modified—creating
hearing loss.

13. Cochlear Microphonics & Hair Cells
 Hair Cell Movement & Electrical Potentials
Of course, when the stereocilia and hair
cells are damaged, hearing loss also
begins to reveal itself.

14. Cochlear Microphonics & Hair Cells
 Cochlea Performance Summary
We have learned that 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).

15. Cochlear Microphonics & Hair Cells
WOW! What do we do when the hair cells
become damaged or missing? What
treatment is available?
No surgery other than cochlear implants
are currently available—just digital hearing
instruments—custom fit by hearing health
care professionals.