The inner ear is composed of an osseous portion containing fluid and structures like the cochlea and semicircular canals, and a membranous portion containing additional fluid and structures such as the saccule and utricle. The inner ear consists of the vestibular portion for balance and the cochlear portion for hearing, with the cochlea being a spiral shaped structure that converts sound into electrical signals through hair cells. The cochlea contains three fluid filled canals and uses traveling waves that vary in amplitude at different points along the basilar membrane to detect different frequencies of sound.

1. The Labyrinth
The inner ear, often referred to as
the labyrinth, is composed of two
separate physical parts.
They are:
1. The osseous (bony) portion.
2. The membranous portion.

2. Osseous portion
Contains the:
1. Perilymph fluid
2. Cochlea
3. Three semicircular canals
4. Vestibule

3. Membranous portion
Contains the:
1. Endolymph fluid
2. Saccule
3. Utricle
4. Cochlear duct

4. The inner ear consists of two major
functional parts. They are:
1. The vestibular portion (balance)
2. The cochlear portion (hearing)

5.  The cochlea is one of the most
complicated organs in the body, but
its purpose is simple.
 It changes sound into electricity and
electricity is the “language” the
BRAIN understands.

6.  The size of the cochlea is relatively
small—about the size of the tip of your
little finger.
 Its diameter is about that of the tympanic
membrane.
 Its snail shape has only two and a half
turns in humans, and varies among other
mammals.

7.  The cochlea is almost completely formed
by the end of the first trimester of
pregnancy.
 Hearing (the cochlea) is one of the first
senses of embryonic development for
electrical stimulation of the brain.

8.  The cochlea is located within the petrous
portion of the temporal bone. The bony
labyrinth is the hardest bone within the
human body.

9. The Cochlea
It is a fluid filled cavity which is
divided into three canals. They are:
1. The scala vestibuli
2. The scala media (cochlear duct)
3. The scala tympani

10.  The top chamber of the scala
vestibuli begins at the oval window
and ends at the helicotrema.

11.  The scala media is the middle
chamber where the hair cells are
located.
 It is separated from the scala
tympani by the basilar membrane
upon which the Organ of Corti
(where the hair cells are) is resting.

12.  The scala tympani begins at the
round window and meets the scala
vestibuli at the helicotrema (the apex
of the cochlea)
 The scala tympani and the scala
vestibuli share the same
perilymphatic fluid (the fluid is mostly
potassium—less sodium)

13.  The scala media has a more thick
endolymph fluid which consists of
exactly the opposite proportions of
potassium and sodium relative to the
perilymph fluid.

14. Perilymph has a viscosity similar to
water. The cochlear duct (scala media)
includes endolymph, hair cells, the
tectorial membrane and the basilar
membrane. It is the consistency of
gelatin.
There is no physical discontinuity
between the cochlear duct (scala media)
and the perilymph fluid.

15. Cochlear blood supply
Is very complex, but primary supplies are
around the scala media area.
The stria vascularis maintains the chemical
composition of the endolymph.
There are a group of small blood vessels
just below the basilar membrane which
supply the organ of Corti with nutrients.

16. Cochlear blood supply
Please note, there is no direct
contact with the vascular system and
the structures of the cochlea.
There is an “intermediate transfer
system” through the intracochlear
fluids of each site.

17. Perilymph fluid movement
Since the bony portion of the labyrinth is
solid, the only release for fluid movement
is the round window.
If the round window was solid bone, the
stapes would be unable to move the fluid
from the oval window side.

18. Characteristics of the Basilar Membrane
 It is about .1mm at its base increasing in
width to about .5mm at its apex.
 Its stiffness is about one hundred times
greater at its base than its apex.
 These characteristics become the
determinants of its frequency response
patterns.

19. Characteristics of the Basilar Membrane
 There are transverse bands (side-to-side)
which are located along the length of the
basilar membrane.
 These transverse bands vary in stiffness as
they are spaced along the basilar
membrane.
 Each band is (frequency) sensitive to the
various waves of energy received along the
traveling wave pathway.

20. Characteristics of the Traveling Wave
The first portion of sound stimulus wave
undulation of the basilar membrane is
generally received close to the stapes. The
wave continues with increased undulation to
a maximum point along the basilar
membrane; this maximum amplitude point is
dependent upon the frequency of the
stimulus.

21. Characteristics of the Traveling Wave
The point of maximum amplitude for high
frequencies is closest to the basal end; while
the maximum amplitude for low frequencies
is closer to the apical end.

22. Characteristics of the Traveling Wave
The velocity and therefore the wavelength
decrease as a function of distance from the
stapes.
This reduction of amplitude, velocity, and
wavelength is commonly found with any
sound transmission through a fluid.

23. Characteristics of the Traveling Wave
This frequency dependent maximum basilar
membrane displacement is a clear indication
that the cochlea performs a mechanical
frequency analysis.
This is defined as the Place Theory of
frequency resolution.

24.  Let’s review figure 1-1 in Venema
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