Anatomy and
Physiology of the Outer and Middle Ears
I. Pinna
A. Anatomy
1. Parts (a series of ridges and depressions)
a. Helix
b. Antihelix
c. Triangular Fossa
d. Scaphoid Fossa
e. Tragus
f. Antitragus
g. Concha
h. Lobule
i. Auditory Meatus
2. Composition
a. A plate of cartilage
b. Covered with skin
3. Muscles
a. 3 extrinsic muscles
b. 6 intrinsic muscles
B. Physiology
1. Localization
a. Localization = determining direction of sound
source
b. Reception of high frequency sounds is necessary
c. The pinna helps in sound localization as the pinna
collects high
frequency energy
d. Reason:
1) High frequency sounds have short wavelengths
and don’t
bend around corners (especially
above 2000
Hz)
2) Recall wavelength formula
3) So pinna helps to collect high frequencies and
funnel into
the ear
II. External auditory meatus
and canal
A. Anatomy
1. Cartilaginous portion
a. Outer (lateral) portion of canal
b. Covered with skin
c. Contains glands that produce cerumen
d. Contains tufts of hairs called goatees
2. Osseous portion
a. Inner (medial) portion of canal
b. Covered with skin
3. Shape
a. Most people’s are sideways s-shaped
b. Usually pull pinna straight out to view tympanic
membrane with an
otoscope
c. Weaknesses in cartilage (fissures of Santorini)
allow you to do
this
4. Isthmus
a. Point where canal switches from cartilaginous to
osseous portion
b. Usually about 6 mm from tympanic membrane
B. Physiology
1. Sound conductor
a. Conducts sounds from pinna to tympanic membrane
2. Resonator
a. Resonance = enhancement of the intensity of certain
frequencies as
they are transmitted
b. Frequencies enhanced are dependent on the length
and diameter of
the tube
c. Increase in intensity of sound transmitted through
external
auditory canal is ~ 10-15 dB SPL for
frequencies from
1500 – 7000 Hz with the peak
resonance (best
enhancement) occuring between
2500 Hz – 3500
Hz depending on the canal
III. Middle Ear – air filled normally, about the size of
an
aspirin
A. Anatomy
1. Box depiction (Walls)
a. Membranous (lateral)
1) Most
lateral wall
2) Primarily
composed of tympanic membrane
b. Labyrinthine
(Medial)
1) Behind
this wall is the cochlea
2) Basically
composed of bone
3) Landmarks
a) Oval
window
b)
Cochlear promontory (“bump” for basal
turn of
cochlea)
c) Round
window
c. Tegmental
(Superior)
1) Ceiling
of the middle ear
2) Composed
of a thin plate of bone called the
tegmen
d. Jugular
(Inferior)
1) Floor of
the middle ear
2) Beneath
is the jugular vein
e. Mastoid
(Posterior)
1) Behind
the mastoid process of the temporal
bone
f. Carotid (Anterior)
1) Front
wall
2) Internal
carotid artery lies in front of this
wall
3)
Eustachian tube found on this wall
2. Tympanic membrane
a. Characteristics
1) Relatively thin but
fairly tough
2) Translucent and silvery
grey
3) Roughly circular and
approx 9mm in diameter
4) Good restorative power –
can heal a small hole
in about 24 hours
5) Appears concave (drawn in slightly) in normal
ear
6) Umbo is the point where tympanic membrane is
most drawn
in towards middle ear space
b. Layers and areas
1) Has three layers
2) Has two areas
a) Pars
Tensa
i) More
stiffness
ii)
Comprises most of tympanic membrane
b) Pars
Flaccida
i) Less
stiffness
ii)
Small portion at top of tympanic membrane
between ~ 11 and 1 o’clock positions
3. Ossicles
a. Malleus - most lateral
1) Manubrium – long process attached to tympanic
membrane
2) Head
a) Attaches
to body of incus in a ball and socket
joint
b) Occupies
most of the attic with connection of
incus
3) Largest ossicle *
4) Connected to tensor tympani muscle by tendon
attached to
manubrium
b. Incus – middle ossicle
1) Short process
2) Long process – bends in laterally towards the
stapes; runs
parallel to manubrium
3) Lenticular process
a) Attaches
to the stapes
b)
Articulates with the head of the stapes
4) Smaller than malleus, larger than stapes
c. Stapes – smallest bone in the body; located most
medially
1) head- connected to lenticular process of incus
(incudostapedial
joint)
2) Neck –connected to stapedius muscle by tendon
3) Crura (2) – legs
4) Footplate – connected to oval window via the
annular
ligament
4. Middle ear muscles
a. Tensor tympani
b. Stapedius
5. Eustachian Tube (not covered in much detail in this
course)
a. Primary purpose is to aerate the middle ear space
b. Opens approximately 1000 x/day to accomplish this
B. Physiology
1. Movement of the tympanic membrane
a. It is a sympathetic vibrator = vibrates at same
frequency as
signal source
1) With a low frequency signal, moves as a unit
2) With a high frequency signal, vibrates
segmentally
b. It is heavily damped = stops vibrating once signal
source stops
2. The concept of impedance (z)
a. Definition = the resistance to motion
b. Calculation
1) z = c x density (i.e. impedance = speed of sound
x density)
2) Impedance in air-filled medium is low
air à density
= 1.21 kg/m3
c =
343 m/s
z =
(1.21 kg/m3)(343 m/s) = 415 Rayls
3) Impedance in fluid filled medium is high
water à density
= 998 kg/m3
c
= 1500 m/s
z
= (998 kg/m3)(1500 m/s)
= ~ 1,497,000 Rayls
c. Impedance mismatch * (changed from web)
1) Without help from the design of the middle ear, a
lot of energy
would be reflected back due to the
differences
in the impedances of the media
2) Calculation
a) Energy
transmitted = (4z1z2) / (z1 + z2)2
b) Using the
above values, this results in an
answer
of 0.001 or 0.1%
c) Converting
to a dB value, we end up with
–30 dB
SPL
d) That is
we lose 30 dB SPL in the transmission
of sound
from an air-filled medium to a fluid-
filled
medium
e) The
author of your text takes some issue with
this
value à read this section
3) Mechanisms to overcome impedance
mismatch
a) Areal
effect - funneling pressure from large
area to
smaller area
1) Tymp memb = 55 mm2
effective area *
2) Oval window = 3.2 mm2
area
3) Ratio is 55/3.2 @ 17
4) 20 log (17) = 24 dB
increase
b) Leverage
effect - effect (lever-ratio
hypothesis)
1) Long
process of manubrium (of the malleus)
is longer than the long process of incus
2) Difference in length
between manubrium
and incus produces a lever system with a
gain in acoustic energy
3) Ratio is 9/7 @ 1.3
4) 20 log (1.3) = 2.3 dB
increase (usually see
rounded to 3 dB)
c) Curvature
of the tympanic membrane
1) Has
to do with curvature of the tympanic
membrane
2) Adds
additional approx 6 dB increase *
3. Ossicular chain movement
a. Tympanic membrane receives acoustic pressure
directed against
the ear
1) Causes movement of the tympanic membrane
a) It moves
more in inferior portion than top (due
to pars flaccida)
b) Its
movement is proportional to incoming
sound
2) With positive pressure on the tympanic
membrane:
a) Manubrium
of malleus moves medially, head
of
malleus swings laterally and pulls head of
incus
with it (rocking motion)
b) This causes lenticular process of incus to move
across the head of the stapes, causing the
stapes to move in and
out of the oval window
c) Stapes movement causes oval window
movement which causes
inner ear fluid
displacement
3) With negative pressure on
the tympanic
membrane all motion is reversed
b. Motion of the stapes footplate changes with
intensity and
frequency of sound
1) With moderate intensity (<70 dB SPL) anterior
portion of
stapes footplate moves further into
oval window
than posterior portion
2) With high intensities (>70 dB SPL) and low
frequencies
(< 150 Hz) :
a) Movement
is perpendicular to previous motion
b) Top half
of stapes footplate moves into oval
window while bottom is pulling out
c) This
motion doesn’t effectively move cochlear
fluids (protective function?)
3) With high intensities and mid-to-high frequencies
both motions
occur simultaneously (still less
effective
than 1)