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THE EARS AND HEARING
Equilibrium and hearing are provided by a
receptor complex called the inner ear
- the receptors are called HAIR CELLS
- the complex structure of the inner ear and
arrangements of accessory structures
account for abilities of hair cells to
respond to different stimuli and to provide
the input for 2 senses:
1. EQUILIBRIUM- informs us of the position
of the body in space by monitoring gravity,
linear acceleration, and rotation
2. HEARING- enables us to detect and
interpret sound waves
ANATOMY OF THE EAR
The ear is organized into 3 parts:
1. External ear- visible portion of the ear,
collects and directs sound waves
toward the middle ear
2. Middle ear- collects and amplifies sound
waves and transmits them to inner ear
3. Inner ear- contains the sensory
organs for hearing and equilibrium
EXTERNAL EAR
The external ear extends beyond the lateral
surface of the head
PARTS:
1. Auricle/Pinna- the visible part of the ear that
has a flexible frame made of cartilage and
covered with skin
- this is the part of the ear that you can see!
2. Auditory Canal- opening in the center of the
pinna
- the canal directs sound waves to the
middle ear
3. Tympanic membrane (tympanum)- closes
the inner end of the auditory canal
- also called eardrum
- because it is S shaped, you cannot
see the tympanic membrane without
a special instrument
4. Ceruminous Glands- the outer portion of
the auditory canal has these waxproducing glands
- EARWAX- helps prevent foreign
objects and insects from getting into
the ear
- earwax also slows the growth of
microorganisms in the ear canal, and
reduces the chances of infection
MIDDLE EAR
- Also called the TYMPANIC CAVITY
- small air-filled chamber located
within the spongy portion of the
temporal bone
- separated from auditory canal by the
tympanic membrane, but communicates
with upper portion of pharynx
(nasopharynx)
- connected to nasopharynx with
EUSTACHIAN TUBE (auditory tube)
OSSICLES- 3 small bones located between
the tympanic membrane and the inner
wall of the middle ear
- Vibrations of sound waves from the
tympanic membrane move to these bones
1. Malleus- hammer
2. Incus- anvil
3. Stapes- stirrups
- these are responsible for transmitting
sound vibrations across the inner ear
cavity
The base of the stapes pulses against a
membrane covering an opening called the
OVAL WINDOW, in the inner wall of the
middle ear
- OVAL MEMBRANE- membrane
covering oval window
- the ossicles are completely formed at
birth and do not change in size
2 muscles are responsible for controlling the
movement of the ossicles:
1. Tensor tympani- when this muscle
contracts, the handle of the malleus is
pulled inward producing tension on the
tympanic membrane (reducing movement)
2. Stapedius- acts in opposition to the
tensor tympani, pulls on the stapes
Tension on either side of the tympanic
membrane must be equal, or the membrane
will not vibrate properly and hearing will be
impaired
EUSTACHIAN TUBE/ AUDITORY TUBEequalizes the air pressure between the
middle and outer ear
- unfortunately can also allow microorganisms
to travel from the nasopharynx into the
tympanic cavity  INFECTION
Swallowing or yawning causes the inner
edges of the tube to open, and air is
allowed to enter or leave  ears “pop”
- if the entrance to the Eustachian tube is
inflamed because of infection, the edges
may not open and the tympanic
membrane will not move properly 
temporary deafness or discomfort
occurs
INNER EAR
Contains receptors that initiate nerve
impulses which the brain interprets as
sound
- most important part of auditory
device
- also contains parts concerned with balance
- divided into 3 canals called the BONY
LABYRINTH
MEMBRANOUS LABYRINTH- fills the Bony
Labyrinth
- separated from the bony wall by a
fluid called PERILYMPH
- the membrane itself is filled with another
fluid called ENDOLYMPH
The Bony Labyrinth is made up of 3 parts:
Vestibule, Cochlea, & Semicircular
Canals
1. Vestibule- connected to the middle ear by the
oval window
- acts as an entrance to the semicircular
canals and the cochlea
Inside the vestibule are 2 membranous sacs filled
with endolymph called the UTRICLE and the
SACCULE
- receptors in these sacs provide sensations
of gravity and linear acceleration
2. Cochlea- snail-shaped organ of the inner
ear
- contains the COCHLEAR DUCT of the
membranous labyrinth
- receptors in this duct provide the sense of
hearing
- the duct is located between a pair of
perilymph-filled chambers
3. Semicircular Canals
- three looped tubes that are 90 degrees
with one another
- enclose the semicircular ducts- tubular
membranes in the semicircular canals
- receptors in these ducts are stimulated
by rotation of the head
- the combination of the vestibule and the
semicircular canals is called the VESTIBULAR
COMPLEX
The bony labyrinth’s walls are dense bone
everywhere except at 2 small areas near
the base of the cochlea:
Round window
Oval window
- both of these are covered with membranes
that separate perilymph in the cochlea
from air in the middle ear
- the membrane of the oval window is firmly
attached to the base of the stapes
- when a sound vibrates the tympanic
membrane, the movements are
conducted over the malleus and incus
to the stapes
- movement of the stapes leads to
stimulation of receptors in the cochlear
duct, and we hear the sound
RECEPTOR FUNCTION IN
INNER EAR
Receptors of the inner ear are called HAIR
CELLS
- each of these hair cells communicates with
a sensory neuron by constantly releasing
small quantities of neurotransmitter
- the free surface of hair cells are
covered with about 80-100 fingerlike STEREOCILIA
- hair cells don’t actively move these
stereocilia; when an external force pushes
the stereocilia, their movement distorts
the cell surface and alters the rate of
neurotransmitter release
- displacement of the stereocilia in one
direction stimulates the hair cells
(increases neurotransmitter release)
- displacement in the opposite direction
inhibits hair cells (decreases
neurotransmitter release)
PHYSIOLOGY OF HEARING
EQUILIBRIUM AND HEARING
EQUILIBRIUM
2 types of equilibrium:
1. DYNAMIC- aids us in maintaining our
balance when the head and body are
moved suddenly
- receptors are the semicircular ducts:
provide info. about rotational
movements of the head
2. STATIC- maintains our posture and
stability when the body is still
- receptors are the utricle and saccule:
provide info. about your position with
respect to gravity
* if you stand with your head to the side,
hair cells in these receptors report the
angle involved, and whether your head
tilts forward or backward
* these receptors are also stimulated
by sudden changes in velocity
Semicircular ducts- Rotational
movement
The 3 semicircular ducts:
Anterior
Posterior
Lateral
are continuous with the utricle
- each duct contains a swollen area
called the AMPULLA, which contains
the sensory receptors
- hair cells attached to the walls of the
ampulla form a raised structure called a
CRISTA
- the stereocilia of these hair cells are
embedded in a gelatin-like substance
called the CUPULA
- when the head rotates, movement of
the endolymph pushes against the
cupula and stimulates the hair cells
Each semicircular duct responds to one of
three possible rotational movements:
- shaking the head “no”
- nodding “yes”
- tilting the head from side to side
Vestibule- Gravity and linear
acceleration
Hair cells of the utricle and saccule are
clustered in oval MACULAE
- as in the ampullae, stereocilia are
embedded in a gelatinous material
- the macular receptors lie under a thin layer
of densely packed calcium carbonate
crystals  the complex is called an
OTOLITH
- when the head is upright, the crystals sit
atop the macula, pushing the stereocilia
downward
- when the head is tilted, the crystals shift
to the side, distorting the stereocilia
this tells the CNS that the head is no
longer level
Otolith crystals are heavy, so when the rest
of the body makes a sudden movement,
they lag behind
Ex: Elevator
- then an elevator starts downward, we
know it right away because the crystals
are no longer pushing so forcefully against
the surfaces of the hair cells
- once the crystals catch up and the
elevator reaches constant speed, we no
longer feel like we are moving
- when the elevator slows, the crystals press
harder against the hair cells and we can feel
the force of gravity increase
HEARING
Receptors of the cochlear duct provide us
with a sense of hearing that enables us to
detect a quiet whisper, yet remain
functional in a noisy room
- receptors for auditory sensation are hair
cells similar to those in the vestibular
complex
In transferring vibrations from the tympanic
membrane to the oval window, the
ossicles convert sound energy in air to
pressure pulses in the perilymph of the
cochlea
- these pulses stimulate hair cells
along the cochlear spiral
- FREQUENCY of sound is determined by
which part of the cochlear duct is
stimulated
- INTENSIY of sound is determined by how
many hair cells are stimulated at that part
of the cochlear duct
Cochlear Duct
If we look at the cochlear duct in cross
section, we can see that it lies between a
pair of perilymph filled chambers:
Vestibular duct
Tympanic duct
- the outer surfaces of these ducts are
encased by the bony labyrinth everywhere
except at the oval window
ORGAN OF CORTI
The hair cells of the cochlear duct are found
in the ORGAN OF CORTI
- this structure sits above the BASILAR
MEMBRANE, which separates the
cochlear duct from the tympanic duct
- in the organ of Corti, hair cells are
arranged in a series of longitudinal rows
with their stereocilia in contact with the
overhanging TECTORIAL MEMBRANE
- this membrane is attached to the inner
wall of the cochlear duct
- when a certain portion of the basilar membrane
bounces up and down, the stereocilia of the
hair cells are distorted as they are pushed
against the tectorial membrane
- the basilar membrane moves in response
to pressure waves in the perilymph
- these waves are produced when sounds
arrive at the tympanic membrane
The Hearing Process
Some hearing terms:
CYCLES- a term used instead of waves
HERTZ (Hz)- number of cycles per
second, represents frequency
PITCH- our sensory response to frequency;
how high or low a sound is
INTENSITY- amount of energy or power of a
sound; volume
- reported in DECIBELS
Ex: Soft whisper = 30 decibels
Jet plane = 140 decibels
6 BASIC STEPS OF HEARING:
1. Sound waves arrive at tympanic membrane
- waves enter the auditory canal and
travel toward the tympanic membrane
2. Movement of the tympanic membrane
causes displacement of the ossicles
- when the membrane vibrates, so does
the malleus, incus, and stapes
3. Movement of the stapes at the oval
window establishes pressure waves in the
perilymph of the vestibular duct
- since the rest of the cochlea is surrounded
by bone, pressure can only be relieved at
the round window- membrane bulges
outward
4. Pressure waves distort the basilar
membrane on their way to the round
window
- the basilar membrane does not have the
same structure throughout its length: near
the oval window it is narrow and stiff; at
the other end it is wider and flexible
- the location of maximum stimulation
varies with frequency
- high frequency sounds vibrate the
membrane near the oval window
- low frequency sounds vibrate the
membrane further away from the oval
window
- the LOUDER the sound, the greater
the movement of the membrane
5. Vibration of the basilar membrane causes
vibration of hair cells against the tectorial
membrane
- the resulting displacement of the
stereocilia stimulates sensory neurons
- the number of hair cells stimulated in a given
region of the organ of Corti gives information
about the intensity of the sound  louder
sound, more hair cells stimulated
6. Information about the region and
intensity of stimulation is relayed to the
CNS over the cochlear branch of the
vestibulocochlear nerves
- this information is carried to the medulla
oblongata for distribution to other centers
in the brain
AUDITORY SENSITIVITY
We never use the full potential of our auditory
system because body movements and our
internal organs produce sounds that are tuned
out by adaptation
- when other environmental noises fade away, the
level of adaptation drops and the system
becomes more sensitive
- if we relax in a quiet room, our heartbeat
gets louder as the auditory system adjusts
to the level of background noise