Hearing
Maddie, Emma, Kelly, Meg
Equilibrium sensations – inform us of the
position of the head in space by
monitoring gravity, linear acceleration,
and rotation
Hearing – enables us to detect and
interpret sound waves
Hair cells – the receptor mechanism for
both equilibrium and hearing (respond to
different stimuli and thus provide input)
Ear is divided into 3 regions:
1.External Ear
2.Middle Ear
3.Inner Ear
External Ear
• Visible portion of the ear
• Collects and directs sound waves toward the
middle ear
• Auricle- protects the opening of the canal and
provides directional sensitivity
• Auricle- blocks sounds from behind while
sounds from the front are channeled into the
external acoustic canal
External Acoustic Canal- passageway that ends
at the tympanic membrane (ear drum)
Tympanic membrane- separates external ear
from middle ear
Ceruminous glands- glands along the external
acoustic canal that secrete a waxy material,
cerumen, to block out foreign objects and
increase sensitivity
Middle Ear aka tympanic cavity
• Communicates with the nasopharynx through
the auditory tube
• With the mastoid air cells through small
connections
• Auditory tube- equalizes the pressures on
either side of the tympanic membrane (ear
drum)
• Otitis media- a middle ear infection caused by
invasion of microorganisms
Middle Ear- contains three tiny ear bones called
auditory ossicles
Auditory ossicles:
1. Malleus (hammer)-attaches to tympanic
membrane
2. Incus (anvil)- attaches malleus to the stapes
3. Stapes –inner ossicle, bound to the oval
window which surrounds the inner ear
Auditory ossicles• Act as levers that conduct vibrations to the
inner ear
• Produces rocking motion
Tensor tympani muscle – contracts and pulls
malleus medially and stiffens the tympanic
membrane and reduces movement for loud
sounds
Stapedius muscle – pulls the stapes, reducing
their movement at the oval window
Inner Ear
4 layers
Outer layer  Bony Labyrinth – made up of dense bone
2nd layer  Perilymph – liquid in between the bony and
membranous labyrinths
3rd layer  Membranous labyrinth – delicate, interconnected
network of fluid filled tubes (receptors of the inner ear are
found within these tubes)
Inner layer  Endolymph – a fluid with electrolyte
concentrations
KEY
Semicircular canal
Membranous
labyrinth
Anterior
Semicircular
ducts
Bony labyrinth
Lateral
Posterior
Vestibule
Cristae within
ampullae
Maculae
Endolymphatic sac
Cochlea
Perilymph
(a)
Bony labyrinth
Utricle
Saccule
Endolymph
Vestibular duct
Membranous
labyrinth
Cochlear duct
(b)
Tympanic
duct
Organ of
Corti
Inner Ear
Bony labyrinth – subdivided into…
1. Vestibule – consists of the saccule and the utricle
(membranous sacs)
2. 3 semicircular canals – enclose semicircular ducts
• Combination of vestibule and semicircular canals is
called the vestibular complex
3. Cochlea – spiral shaped bony chamber that contains the
cochlear duct
Inner Ear
• Bony labyrinth consists of dense bone
everywhere except the round window and
oval window
Equilibrium
• Equilibrium sensations provided by receptors of the
vestibular complex
Equilibrium
• Semicircular Ducts (Anterior, Posterior, Lateral semicircular)
• Sensory receptors in the semicircular ducts respond to rotation movements
of the head
• Each semicircular duct contains an ampulla (expanded region that contains
the receptors)
• Crista – region in the wall of the ampulla that contains the receptors
• Bound to cupula
• Each hair cell in the vestibule contains
a kinocilium (single large cilium)
Equilibrium
• Hair cells (receptors) are active during a movement,
quiet when the body is motionless
• Free surface of each hair cell supports 80-100
long stereocilia (resemble microvilli)
• Hair cells provide information about the direction
and strength of mechanical stimuli
• Stimuli involved varies by hair cell’s location
• Gravity or acceleration in the vestibule
• Rotation in the semicircular canals
• Sound in the cochlea
Equilibrium
• Movement of receptors controlled by three
rotational planes
• Horizontal rotation (ex. Shaking your head
no) stimulates the hair cells of the lateral
semicircular duct
• Vertical movement (ex. Nodding “yes”)
excites the anterior duct
• Tilting your head from side to side activates
receptors in the posterior duct
The Utricle and Saccule
• Function – provide equilibrium sensations
• Utricle and Saccule are connected by a slender
passageway that is continuous with the narrow
endolymphatic duct, which ends in the endolymphatic
sac
The Utricle and Saccule
• Hair cells of utricle and saccule are clustered in oval
structures called maculae
• Hair cell processes are embedded in a gelatinous mass
(contains densely packed calcium carbonate crystals
known as statoconia)
• Otolith  Whole complex (gelatinous matrix +
statoconia)
STEP
1
Head in the anatomical position
Gravity
(a)
Otolith
Gelatinous material
STEP
Head tilted posteriorly
2
Gravity
Statoconia
Nerve fibers
Receptor
output increases
(b) Structure of a macula
(c)
“Otolith
moves
downhill,”
distorting
hair cell
processes
Macula of Saccule
• When your head is in the normal, upright position, the
statoconia sit atop the macula (their weight pushes the
hair cell processes down rather than one side or another)
• When your head is tilted, the pull of gravity on the
statoconia shifts them to the side, distorting the hair cell
processes (alerts the central nervous system that the head
is no longer level)
Macula of Saccule
• Under normal circumstances, body can distinguish
between sensations of tilting and linear acceleration
through visual information (amusement park rides confuse
your sense of equilibrium because of the change in
position and acceleration with restricted/misleading visual
information)
Pathways for Equilibrium Sensations
• Sensory fibers contained within the vestibular nuclei  4 functions
of the 2 vestibular nuclei
• Integrating sensory information about balance and equilibrium
that arrives from both sides of the head
• Send information to cerebral cortex and cerebellum of brain
Pathways for Equilibrium Sensations
• Reflexive motor commands issued by vestibular nuclei are
distributed to motor nuclei for cranial nerves involved with eye,
head, and neck movements
• Automatic movements of eye that occur in response to
sensations of motion
• directed by the superior colliculi of the mesencephalon (in
an attempt to keep your gaze focused on a specific point,
despite changes in body position and orientation)
• Nystagmus  condition in which people have trouble
controlling their eye movements
To ipsilateral superior colliculus
and relay to cerebral cortex
Red nucleus
III
Vestibular
ganglion
IV
Semicircular
canals
Vestibular
branch
Vestibular
nucleus
VI
To
cerebellum
Vestibule
XI
Cochlear
branch
Vestibulocochlear
nerve (VIII)
Vestibulospinal
tracts
Hearing
• Receptors responsible for hearing are hair cells in the
cochlear duct
• Auditory ossicles convert pressure fluctuation in the
air into fluctuation in the perilymph of the cochlea
(outside pressure to inside pressure)
Hearing
• Frequency of sound determined from which part of
cochlear duct is stimulated
• Volume is determined from how many hair cells are
stimulated
The Cochlear Duct
• Cochlear duct is between perilymph ducts:
vestibular duct and tympanic duct
• Outer surfaces encased by bony labyrinth
everywhere except bases of ducts
• Ducts are connected and actually form one
long duct
The Cochlear Duct
• Hairs are located in the organ of Corti in longitudinal
rows
• When the basilar membrane (which the hairs are
located on) bounces, the hair cells are distorted by
pressing against the upper membrane (tectorial
membrane)
An Introduction to Sound
• Hearing is perception of sound
• Sine waves: S-shaped curves created by high and
low pressure, travel in cycles
• Travel at about 768 mph: speed of sound
An Introduction to Sound
• Wavelength inversely related to frequency (number of
waves that pass through reference point for certain
amount of time)
• Pitch=sensory response to frequency
• Amplitude=intensity of sound, energy content
• Cycles per second=hertz, Hz
• Sound energy reported in decibels
An Introduction to Sound
• With the right combination of frequency and
amplitude, object will vibrate at same frequency
as sound: called resonance
• To hear sound, tympanic membrane must vibrate
in resonance with sound waves
The Hearing Process
Sound waves arrive at the tympanic membrane…
1. Enter external acoustic canal and travel to tympanic membrane
2. Movement of the tympanic membrane causes displacement of
the auditory ossicles
a) Tympanic membrane is the surface for sound collection
b) Resonate with frequencies ~20-20,000 Hz
c) When tympanic membrane vibrates, inner ossicles also
vibrate= amplify the sound
The Hearing Process
3. Movement of the stapes at the oval window establishes
pressure waves in the perilymph of the vestibular duct
a) Because liquid is incompressible, pressure can only be
relieved at the round window
b) Stapes vibrate and creates pressure waves in the
perilymph
The Hearing Process
4. The pressure waves distort the basilar membrane on their
way to the round window of the tympanic duct
a) Pressure waves travel around perilymph and reach
round window
b) As they do this, they disrupt the basilar membrane
c) High frequencies vibrate the basilar membrane near
oval window
d) Lower the frequency, longer wavelength and further
from oval window is the maximum distortion
e) Frequency translated to position along basilar
membrane
f) Amount of movement depends on force of sound
The Hearing Process
5. Vibration of the basilar membrane causes vibration of hair
cells against the tectorial membrane
a) Vibration of basilar membrane moves hair cells against
tectorial membrane
b) Ion channels open, depolarizes hair cells
c) Leads to release of neurotransmitters/ stimulates sensory
organs
d) Hairs are stimulated in rows
e) Number of cells responding indicates intensity of sound
The Hearing Process
Region and intensity of stimulated area is relayed to the
CNS over the Cochlear branch of the vestibulocochlear
nerve
a) Cell bodies of sensory neurons located in spiral
ganglion
b) Vestibulocochlear nerve is responsible for
transmitting sound and equilibrium to the brain for
further distribution
Auditory pathways
• Vestibulocochlear nerve formed by neurons
• Info then goes to opposite side of brain to processing
center which coordinates reflexes such as turning your
head from a loud noise
• Auditory cortex in temporal lobe maps out the organ of
Corti
Auditory pathways
• Frequency to position of basilar membrane is projected
onto auditory cortex
• Creates sensation of pitch
• Damaged auditory cortex-responds to sound, but cannot
interpret sounds or find patterns
Auditory sensitivity
• Difficult to assess the absolute sensitivity of the system
• We could, in theory hear air molecules, but full potential is
never reached because of our own body and other
peripheral sounds
• We adapt to environment which affects hearing i.e.
Relaxing in a quiet room
Auditory sensitivity
• Young children have the greatest hearing range
• Declines with age due to damage or other accumulated
injuries
• Tympanic membrane is less flexible, articulations
between ossicles stiffen and round window may begin to
ossify
• Result: older individuals exhibit hearing loss
http://www.youtube.com/watch?v=Jk-4YiiPwBc&safe=active