Download Hearing

Anatomy and Physiology of Hearing
© 2013 Pearson Education, Inc.
The Ear: Hearing and Balance
•
Three major areas of ear
1. External (outer) ear – hearing only
2. Middle ear (tympanic cavity) – hearing only
3. Internal (inner) ear – hearing and
equilibrium
•
•
© 2013 Pearson Education, Inc.
Receptors for hearing and balance respond to
separate stimuli
Are activated independently
Figure 15.24a Structure of the ear.
Middle Internal ear
External ear
(labyrinth)
ear
Auricle
(pinna)
Helix
Lobule
External
acoustic Tympanic Pharyngotympanic
meatus membrane (auditory) tube
The three regions of the ear
© 2013 Pearson Education, Inc.
External Ear
• Auricle (pinna)Composed of
– Helix (rim); Lobule (earlobe)
– Funnels sound waves into auditory canal
• External acoustic meatus (auditory
canal)
– Short, curved tube lined with skin bearing
hairs, sebaceous glands, and ceruminous
glands
– Transmits sound waves to eardrum
© 2013 Pearson Education, Inc.
External Ear
• Tympanic membrane (eardrum)
– Boundary between external and middle ears
– Connective tissue membrane that vibrates in
response to sound
– Transfers sound energy to bones of middle
ear
© 2013 Pearson Education, Inc.
Middle Ear (Tympanic Cavity)
• A small, air-filled, mucosa-lined cavity in
temporal bone
– Flanked laterally by eardrum
– Flanked medially by bony wall containing oval
(vestibular) and round (cochlear) windows
© 2013 Pearson Education, Inc.
Middle Ear
• Epitympanic recess—superior portion of
middle ear
• Mastoid antrum
– Canal for communication with mastoid air
cells
• Pharyngotympanic (auditory) tube—
connects middle ear to nasopharynx
– Equalizes pressure in middle ear cavity with
external air pressure
© 2013 Pearson Education, Inc.
Figure 15.24b Structure of the ear.
Oval window
(deep to stapes)
Entrance to mastoid
antrum in the
epitympanic recess
Malleus
(hammer)
Incus
Auditory
(anvil)
ossicles
Stapes
(stirrup)
Tympanic membrane
Semicircular
canals
Vestibule
Vestibular
nerve
Cochlear
nerve
Cochlea
Round window
Middle and internal ear
© 2013 Pearson Education, Inc.
Pharyngotympanic
(auditory) tube
Otitis Media
• Middle ear inflammation
– Especially in children
• Shorter, more horizontal pharyngotympanic tubes
• Most frequent cause of hearing loss in children
– Most treated with antibiotics
– Myringotomy to relieve pressure if severe
© 2013 Pearson Education, Inc.
Ear Ossicles
• Three small bones in tympanic cavity: the
malleus, incus, and stapes
– Suspended by ligaments and joined by
synovial joints
– Transmit vibratory motion of eardrum to oval
window
– Tensor tympani and stapedius muscles
contract reflexively in response to loud
sounds to prevent damage to hearing
receptors
© 2013 Pearson Education, Inc.
Figure 15.25 The three auditory ossicles and associated skeletal muscles.
View
Superior
Malleus
Incus Epitympanic recess
Lateral
Anterior
© 2013 Pearson Education, Inc.
Pharyngotym- Tensor
tympani
panic tube
muscle
Tympanic Stapes Stapedius
membrane
muscle
(medial view)
Two Major Divisions of Internal Ear
• Bony labyrinth
– Tortuous channels in temporal bone
– Three regions: vestibule, semicircular
canals, and cochlea
– Filled with perilymph – similar to CSF
• Membranous labyrinth
– Series of membranous sacs and ducts
– Filled with potassium-rich endolymph
© 2013 Pearson Education, Inc.
Figure 15.26 Membranous labyrinth of the internal ear.
Temporal
bone
Semicircular ducts
in semicircular
canals
Anterior
Posterior
Lateral
Facial nerve
Vestibular nerve
Cristae ampullares
in the membranous
ampullae
Superior vestibular
ganglion
Inferior vestibular
ganglion
Cochlear nerve
Maculae
Spiral organ
Utricle in
vestibule
Cochlear duct
in cochlea
Saccule in
vestibule
© 2013 Pearson Education, Inc.
Stapes in
oval window
Round window
Vestibule
• Central egg-shaped cavity of bony
labyrinth
• Contains two membranous sacs
1. Saccule is continuous with cochlear duct
2. Utricle is continuous with semicircular
canals
• These sacs
– House equilibrium receptor regions
(maculae)
– Respond to gravity and changes in position
of head
© 2013 Pearson Education, Inc.
Semicircular Canals
• Three canals (anterior, lateral, and
posterior) that each define ⅔ circle
– Lie in three planes of space
• Membranous semicircular ducts line each
canal and communicate with utricle
• Ampulla of each canal houses equilibrium
receptor region called the crista
ampullaris
– Receptors respond to angular (rotational)
movements of the head
© 2013 Pearson Education, Inc.
Figure 15.26 Membranous labyrinth of the internal ear.
Temporal
bone
Semicircular ducts
in semicircular
canals
Anterior
Posterior
Lateral
Facial nerve
Vestibular nerve
Cristae ampullares
in the membranous
ampullae
Superior vestibular
ganglion
Inferior vestibular
ganglion
Cochlear nerve
Maculae
Spiral organ
Utricle in
vestibule
Cochlear duct
in cochlea
Saccule in
vestibule
© 2013 Pearson Education, Inc.
Stapes in
oval window
Round window
The Cochlea
• A spiral, conical, bony chamber
– Size of split pea
– Extends from vestibule
– Coils around bony pillar (modiolus)
– Contains cochlear duct, which houses spiral
organ (organ of Corti) and ends at cochlear
apex
© 2013 Pearson Education, Inc.
The Cochlea
• Cavity of cochlea divided into three
chambers
– Scala vestibuli—abuts oval window, contains
perilymph
– Scala media (cochlear duct)—contains
endolymph
– Scala tympani—terminates at round window;
contains perilymph
• Scalae tympani and vestibuli are
continuous with each other at helicotrema
(apex)
© 2013 Pearson Education, Inc.
The Cochlea
• The "roof" of cochlear duct is vestibular
membrane
• External wall is stria vascularis – secretes
endolymph
• "Floor" of cochlear duct composed of
– Bony spiral lamina
– Basilar membrane, which supports spiral
organ
• The cochlear branch of nerve VIII runs
from spiral organ to brain
© 2013 Pearson Education, Inc.
Figure 15.27a Anatomy of the cochlea.
Helicotrema
at apex
Modiolus
Cochlear nerve,
division of the
vestibulocochlear
nerve (VIII)
Spiral ganglion
Osseous spiral lamina
Vestibular membrane
Cochlear duct
(scala media)
© 2013 Pearson Education, Inc.
Figure 15.27b Anatomy of the cochlea.
Vestibular membrane
Tectorial membrane
Cochlear duct
(scala media;
contains
endolymph)
Stria
vascularis
Spiral organ
Basilar
membrane
© 2013 Pearson Education, Inc.
Osseous spiral lamina
Scala
vestibuli
(contains
perilymph)
Scala
tympani
(contains
perilymph)
Spiral
ganglion
Figure 15.27c Anatomy of the cochlea.
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
© 2013 Pearson Education, Inc.
Figure 15.27d Anatomy of the cochlea.
Inner
hair
cell
Outer
hair
cell
© 2013 Pearson Education, Inc.
Properties of Sound
• Sound is
– Pressure disturbance (alternating areas of
high and low pressure) produced by vibrating
object
• Sound wave
– Moves outward in all directions
– Illustrated as an S-shaped curve or sine wave
© 2013 Pearson Education, Inc.
Figure 15.28 Sound: Source and propagation.
Area of
high pressure
(compressed
molecules)
Air pressure
Wavelength
Area of
low pressure
(rarefaction)
Crest
Trough
Distance Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it, creating
alternate zones of high and low pressure.
© 2013 Pearson Education, Inc.
Sound waves radiate
outward in all
directions.
Properties of Sound Waves
• Frequency
– Number of waves that pass given point in
given time
– Pure tone has repeating crests and troughs
– Wavelength
• Distance between two consecutive crests
• Shorter wavelength = higher frequency of sound
© 2013 Pearson Education, Inc.
Properties of Sound
• Pitch
– Perception of different frequencies
– Normal range 20–20,000 hertz (Hz)
– Higher frequency = higher pitch
• Quality
– Most sounds mixtures of different frequencies
– Richness and complexity of sounds (music)
© 2013 Pearson Education, Inc.
Properties of Sound
• Amplitude
– Height of crests
• Amplitude perceived as loudness
– Subjective interpretation of sound intensity
– Normal range is 0–120 decibels (dB)
– Severe hearing loss with prolonged exposure
above 90 dB
• Amplified rock music is 120 dB or more
© 2013 Pearson Education, Inc.
Figure 15.29 Frequency and amplitude of sound waves.
Pressure
High frequency (short wavelength) = high pitch
Low frequency (long wavelength) = low pitch
0.01
0.02
Time (s)
0.03
Frequency is perceived as pitch.
Pressure
High amplitude = loud
Low amplitude = soft
0.01
© 2013 Pearson Education, Inc.
0.02
Time (s)
0.03
Amplitude (size or intensity) is perceived as loudness.
Transmission of Sound to the Internal Ear
• Sound waves vibrate tympanic membrane
• Ossicles vibrate and amplify pressure at
oval window
• Cochlear fluid set into wave motion
• Pressure waves move through perilymph
of scala vestibuli
© 2013 Pearson Education, Inc.
Transmission of Sound to the Internal Ear
• Waves with frequencies below threshold of
hearing travel through helicotrema and
scali tympani to round window
• Sounds in hearing range go through
cochlear duct, vibrating basilar membrane
at specific location, according to frequency
of sound
© 2013 Pearson Education, Inc.
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Slide 1
Auditory ossicles
Malleus Incus
Stapes
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
4a
Scala tympani
Cochlear duct
2
3
4b
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
3 Pressure waves
created by the stapes
vibrate the tympanic vibrate. Pressure is
pushing on the oval
amplified.
membrane.
window move through
fluid in the scala
© 2013 Pearson Education, Inc.
vestibuli.
4a Sounds with
frequencies below hearing
travel through the
helicotrema and do not
excite hair cells.
4b Sounds in the hearing
range go through the
cochlear duct, vibrating the
basilar membrane and
deflecting hairs on inner hair
cells.
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Auditory ossicles
Malleus Incus
Stapes
Slide 2
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
Scala tympani
Cochlear duct
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
vibrate the tympanic
membrane.
© 2013 Pearson Education, Inc.
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Auditory ossicles
Malleus Incus
Stapes
Slide 3
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
Scala tympani
Cochlear duct
2
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
vibrate the tympanic vibrate. Pressure is
amplified.
membrane.
© 2013 Pearson Education, Inc.
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Auditory ossicles
Malleus Incus
Stapes
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
Scala tympani
Cochlear duct
2
3
1
Tympanic
membrane
Slide 4
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
3 Pressure waves
created by the stapes
vibrate the tympanic vibrate. Pressure is
pushing on the oval
amplified.
membrane.
window move through
fluid in the scala
© 2013 Pearson Education, Inc.
vestibuli.
Basilar
membrane
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Auditory ossicles
Malleus Incus
Stapes
Slide 5
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
4a
Scala tympani
Cochlear duct
2
3
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
3 Pressure waves
created by the stapes
vibrate the tympanic vibrate. Pressure is
pushing on the oval
amplified.
membrane.
window move through
fluid in the scala
© 2013 Pearson Education, Inc.
vestibuli.
4a Sounds with
frequencies below hearing
travel through the
helicotrema and do not
excite hair cells.
Figure 15.30a Pathway of sound waves and resonance of the basilar membrane.
Slide 6
Auditory ossicles
Malleus Incus
Stapes
Cochlear nerve
Oval
window
Scala vestibuli
Helicotrema
4a
Scala tympani
Cochlear duct
2
3
4b
Basilar
membrane
1
Tympanic
membrane
Round
window
Route of sound waves through the ear
1 Sound waves
2 Auditory ossicles
3 Pressure waves
created by the stapes
vibrate the tympanic vibrate. Pressure is
pushing on the oval
amplified.
membrane.
window move through
fluid in the scala
© 2013 Pearson Education, Inc.
vestibuli.
4a Sounds with
frequencies below hearing
travel through the
helicotrema and do not
excite hair cells.
4b Sounds in the hearing
range go through the
cochlear duct, vibrating the
basilar membrane and
deflecting hairs on inner hair
cells.
Resonance of the Basilar Membrane
• Fibers near oval window short and stiff
– Resonate with high-frequency pressure
waves
• Fibers near cochlear apex longer, more
floppy
– Resonate with lower-frequency pressure
waves
• This mechanically processes sound before
signals reach receptors
© 2013 Pearson Education, Inc.
Figure 15.30b Pathway of sound waves and resonance of the basilar membrane.
Basilar membrane
High-frequency sounds displace the
basilar membrane near the base.
Medium-frequency sounds displace the
basilar membrane near the middle.
Low-frequency sounds displace the
basilar membrane near the apex.
Fibers of basilar membrane
Apex
(long,
floppy
fibers)
Base (short,
stiff fibers)
20,000
© 2013 Pearson Education, Inc.
2000
200
Frequency (Hz)
20
Different sound frequencies cross the basilar membrane at
different locations.
Excitation of Hair Cells in the Spiral Organ
• Cells of spiral organ
– Supporting cells
– Cochlear hair cells
• One row of inner hair cells
• Three rows of outer hair cells
• Have many stereocilia and one kinocilium
• Afferent fibers of cochlear nerve coil about
bases of hair cells
© 2013 Pearson Education, Inc.
Figure 15.27c Anatomy of the cochlea.
Tectorial membrane
Inner hair cell
Hairs (stereocilia)
Afferent nerve
fibers
Outer hair cells
Supporting cells
Fibers of
cochlear
nerve
Basilar
membrane
© 2013 Pearson Education, Inc.
Excitation of Hair Cells in the Spiral Organ
• Stereocilia
– Protrude into endolymph
– Longest enmeshed in gel-like tectorial
membrane
• Sound bending these toward kinocilium
– Opens mechanically gated ion channels
– Inward K+ and Ca2+ current causes graded potential and
release of neurotransmitter glutamate
– Cochlear fibers transmit impulses to brain
© 2013 Pearson Education, Inc.
Auditory Pathways to the Brain
• Impulses from cochlea pass via spiral ganglion
to cochlear nuclei of medulla
• From there, impulses sent
– To superior olivary nucleus
– Via lateral lemniscus to Inferior colliculus (auditory
reflex center)
• From there, impulses pass to medial geniculate
nucleus of thalamus, then to primary auditory
cortex
• Auditory pathways decussate so that both
cortices receive input from both ears
© 2013 Pearson Education, Inc.
Figure 15.32 The auditory pathway.
Medial geniculate
nucleus of thalamus
Primary auditory
cortex in temporal lobe
Inferior colliculus
Lateral lemniscus
Superior olivary
nucleus (ponsmedulla junction)
Midbrain
Cochlear nuclei
Vibrations
Medulla
Vestibulocochlear
nerve
Vibrations
Spiral ganglion
of cochlear nerve
Bipolar cell
Spiral organ
© 2013 Pearson Education, Inc.
Auditory Processing
• Pitch perceived by impulses from specific
hair cells in different positions along
basilar membrane
• Loudness detected by increased numbers
of action potentials that result when hair
cells experience larger deflections
• Localization of sound depends on relative
intensity and relative timing of sound
waves reaching both ears
© 2013 Pearson Education, Inc.
Equilibrium and Orientation
• Vestibular apparatus
– Equilibrium receptors in semicircular canals
and vestibule
– Vestibular receptors monitor static equilibrium
– Semicircular canal receptors monitor dynamic
equilibrium
© 2013 Pearson Education, Inc.
Maculae
• Sensory receptors for static equilibrium
• One in each saccule wall and one in each utricle
wall
• Monitor the position of head in space, necessary
for control of posture
• Respond to linear acceleration forces, but not
rotation
• Contain supporting cells and hair cells
• Stereocilia and kinocilia are embedded in the
otolith membrane studded with otoliths (tiny
CaCO3 stones)
© 2013 Pearson Education, Inc.
Figure 15.33 Structure of a macula.
Kinocilium
Stereocilia
Macula of
utricle
Macula of
saccule
Otolith
Otoliths membrane
Hair bundle
Hair cells
© 2013 Pearson Education, Inc.
Vestibular
nerve fibers
Supporting
cells
Maculae
• Maculae in utricle respond to horizontal
movements and tilting head side to side
• Maculae in saccule respond to vertical
movements
• Hair cells synapse with vestibular nerve
fibers
© 2013 Pearson Education, Inc.
Activating Maculae Receptors
• Hair cells release neurotransmitter
continuously
– Movement modifies amount they release
• Bending of hairs in direction of kinocilia
– Depolarizes hair cells
– Increases amount of neurotransmitter release
– More impulses travel up vestibular nerve to
brain
© 2013 Pearson Education, Inc.
Activating Maculae Receptors
• Bending away from kinocilium
– Hyperpolarizes receptors
– Less neurotransmitter released
– Reduces rate of impulse generation
• Thus brain informed of changing position
of head
© 2013 Pearson Education, Inc.
Figure 15.34 The effect of gravitational pull on a macula receptor cell in the utricle.
Otolith
membrane
Kinocilium
Stereocilia
Receptor potential
Nerve impulses generated
in vestibular fiber
© 2013 Pearson Education, Inc.
Depolarization
When hairs bend toward
the kinocilium, the hair cell
depolarizes, exciting the
nerve fiber, which generates
more frequent action potentials.
Hyperpolarization
When hairs bend away
from the kinocilium, the hair cell
hyperpolarizes, inhibiting the nerve
fiber, and decreasing the action
potential frequency.
The Crista Ampullares (Crista)
• Sensory receptor for rotational
acceleration
– One in ampulla of each semicircular canal
– Major stimuli are rotational movements
• Each crista has supporting cells and hair
cells that extend into gel-like mass called
ampullary cupula
• Dendrites of vestibular nerve fibers
encircle base of hair cells
© 2013 Pearson Education, Inc.
Figure 15.35a–b Location, structure, and function of a crista ampullaris in the internal ear.
Ampullary cupula
Crista
ampullaris
Endolymph
Hair bundle
(kinocilium
plus stereocilia)
Membranous
labyrinth
Crista
ampullaris
Fibers of vestibular nerve
Hair cell
Supporting
cell
Anatomy of a crista ampullaris in a semicircular canal
Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
At rest, the cupula stands upright.
© 2013 Pearson Education, Inc.
Scanning electron micrograph
of a crista ampullaris (200x)
Flow of endolymph
During rotational acceleration,
As rotational movement slows,
endolymph moves inside the
endolymph keeps moving in the
semicircular canals in the direction
direction of rotation. Endolymph flow
opposite the rotation (it lags behind due bends the cupula in the opposite
to inertia). Endolymph flow bends the
direction from acceleration and
cupula and excites the hair cells.
inhibits the hair cells.
Movement of the ampullary cupula during rotational acceleration and deceleration
Activating Crista Ampullaris Receptors
• Cristae respond to changes in velocity of
rotational movements of the head
• Bending of hairs in cristae causes
– Depolarizations, and rapid impulses reach
brain at faster rate
© 2013 Pearson Education, Inc.
Activating Crista Ampullaris Receptors
• Bending of hairs in the opposite direction
causes
– Hyperpolarizations, and fewer impulses reach
the brain
• Thus brain informed of rotational
movements of head
© 2013 Pearson Education, Inc.
Figure 15.35c Location, structure, and function of a crista ampullaris in the internal ear.
Section of
ampulla,
filled with
endolymph
Cupula
Fibers of
vestibular
nerve
At rest, the cupula stands upright.
Flow of endolymph
During rotational acceleration,
As rotational movement slows,
endolymph moves inside the
endolymph keeps moving in the
semicircular canals in the direction
direction of rotation. Endolymph flow
opposite the rotation (it lags behind due
bends the cupula in the opposite
to inertia). Endolymph flow bends the
direction from acceleration and
cupula and excites the hair cells.
inhibits the hair cells.
Movement of the ampullary cupula during rotational acceleration and deceleration
© 2013 Pearson Education, Inc.
Vestibular Nystagmus
• Strange eye movements during and
immediately after rotation
– Often accompanied by vertigo
• As rotation begins eyes drift in direction
opposite to rotation, then CNS
compensation causes rapid jump toward
direction of rotation
• As rotation ends eyes continue in
direction of spin then jerk rapidly in
opposite direction
© 2013 Pearson Education, Inc.
Equilibrium Pathway to the Brain
• Equilibrium information goes to reflex centers in
brain stem
– Allows fast, reflexive responses to imbalance
• Impulses travel to vestibular nuclei in brain stem
or cerebellum, both of which receive other input
• Three modes of input for balance and
orientation:
– Vestibular receptors
– Visual receptors
– Somatic receptors
© 2013 Pearson Education, Inc.
Figure 15.36 Neural pathways of the balance and orientation system.
Input: Information about the body’s position in space comes from
three main sources and is fed into two major processing areas in the
central nervous system.
Somatic receptors
(skin, muscle
and joints)
Vestibular
receptors
Visual
receptors
Cerebellum
Vestibular
nuclei
(brain stem)
Central nervous
system processing
Oculomotor control
(cranial nerve nuclei
III, IV, VI)
Spinal motor control
(cranial nerve XI nuclei
and vestibulospinal tracts)
(eye movements)
(neck, limb, and trunk
movements)
Output: Responses by the central nervous system provide fast
reflexive control of the muscles serving the eyes, neck, limbs, and
trunk.
© 2013 Pearson Education, Inc.
Motion Sickness
• Sensory input mismatches
– Visual input differs from equilibrium input
– Conflicting information causes motion
sickness
• Warning signs are excess salivation,
pallor, rapid deep breathing, profuse
sweating
• Treatment with antimotion drugs that
depress vestibular input such as meclizine
and scopolamine
© 2013 Pearson Education, Inc.
Homeostatic Imbalances of Hearing
• Conduction deafness
– Blocked sound conduction to fluids of internal
ear
• Impacted earwax, perforated eardrum, otitis media,
otosclerosis of the ossicles
• Sensorineural deafness
– Damage to neural structures at any point from
cochlear hair cells to auditory cortical cells
– Typically from gradual hair cell loss
© 2013 Pearson Education, Inc.
Treating Deafness
• Research trying to prod supporting cell
differentiation into hair cells to treat
sensorineural deafness
• Cochlear implants for congenital or
age/noise cochlear damage
– Convert sound energy into electrical signals
– Inserted into drilled recess in temporal bone
– So effective that deaf children can learn to
speak
© 2013 Pearson Education, Inc.
Homeostatic Imbalances of Hearing
• Tinnitus
– Ringing or clicking sound in ears in absence
of auditory stimuli
– Due to cochlear nerve degeneration,
inflammation of middle or internal ears, side
effects of aspirin
• Ménière's syndrome: labyrinth disorder
that affects cochlea and semicircular
canals
– Causes vertigo, nausea, and vomiting
© 2013 Pearson Education, Inc.
Developmental Aspects
• All special senses are functional at birth
• Chemical senses—few problems occur until
fourth decade, when these senses begin to
decline
– Odor and taste detection poor after 65
• Vision—optic vesicles protrude from
diencephalon during fourth week of development
– Vesicles indent to form optic cups; their stalks form
optic nerves
– Later, lens forms from ectoderm
© 2013 Pearson Education, Inc.
Developmental Aspects
• Vision not fully functional at birth
• Babies hyperopic, see only gray tones, eye
movements uncoordinated, tearless for 2 weeks
• Depth perception, color vision well developed by
age three; emmetropic eyes developed by year
six
• With age, lens loses clarity, dilator muscles less
efficient, visual acuity drastically decreased by
age 70 and lacrimal glands less active so eyes
dry, more prone to infection
© 2013 Pearson Education, Inc.
Developmental Aspects
• Ear development begins in three-week embryo
• Inner ears develop from otic placodes, which
invaginate into otic pit and otic vesicle
• Otic vesicle becomes membranous labyrinth,
and surrounding mesenchyme becomes bony
labyrinth
• Middle ear structures develop from pharyngeal
pouches
• Branchial groove develops into outer ear
structures
© 2013 Pearson Education, Inc.
Developmental Aspects
• Newborns can hear but early responses
reflexive
• Language skills tied to ability to hear well
• Congenital abnormalities common
– Missing pinnae, closed or absent external
acoustic meatuses
– Maternal rubella causes sensorineural
deafness
© 2013 Pearson Education, Inc.
Developmental Aspects
• Few ear problems until 60s when
deterioration of spiral organ noticeable
• Hair cell numbers decline with age
– Presbycusis occurs first
• Loss of high pitch perception
• Type of sensorineural deafness
© 2013 Pearson Education, Inc.