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Transcript
Chapter 15 Part D
The Special
Senses
© Annie Leibovitz/Contact Press Images
© 2016 Pearson Education, Inc.
PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
Hearing
15.8 Sound Detection
• Hearing is the reception of an air sound wave
that is converted to a fluid wave that ultimately
stimulates mechanosensitive cochlear hair cells
that send impulses to the brain for interpretation
Properties of Sound
• Sound is a pressure disturbance (alternating
areas of high and low pressure) produced by a
vibrating object and propagated by molecules of
the medium (air)
© 2016 Pearson Education, Inc.
Properties of Sound (cont.)
• Sound waves are created when an object
moves:
– Air molecules that are displaced by object
movement are pushed forward into adjacent
area, adding to air molecules already there
• Creates an area of high pressure due to compression
of molecules together
– As object returns to original position, the area it
leaves now has fewer air molecules
• Creates an area of low pressure due to presence of
fewer air molecules
• Referred to as rarefaction
© 2016 Pearson Education, Inc.
Properties of Sound (cont.)
• Sound waves are alternating areas (waves) of
compressions and rarefactions
• Object vibrating causes waves to move outward
in all directions as air all around it is
compressed and rarefied
• Kinetic energy of object is transferred to air
molecules, which then transfer it to other air
molecules
– Wave energy declines with time and distance
© 2016 Pearson Education, Inc.
Properties of Sound (cont.)
• Illustrated as an S-shaped curve, or sine wave
– Compressions shown as crests, rarefactions as
troughs
© 2016 Pearson Education, Inc.
Figure 15.28a Sound: Source and propagation.
Area of
high pressure
(compressed
molecules)
Air pressure
Wavelength
Crest
Trough
Distance Amplitude
A struck tuning fork alternately compresses
and rarefies the air molecules around it.
© 2016 Pearson Education, Inc.
Area of
low pressure
(rarefaction)
Figure 15.28b Sound: Source and propagation.
© 2016 Pearson Education, Inc.
Sound waves
radiate outward
in all directions.
Properties of Sound (cont.)
• Sound can be described by two physical
properties: frequency and amplitude
1. Frequency
• Number of waves that pass given point in a given time
• Pure tone has repeating crests and troughs
• Wavelength
– Distance between two consecutive crests
– Shorter wavelength  higher frequency of sound
– Wavelength is consistent for a particular sound
© 2016 Pearson Education, Inc.
Properties of Sound (cont.)
1. Frequency (cont.)
• Frequency range of human hearing is 20–20,000 hertz
(Hz  waves per second), but most sensitive between
1500 and 4000 Hz
– Pitch: perception of different frequencies
» Higher the frequency, higher the pitch
– Quality: characteristic of sounds
» Most sounds are mixtures of different frequencies
» Tone: one frequency (ex: tuning fork)
» Sound quality provides richness and complexity of
sounds (music)
© 2016 Pearson Education, Inc.
Pressure
Figure 15.29a Frequency and amplitude of sound waves.
High frequency
(short wavelength)
 high pitch
Low frequency
(long wavelength)
 low pitch
0.01
Time (s)
0.02
Frequency is perceived as pitch.
© 2016 Pearson Education, Inc.
0.03
Properties of Sound (cont.)
2. Amplitude
• Height of crests
• Amplitude perceived as loudness: subjective
interpretation of sound intensity
– Measured in decibels (dB)
– Normal range is 0–120 decibels (dB)
» Normal conversation is around 50 dB
» Threshold of pain is 120 dB
– Severe hearing loss can occur with prolonged exposure
above 90 dB
» Amplified rock music is 120 dB or more
© 2016 Pearson Education, Inc.
Pressure
Figure 15.29b Frequency and amplitude of sound waves.
High amplitude
 loud
Low amplitude
 soft
0.01
Time (s)
0.02
0.03
Amplitude (size or intensity) is perceived as loudness.
© 2016 Pearson Education, Inc.
Transmission of Sound to Internal Ear
• Pathway of sound
1. Tympanic membrane: sound waves enter
external acoustic meatus and strike tympanic
membrane, causing it to vibrate
• The higher the intensity, the more vibration
2. Auditory ossicles: transfer vibration of
eardrum to oval window
• Tympanic membrane is about 20 larger than oval
window, so vibration transferred to oval window is
amplified about 20
© 2016 Pearson Education, Inc.
Transmission of Sound to Internal Ear (cont.)
3. Scala vestibuli: stapes rocks back and forth
on oval window with each vibration, causing
wave motions in perilymph
• Wave ends at round window, causing it to bulge
outward into middle ear cavity
4a. Helicotrema path: waves with frequencies
below threshold of hearing travel through
helicotrema and scali tympani to round window
4b. Basilar membrane path: sounds in hearing
range go through cochlear duct, vibrating
basilar membrane at specific location, according
to frequency of sound
© 2016 Pearson Education, Inc.
Slide 2
Figure 15.30 Pathway of sound waves.
Auditory ossicles
Malleus Incus Stapes
Oval
window
Cochlear
nerve
1 Sound waves vibrate the
tympanic membrane.
Scala vestibuli
Helicotrema
Scala
tympani
Cochlear
duct
Basilar
membrane
1
Tympanic
membrane
© 2016 Pearson Education, Inc.
Round
window
Slide 3
Figure 15.30 Pathway of sound waves.
Auditory ossicles
Malleus Incus Stapes
Oval
window
Cochlear
nerve
1 Sound waves vibrate the
tympanic membrane.
Scala vestibuli
Helicotrema
2 Auditory ossicles vibrate.
Scala
tympani
Cochlear
duct
2
Basilar
membrane
1
Tympanic
membrane
© 2016 Pearson Education, Inc.
Round
window
Pressure is amplified.
Slide 4
Figure 15.30 Pathway of sound waves.
Auditory ossicles
Cochlear
nerve
Malleus Incus Stapes
Oval
window
Scala vestibuli
Helicotrema
2
3
1 Sound waves vibrate the
tympanic membrane.
2 Auditory ossicles vibrate.
Scala
tympani
Cochlear
duct
Basilar
membrane
1
Tympanic
membrane
© 2016 Pearson Education, Inc.
Round
window
Pressure is amplified.
3 Pressure waves created by the
stapes pushing on the oval window
move through fluid in the scala
vestibuli.
Slide 5
Figure 15.30 Pathway of sound waves.
Auditory ossicles
Cochlear
nerve
Malleus Incus Stapes
Oval
window
Scala vestibuli
Helicotrema
2
3
1 Sound waves vibrate the
tympanic membrane.
2 Auditory ossicles vibrate.
4a
Scala
tympani
Cochlear
duct
Basilar
membrane
1
Tympanic
membrane
© 2016 Pearson Education, Inc.
Round
window
Pressure is amplified.
3 Pressure waves created by the
stapes pushing on the oval window
move through fluid in the scala
vestibuli.
4a Sounds with frequencies below
hearing travel through the
helicotrema and do not excite hair
cells.
Slide 6
Figure 15.30 Pathway of sound waves.
Auditory ossicles
Cochlear
nerve
Malleus Incus Stapes
Oval
window
Scala vestibuli
Helicotrema
2
1 Sound waves vibrate the
tympanic membrane.
2 Auditory ossicles vibrate.
Scala
tympani
4a
Cochlear
duct
3
4b
1
Basilar
membrane
Pressure is amplified.
3 Pressure waves created by the
stapes pushing on the oval window
move through fluid in the scala
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.
Tympanic
membrane
© 2016 Pearson Education, Inc.
Round
window
Resonance of the Basilar Membrane
• Resonance: movement of different areas of
basilar membrane in response to a particular
frequency
• Basilar membrane changes along its length:
– Fibers near oval window are short and stiff
• Resonate with high-frequency waves
– Fibers near cochlear apex are longer, floppier
• Resonate with lower-frequency waves
• So basilar membrane mechanically processes
sound even before signals reach receptors
© 2016 Pearson Education, Inc.
Figure 15.31-1 Basilar membrane function.
Let’s uncoil the cochlea to see how it
separates different frequencies of sound
so that we can hear different pitches.
Stapes
Basilar membrane
© 2016 Pearson Education, Inc.
Figure 15.31-2 Basilar membrane function.
The properties of the basilar membrane change along its length.
Short, stiff fibers
Long, floppy fibers
Base
As a result, different frequencies vibrate the basilar
membrane in different places.
© 2016 Pearson Education, Inc.
Apex
Sound Transduction
• Excitation of inner hair cells
– Movement of basilar membrane deflects hairs of
inner hair cells
• Cochlear hair cells have microvilli that contain many
stereocilia (hairs) that bend at their base
• Longest hair cells are connected to shortest hair cells
via tip links
– Tip links, when pulled on, open ion channels they are
connected to
– Stereocilia project into K+-rich endolymph, with
longest hairs enmeshed in gel-like tectorial
membrane
© 2016 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
© 2016 Pearson Education, Inc.
Sound Transduction (cont.)
• Excitation of inner hair cells (cont.)
– Bending of stereocilia toward tallest ones pull on
tip links, causing K+ and Ca2+ ion channels in
shorter stereocilia to open
• K+ and Ca2+ flow into cell, causing receptor potential
that can lead to release of neurotransmitter
(glutamate)
– Can trigger AP in afferent neurons of cochlear nerve
– Bending of stereocilia toward shorter ones
causes tip links to relax
• Ion channels close, leading to repolarization (and
even hyperpolarization)
© 2016 Pearson Education, Inc.
Figure 15.32 Bending of stereocilia opens or closes mechanically gated ion channels in hair cells.
Basilar membrane at rest
Tectorial
membrane
Tip link
Stereocilia
A few
channels
are open;
cell slightly
depolarized
Hairs bent toward tallest stereocilium
K, Ca2
1 Tip links tighten,
opening
mechanically
gated ion channels.
2 More cations
Hairs bent away from tallest stereocilium
1 Tip links loosen,
closing mechanically
gated ion channels.
2 No cations enter;
cell hyperpolarizes.
enter; cell
depolarizes.
Hair cell
Basilar
membrane
Cochlear nerve axon
© 2016 Pearson Education, Inc.
3
Neurotransmitter
release.
3
Neurotransmitter
release.
4
Action potentials
in cochlear nerve.
4
Action potentials
in cochlear nerve.
Sound Transduction (cont.)
• Role of outer hair cells
– Nerve fibers coiled around hair cells of outer row
are efferent neurons that convey messages from
brain to ear
– Outer hair cells can contract and stretch, which
changes stiffness of basilar membrane
– This ability serves two functions:
• Increase “fine-tuning” responsiveness of inner hair
cells by amplifying motion of basilar membrane
• Protect inner hair cells from loud noises by decreasing
motion of basilar membrane
© 2016 Pearson Education, Inc.
15.9 Auditory Pathways to Brain
Auditory Pathway
• Neural impulses from cochlear bipolar cells
reach auditory cortex via following pathway:
– Spiral ganglion 
– Cochlear nuclei (medulla) 
– Superior olivary nucleus (pons-medulla) 
– Lateral lemniscus (tract) 
– Inferior colliculus (midbrain auditory reflex
center 
– Medial geniculate nucleus (thalamus) 
– Primary auditory cortex
© 2016 Pearson Education, Inc.
Auditory Pathway (cont.)
• Some fibers cross over, some do not; so both
auditory cortices receive input from both ears
© 2016 Pearson Education, Inc.
Figure 15.33 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
© 2016 Pearson Education, Inc.
Auditory Processing
• Perception of pitch: impulses from hair cells in
different positions along basilar membrane are
interpreted by brain as specific pitches
• Detection of loudness is determined by brain
as an increase in the number of action
potentials (frequency) that result when hair cells
experience larger deflections
© 2016 Pearson Education, Inc.
Auditory Processing (cont.)
• Localization of sound depends on relative
intensity and relative timing of sound waves
reaching both ears
– If timing is increased on one side, brain
interprets sound as coming from that side
© 2016 Pearson Education, Inc.
Equilibrium
15.10 Maintenance of Equilibrium
• Equilibrium is response to various movements
of head that rely on input from inner ear, eyes,
and stretch receptors
• Vestibular apparatus: equilibrium receptors in
semicircular canals and vestibule
– Vestibular receptors monitor static equilibrium
– Semicircular canal receptors monitor dynamic
equilibrium
© 2016 Pearson Education, Inc.
Figure 15.24b Structure of the ear.
Oval window
(deep to stapes)
Entrance to mastoid
antrum in the
epitympanic recess
Auditory
ossicles
Semicircular
canals
Malleus
(hammer)
Vestibule
Incus
(anvil)
Vestibular
nerve
Stapes
(stirrup)
Tympanic
membrane
Round window
Middle and internal ear
© 2016 Pearson Education, Inc.
Cochlear
nerve
Cochlea
Pharyngotympanic
(auditory) tube
The Maculae
• Maculae: sensory receptor organs that monitor
static equilibrium
– One organ located in each saccule wall and one
in each utricle wall
• Monitor the position of head in space
• Play a key role in control of posture
• Respond to linear acceleration forces, but not
rotation
© 2016 Pearson Education, Inc.
Figure 15.26 Membranous labyrinth of the internal ear.
Temporal
bone
Facial nerve
Semicircular ducts in
semicircular canals
• Anterior
• Posterior
• Lateral
Vestibular nerve
Superior vestibular ganglion
Inferior vestibular ganglion
Cochlear nerve
Cristae ampullares
in the membranous
ampullae
Maculae
Utricle in vestibule
Spiral organ
Saccule in vestibule
Cochlear duct
in cochlea
Stapes in
oval window
© 2016 Pearson Education, Inc.
Round window
The Maculae (cont.)
• Anatomy of a macula
– Each is a flat epithelium patch containing hair
cells with supporting cells
– Hair cells have stereocilia and special “true
stereocilium” called kinocilium
• Located next to tallest stereocilia
– Stereocilia are embedded in otolith membrane,
jelly-like mass studded with otoliths (tiny CaCO3
stones)
• Otoliths increase membrane’s weight and increase its
inertia (resistance to changes in motion)
© 2016 Pearson Education, Inc.
The Maculae (cont.)
• Anatomy of a macula (cont.)
– Utricle maculae are horizontal with vertical hairs
• Respond to change along a horizontal plane, such as
tilting head
– Forward/backward movements stimulate utricle
– Saccule maculae are vertical with horizontal
hairs
• Respond to change along a vertical plane
– Up/down movements stimulate saccule (Example:
acceleration of an elevator)
© 2016 Pearson Education, Inc.
The Maculae (cont.)
• Anatomy of a macula (cont.)
– Hair cells synapse with fibers of vestibular
nerve whose cell bodies are located in superior
and inferior vestibular ganglia
• Part of vestibulocochlear cranial nerve (VIII)
© 2016 Pearson Education, Inc.
Figure 15.34a Structure and function of a macula.
Macula of
utricle
Macula of
saccule
Kinocilium
Stereocilia
Otoliths Otolith
membrane
Hair
cells
Supporting
cells
Vestibular
nerve fibers
© 2016 Pearson Education, Inc.
The Maculae (cont.)
• Activating receptors of a macula
– Hair cells release neurotransmitter continuously
• Acceleration/deceleration causes a change in amount
of neurotransmitter released
– Leads to change in AP frequency to brain
– Density of otolith membrane causes it to lag
behind movement of hair cells when head
changes positions
• Base of stereocilia moves at same rate as head, but
tips embedded in otolith are pulled by lagging
membrane, causing hair to bend
• Ion channels open, and depolarization occurs
© 2016 Pearson Education, Inc.
The Maculae (cont.)
• Activating receptors of a macula (cont.)
– Bending of hairs in direction of kinocilia:
• Depolarizes hair cells
• Increases amount of neurotransmitter release
• More impulses travel up vestibular nerve to brain
– Bending of hairs away from kinocilia:
• Hyperpolarizes receptors
• Less neurotransmitter released
• Reduces rate of impulse generation
– Thus brain is informed of changing position of
head
© 2016 Pearson Education, Inc.
Figure 15.34b Structure and function of a macula.
Head Upright
Gravity
Steady stream of
action potentials in
vestibular nerve
Force
Force
© 2016 Pearson Education, Inc.
Head tilted forward
• Hairs bend toward
kinocilium
• Hair cell depolarizes
• Nerve fiber excited
• Action potentials
in vestibular nerve
Head tilted backwards
• Hairs bend away from
kinocilium
• Hair cell hyperpolarizes
• Nerve fiber inhibited
• Action potentials in
vestibular nerve
The Cristae Ampullares
• Receptor for rotational acceleration is crista
ampullaris (crista)
– Small elevation in ampulla of each semicircular
canal
• Cristae are excited by acceleration and
deceleration of head
– Major stimuli are rotational (angular) movements,
such as twirling of the body
– Semicircular canals are located in all three
planes of space, so cristae can pick up on all
rotational movements of head
© 2016 Pearson Education, Inc.
The Cristae Ampullares (cont.)
• Anatomy of a crista ampullaris
– 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
• Activating receptors of crista ampullaris
– Cristae respond to changes in velocity of
rotational movements of head
– Inertia in ampullary cupula causes endolymph in
semicircular ducts to move in direction opposite
body’s rotation, causing hair cells to bend
© 2016 Pearson Education, Inc.
Figure 15.35a 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
Anatomy of a crista ampullaris in a semicircular canal
© 2016 Pearson Education, Inc.
Hair cell
Supporting
cell
Figure 15.35b Location, structure, and function of a crista ampullaris in the internal ear.
Ampullary cupula
Scanning electron micrograph
of a crista ampullaris (200×)
© 2016 Pearson Education, Inc.
The Cristae Ampullares (cont.)
• Activating receptors of crista ampullaris
(cont.)
– Bending hairs in cristae causes depolarization
• Rapid impulses reach brain at faster rate
– Bending of hairs in opposite direction causes
hyperpolarizations
• Fewer impulses reach brain
– Thus brain is informed of head rotations
© 2016 Pearson Education, Inc.
The Cristae Ampullares (cont.)
• Activating receptors of crista ampullaris
(cont.)
– Axes of hair cells in complementary semicircular
ducts are opposite
• Depolarization occurs in one ear, while
hyperpolarization occurs in other ear
– Endolymph will come to rest after a while, so this
system detects only changes in movements
© 2016 Pearson Education, Inc.
Figure 15.35c Location, structure, and function of a crista ampullaris in the internal ear.
Section of
ampulla,
filled with
endolymph
Ampullary cupula
Fibers of
vestibular
nerve
At rest, the cupula stands upright.
Flow of endolymph
During rotational acceleration, endolymph
moves inside the semicircular canals in the
direction opposite the rotation (it lags
behind due to inertia). Endolymph flow
bends the cupula and excites the hair cells.
Movement of the ampullary cupula during rotational acceleration and deceleration
© 2016 Pearson Education, Inc.
As rotational movement slows,
endolymph keeps moving in the direction
of rotation. Endolymph flow bends the
cupula in the opposite direction from
acceleration and inhibits the hair cells.
The Cristae Ampullares (cont.)
• Vestibular nystagmus
– Semicircular canal impulses are linked to reflex
movements of eyes
– Nystagmus is 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
© 2016 Pearson Education, Inc.
Equilibrium Pathway to the Brain
• Equilibrium information goes to reflex centers in
brain stem
– Allows fast, reflexive responses to imbalance so
we don’t fall down
• Impulses from activated vestibular receptors
travel to either vestibular nuclei in brain stem or
to cerebellum
• Three modes of input for balance and
orientation:
– Vestibular receptors
– Visual receptors
– Somatic receptors
© 2016 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.
© 2016 Pearson Education, Inc.
Clinical – Homeostatic Imbalance 15.15
• Equilibrium problems are usually unpleasant
and can cause nausea, dizziness, and loss of
balance
• Nystagmus in the absence of rotational stimuli
may be present
© 2016 Pearson Education, Inc.
Clinical – Homeostatic Imbalance 15.15
• Motion sickness: sensory inputs are
mismatched
– 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
© 2016 Pearson Education, Inc.
15.11 Homeostatic Imbalances of Hearing
Deafness
• Conduction deafness
– Blocked sound conduction to fluids of internal
ear
• Causes include 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
© 2016 Pearson Education, Inc.
Deafness (cont.)
• Sensorineural deafness research is under way
to prod supporting cells to differentiate into hair
cells
• Cochlear implants that convert sound energy
into electrical signals are effective for congenital
or age/noise cochlear damage
– Inserted into drilled recess in temporal bone
– So effective that deaf children can learn to speak
© 2016 Pearson Education, Inc.
Figure 15.37 Boy with a cochlear implant.
© 2016 Pearson Education, Inc.
Tinnitus
• Ringing, buzzing, 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
© 2016 Pearson Education, Inc.
Ménière’s Syndrome
• Labyrinth disorder that affects cochlea and
semicircular canals
• Causes vertigo, nausea, and vomiting
• Treatment: anti–motion sickness drugs in mild
cases or surgical removal of labyrinth in severe
cases
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Taste and Smell
• 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 is poor after 65
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Vision
• Optic vesicles protrude from diencephalon
during week 4 of development
– Vesicles indent to form optic cups
• Stalks form optic nerves
– Later, lens forms from ectoderm
• Vision is not fully functional at birth; babies are
hyperopic because eyes are shortened
– See only gray tones
– Eye movements are uncoordinated
– Tearless for about 2 weeks
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Vision (cont.)
• By 5 months of age, infants can follow objects,
but acuity is still poor
• Depth perception and color vision develop by
age 3
• Adult eye size reached around 8–9 years of age
• Around year 40, lenses start to lose elasticity,
resulting in presbyopia
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Vision (cont.)
• With age, lens loses clarity, dilator muscles are
less efficient; visual acuity is drastically
decreased by age 70
– Lacrimal glands less active, so eyes are dry,
more prone to infection
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Hearing and Balance
• Ear development begins in 3-week embryo
• Inner ears develop from thickening of ectoderm
called otic placodes, which invaginate into otic
pit and otic vesicle
• Otic vesicle becomes membranous labyrinth,
and surrounding mesenchyme becomes bony
labyrinth
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Hearing and Balance (cont.)
• Middle ear structures develop from endodermal
pharyngeal pouches, ossicles from neural
crest cells, and pharyngeal cleft (branchial
groove) develops into outer ear structures
• Newborns can hear, but early responses are
reflexive in nature
– By month 4, infants can turn head toward voices
of family members
© 2016 Pearson Education, Inc.
Developmental Aspects of the Special
Senses
Hearing and Balance (cont.)
• Language skills tied to ability to hear well
• Few ear problems until 60s, when deterioration
of spiral organ becomes noticeable
• Hair cell numbers decline with age
– Presbycusis: loss of high-pitch perception
occurs first
• Type of sensorineural deafness
© 2016 Pearson Education, Inc.
Clinical – Homeostatic Imbalance 15.16
• Congenital problems of eyes are relatively
uncommon, but incidence is increased by
certain maternal infections
• Rubella (German measles) is dangerous,
especially during critical first 3 months of
pregnancy
• Common problems associated with rubella
infections are blindness and cataracts
© 2016 Pearson Education, Inc.
Clinical – Homeostatic Imbalance 15.17
• Congenital abnormalities are common
– Missing pinnae, closed or absent external
acoustic meatuses
– Maternal rubella causes sensorineural deafness
© 2016 Pearson Education, Inc.