Download Structure of the Eye

Chapter 8
Special Senses
Lecture Presentation by
Patty Bostwick-Taylor
Florence-Darlington Technical College
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The Senses
 Special senses
 Smell
 Taste
 Sight
 Hearing
 Equilibrium
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The Eye and Vision
 70 percent of all sensory receptors are in
the eyes
 Each eye has over 1 million nerve fibers
 Protection for the eye
 Most of the eye is enclosed in a bony orbit
 A cushion of fat surrounds most of the eye
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Accessory Structures of the Eye
 Eyelids and eyelashes
 Conjunctiva
 Lacrimal apparatus
 Extrinsic eye muscles
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Figure 8.1 Surface anatomy of the eye and accessory structures.
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
(canthus)
Iris
Eyelid
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Eyebrow
Eyelid
Eyelashes
Pupil
Lacrimal
caruncle
Medial
commissure
(canthus)
Sclera
(covered by
conjunctiva)
Accessory Structures of the Eye
 Eyelids
 Meet at the medial and lateral commissure (canthus)
 Eyelashes
 Tarsal glands produce an oily secretion that
lubricates the eye
 Ciliary glands are located between the eyelashes
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Accessory Structures of the Eye
 Conjunctiva
 Membrane that lines the eyelids
 Connects to the outer surface of the eye
 Secretes mucus to lubricate the eye and keep it
moist
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Accessory Structures of the Eye
 Lacrimal apparatus  lacrimal gland and ducts
 Lacrimal gland—produces lacrimal fluid; situated on
lateral aspect of each eye
 Lacrimal canaliculi—drain lacrimal fluid from eyes
medially
 Lacrimal sac—provides passage of lacrimal fluid
towards nasal cavity
 Nasolacrimal duct—empties lacrimal fluid into the
nasal cavity
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Accessory Structures of the Eye
 Function of the lacrimal apparatus
 Protects, moistens, and lubricates the eye
 Empties into the nasal cavity
 Lacrimal secretions (tears) contain:




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Dilute salt solution
Mucus
Antibodies
Lysozyme (enzyme that destroys bacteria)
Figure 8.2a Accessory structures of the eye.
Lacrimal
gland
Excretory duct
of lacrimal gland
Conjunctiva
Anterior
aspect
Eyelid
Eyelashes
Tarsal
glands
(a)
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Eyelid
Figure 8.2b Accessory structures of the eye.
Lacrimal
gland
Excretory ducts
of lacrimal gland
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
(b)
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Lacrimal sac
Structure of the Eye
 Layers forming the wall of the eyeball
 Fibrous layer: outside layer
 Vascular layer: middle layer
 Sensory layer: inside layer
 Humors are fluids that fill the interior of the eyeball
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Figure 8.4a Internal anatomy of the eye (sagittal section).
Ciliary body
Ciliary zonule
Cornea
Iris
Pupil
Aqueous humor
(in anterior segment)
Lens
Scleral venous sinus
(canal of Schlemm)
Vitreous humor
(in posterior segment)
(a)
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Sclera
Choroid
Retina
Fovea centralis
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
Figure 8.4b Internal anatomy of the eye (sagittal section).
Ciliary body
Iris
Margin of pupil
Aqueous humor
(in anterior segment)
(b)
Lens
Cornea
Ciliary zonule
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Vitreous humor
in posterior segment
Retina
Choroid
Sclera
Fovea centralis
Optic disc
Optic nerve
Structure of the Eye: The Fibrous Layer
 Sclera
 White connective tissue layer
 Seen anteriorly as the “white of the eye”
 Cornea
 Transparent, central anterior portion
 Allows for light to pass through
 Repairs itself easily
 The only human tissue that can be transplanted
without fear of rejection
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Structure of the Eye: Vascular Layer
 Choroid is a blood-rich nutritive layer in the posterior
of the eye
 Pigment prevents light from scattering
 Modified anteriorly into two structures:
1. Ciliary body—smooth muscle attached to lens by
ciliary zonule (suspensory ligament)
2. Iris—regulates amount of light entering eye
 Pigmented layer that gives eye color
 Pupil—rounded opening in the iris
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Structure of the Eye: Sensory Layer
 Retina contains two layers
1. Outer pigmented layer absorbs light and prevents
it from scattering
2. Inner neural layer
 Contains receptor cells (photoreceptors)
 Rods
 Cones
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Structure of the Eye: Sensory Layer
 Signals pass from photoreceptors via a two-neuron
chain
 Bipolar neurons
 Ganglion cells
 Signals leave the retina toward the brain through
the optic nerve
 Optic disc (blind spot) is where the optic nerve
leaves the eyeball
 Cannot see images focused on the optic disc
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Figure 8.5a The three major types of neurons composing the retina.
Pigmented
layer of retina
Rod
Cone
Bipolar
cells
Ganglion
cells
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(a)
Pathway
of light
Figure 8.5b The three major types of neurons composing the retina.
Pigmented
layer of
retina
Neural layer
of retina
Central
artery
and vein
of retina
Optic
disc
(b)
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Optic
nerve
Sclera
Choroid
Structure of the Eye: Sensory Layer
 Neurons of the retina and vision
 Rods
 Most are found toward the edges of the retina
 Allow vision in dim light and peripheral vision
 All perception is in gray tones
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Structure of the Eye: Sensory Layer
 Neurons of the retina and vision
 Cones
 Allow for detailed color vision
 Densest in the center of the retina
 Fovea centralis–lateral to blind spot
 Area of the retina with only cones
 Visual acuity (sharpest vision) is here
 No photoreceptor cells are at the optic disc, or blind
spot
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Structure of the Eye: Sensory Layer
 Cone sensitivity
 Three types of cones
 Different cones are sensitive to different wavelengths
 Color blindness is the result of the lack of one cone
type
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Figure 8.6 Sensitivities of the three cone types to the different wavelengths of visible light.
Light absorption by cone populations
Visible light
530 nm
(green cones)
560 nm
(red cones)
420 nm
(blue cones)
380
450
500
550
600
650
Wavelength (nanometers)
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700
750
Lens
 Biconvex crystal-like structure
 Held in place by a suspensory ligament attached to
the ciliary body
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Lens
 Cataracts result when the lens becomes hard and
opaque with age
 Vision becomes hazy and distorted
 Eventually causes blindness in affected eye
 Risk factors include:
 Diabetes mellitus
 Frequent exposure to intense sunlight
 Heavy smoking
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Figure 8.7 Photograph of a cataract.
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Two Segments, or Chambers, of the Eye
 Lens divides the eye into two chambers:
1. Anterior (aqueous) segment
 Anterior to the lens
 Contains aqueous humor
2. Posterior (vitreous) segment
 Posterior to the lens
 Contains vitreous humor
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Anterior Segment
 Aqueous humor
 Watery fluid found between lens and cornea
 Similar to blood plasma
 Helps maintain intraocular pressure
 Provides nutrients for the lens and cornea
 Reabsorbed into venous blood through the scleral
venous sinus, or canal of Schlemm
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Posterior Segment
 Vitreous humor
 Gel-like substance posterior to the lens
 Prevents the eye from collapsing
 Helps maintain intraocular pressure
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Ophthalmoscope
 Instrument used to illuminate the interior of the
eyeball and fundus (posterior wall)
 Can detect diabetes, arteriosclerosis, degeneration
of the optic nerve and retina
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Figure 8.8 The posterior wall (fundus) of the retina as seen with an ophthalmoscope.
Fovea
centralis
Macula
Lateral
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Blood
vessels
Optic disc Retina
Medial
Pathway of Light Through the Eye
 Light must be focused to a point on the retina for
optimal vision
 Light is bent, or refracted, by the cornea, aqueous
humor, lens, and vitreous humor
 The eye is set for distance vision
(over 20 feet away)
 Accommodation—the lens must change shape to
focus on closer objects (less than 20 feet away)
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Figure 8.9 Relative convexity of the lens during focusing for distant and close vision.
Retina
Light from distant source
Focal point
(a)
Light from near source
Focal point
Retina
(b)
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Pathway of Light Through the Eye
 Image formed on the retina is a real image
 Real images are:
 Reversed from left to right
 Upside down
 Smaller than the object
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Pathway of Light Through the Eye
 The pathway of light through the eye:
1.
2.
3.
4.
5.
6.
7.
Cornea
Aqueous humor
Through pupil
Aqueous humor
Lens
Vitreous humor
Retina
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Figure 8.10 Real image (reversed left to right, and upside down) formed on the retina.
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Visual Fields and Visual Pathways
 Optic chiasma
 Location where the optic nerves cross
 Fibers from the medial side of each eye cross over to
the opposite side of the brain
 Optic tracts
 Contain fibers from the lateral side of the eye on the
same side and the medial side of the opposite eye
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Visual Fields and Visual Pathways
 Overlap of the visual fields, and inputs from both
eyes to each optic cortex provide for depth
perception
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Pathway of Nerve Impulses into Brain
 The pathway of nerve impulses from the retina of
the eye into the brain:
1.
2.
3.
4.
5.
6.
Optic nerve
Optic chiasma
Optic tract
Thalamus
Optic radiation
Visual cortex in occipital lobe of brain
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Figure 8.11 Visual fields of the eyes and visual pathway to the brain.
Fixation point
Right eye
Left eye
Optic
nerve
Optic
tract
Optic
chiasma
Optic
radiation
Thalamus
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Occipital lobe
(visual cortex)
Eye Reflexes
 Internal muscles are controlled by the autonomic
nervous system
 Photopupillary reflex: bright light causes pupils to
constrict through action of radial, circular, and ciliary
muscles
 Accommodation pupillary reflex: viewing close
objects causes accommodation
 Viewing close objects causes convergence (eyes
moving medially)
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A Closer Look
 Emmetropia—eye focuses images correctly on the
retina
 Myopia (nearsightedness)
 Distant objects appear blurry
 Light from those objects fails to reach the retina and
are focused in front of it
 Results from an eyeball that is too long
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A Closer Look
 Hyperopia (farsightedness)
 Near objects are blurry, whereas distant objects are
clear
 Distant objects are focused behind the retina
 Results from an eyeball that is too short or from a
“lazy lens”
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A Closer Look 8.2 Bringing Things into Focus.
Focal
plane
Correction
None required
Concave lens
(a) Emmetropic
eye
(b) Myopic eye
(nearsighted)
(c) Hyperopic eye
(farsighted)
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Convex lens
A Closer Look
 Astigmatism
 Images are blurry
 Results from light focusing as lines, not points, on
the retina because of unequal curvatures of the
cornea or lens
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Homeostatic Imbalances of the Eyes
 Night blindness—inhibited rod function that hinders
the ability to see at night
 Color blindness—genetic conditions that result in
the inability to see certain colors
 Due to the lack of one type of cone (partial color
blindness)
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Homeostatic Imbalances of the Eyes
 Glaucoma—can cause blindness due to increasing
pressure within the eye
 Hemianopia—loss of the same side of the visual
field of both eyes; results from damage to the visual
cortex on one side only
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The Ear
 Houses two senses:
1. Hearing
2. Equilibrium (balance)
 Receptors are mechanoreceptors
 Different organs house receptors for each sense
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Anatomy of the Ear
 The ear is divided into three areas:
1. External (outer) ear
2. Middle ear (tympanic cavity)
3. Inner ear (bony labyrinth)
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Figure 8.12 Anatomy of the ear.
External (outer) ear
Middle ear
Internal (inner) ear
Vestibulocochlear
nerve
Auricle
(pinna)
Semicircular
canals
Oval window
Cochlea
Vestibule
Round window
Pharyngotympanic
(auditory) tube
Tympanic
membrane
(eardrum) Hammer Anvil Stirrup
(malleus) (incus) (stapes)
External acoustic
meatus
Auditory ossicles
(auditory canal)
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The External Ear
 Involved in hearing only
 Structures of the external ear
 Auricle (pinna)
 External acoustic meatus (auditory canal)
 Narrow chamber in the temporal bone
 Lined with skin and ceruminous (wax) glands
 Ends at the tympanic membrane (eardrum)
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The Middle Ear (Tympanic Cavity)
 Air-filled cavity within the temporal bone
 Involved only in the sense of hearing
 Located between tympanic membrane and oval
window and round window
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The Middle Ear (Tympanic Cavity)
 Two tubes are associated with the inner ear:
1. The opening from the auditory canal is covered by
the tympanic membrane
2. The pharyngotympanic, or auditory, tube connects
the middle ear with the throat
 Allows for equalizing pressure during yawning or
swallowing
 This tube is otherwise collapsed
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Bones of the Middle Ear (Tympanic Cavity)
 Three bones (ossicles) span the cavity:
1. Malleus (hammer)
2. Incus (anvil)
3. Stapes (stirrup)
 Function
 Vibrations from tympanic membrane move the
hammer  anvil  stirrup  oval window of inner
ear
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Inner Ear or Bony Labyrinth
 Includes sense organs for hearing and balance
 Filled with perilymph
 Contains a maze of bony chambers within the
temporal bone:
 Cochlea
 Vestibule
 Semicircular canals
 Membranous labyrinth is suspended in perilymph
and contains endolymph
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Organs of Equilibrium
 Equilibrium receptors of the inner ear are called the
vestibular apparatus
 Vestibular apparatus has two functional parts:
1. Static equilibrium
2. Dynamic equilibrium
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Figure 8.14a Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Semicircular
canals
Ampulla
Vestibular
nerve
Vestibule
(a)
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Static Equilibrium
 Maculae—receptors in the vestibule
 Report on the position of the head
 Send information via the vestibular nerve
 Anatomy of the maculae
 Hair cells are embedded in the otolithic membrane
 Otoliths (tiny stones) float in a gel around the hair
cells
 Movements cause otoliths to bend the hair cells
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Figure 8.13a Structure and function of maculae (static equilibrium receptors).
Membranes in vestibule
Otoliths
Otolithic
membrane
Hair tuft
Hair cell
Supporting cell
(a)
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Nerve fibers of
vestibular division
of cranial nerve VIII
Figure 8.13b Structure and function of maculae (static equilibrium receptors).
Otolithic
membrane
Head upright
(b)
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Otoliths
Hair cell
Force of
gravity
Head tilted
Dynamic Equilibrium
 These receptors respond to angular or rotary
movements
 Crista ampullaris (in the ampulla of each
semicircular canal)—dynamic equilibrium receptors
are located in the semicircular canals
 Tuft of hair cells covered with cupula (gelatinous cap)
 If the head moves, the cupula drags against the
endolymph
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Figure 8.14a Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Semicircular
canals
Ampulla
Vestibular
nerve
Vestibule
(a)
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Figure 8.14b Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Ampulla
Endolymph
(b)
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Cupula of crista
ampullaris
Figure 8.14c Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Flow of
endolymph
Cupula
(c)
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Direction of body
movement
Nerve
fibers
Dynamic Equilibrium
 Action of angular head movements
 The movement of the cupula stimulates the hair cells
 An impulse is sent via the vestibular nerve to the
cerebellum
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Organs of Hearing
 Spiral organ of Corti
 Located within the cochlear duct
 Receptors  hair cells on the basilar membrane
 Gel-like tectorial membrane is capable of bending
hair cells
 Cochlear nerve attached to hair cells transmits nerve
impulses to auditory cortex on temporal lobe
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Figure 8.15a Anatomy of the cochlea.
Spiral
organ of
Corti
Temporal
bone
Cochlea
duct (contains
endolymph)
(a)
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Perilymph in scala vestibuli
Vestibular
membrane
Afferent fibers
of the cochlear
nerve
Temporal
bone
Perilymph in
scala tympani
Figure 8.15b Anatomy of the cochlea.
Hair (receptor)
cells of spiral
organ of Corti
Basilar
membrane
(b)
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Tectorial
membrane
Supporting
cells
Vestibular
membrane
Fibers of
the cochlear
nerve
Mechanism of Hearing
 Vibrations from sound waves move tectorial
membrane
 Hair cells are bent by the membrane
 An action potential starts in the cochlear nerve
 Impulse travels to the temporal lobe
 Continued stimulation can lead to adaptation
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Mechanism of Hearing
 High-pitched sounds disturb the short, stiff fibers of
the basilar membrane
 Receptor cells close to the oval window are
stimulated
 Low-pitched sounds disturb the long, floppy fibers of
the basilar membrane
 Specific hair cells further along the cochlea are
affected
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Figure 8.16 Route of sound waves through the ear.
EXTERNAL EAR
Auditory
canal
Hammer,
Eardrum anvil, stirrup
INTERNAL EAR
Oval
window
Fluids in cochlear canals
lower
Upper and middle
Pressure
Pinna
MIDDLE EAR
Amplitude
One
vibration
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Amplification
in middle ear
Spiral
organ
of Corti
stimulated
Time
Figure 8.17 Activation of the cochlear hair cells.
Stapes
Fibers of
sensory
Scala
neurons
vestibuli
Perilymph
Oval
window
(a)
Round
window
Scala
tympani
Basilar
membrane
Cochlear
duct
Fibers of basilar membrane
Apex
(long,
floppy
fibers)
Base (short,
stiff fibers)
(b)
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20,000
(High notes)
2,000
200
Frequency (Hz)
20
(Low notes)
Hearing and Equilibrium Deficits
 Deafness is any degree of hearing loss
 Conduction deafness results when the transmission
of sound vibrations through the external and middle
ears is hindered
 Sensorineural deafness results from damage to the
nervous system structures involved in hearing
 Ménière’s syndrome affects the inner ear and causes
progressive deafness and perhaps vertigo (sensation
of spinning)
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Chemical Senses: Taste and Smell
 Both senses use chemoreceptors
 Stimulated by chemicals in solution
 Taste has four types of receptors
 Smell can differentiate a large range of chemicals
 Both senses complement each other and respond
to many of the same stimuli
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Olfaction—The Sense of Smell
 Olfactory receptors are in roof of nasal cavity
 Olfactory receptors cells (neurons) with long cilia
known as olfactory hairs detect chemicals
 Chemicals must be dissolved in mucus for detection
by chemoreceptors called olfactory receptors
 Impulses are transmitted via the olfactory filaments
to the olfactory nerve
 Interpretation of smells is made in the cortex
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Figure 8.18 Location and cellular makeup of the olfactory epithelium.
Olfactory bulb
Cribriform plate
of ethmoid bone
Olfactory tract
Olfactory
filaments of the
olfactory nerve
Supporting cell
Olfactory
mucosa
(a)
Olfactory
receptor cell
Olfactory hairs
(cilia)
Mucus layer
Route of inhaled air
containing odor molecules
(b)
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Taste Buds and the Sense of Taste
 Taste buds house the receptor organs
 Locations of taste buds
 Most are on the tongue
 Soft palate
 Cheeks
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Taste Buds and the Sense of Taste
 The tongue is covered with projections called
papillae
 Filiform papillae—sharp with no taste buds
 Fungiform papillae—rounded with taste buds
 Circumvallate papillae—large papillae with taste
buds
 Taste buds are found on the sides of papillae
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Figure 8.19a Location and structure of taste buds.
Epiglottis
Palatine tonsil
Lingual tonsil
Fungiform
papillae
(a)
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Figure 8.19b Location and structure of taste buds.
Vallate papilla
Taste buds
(b)
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Structure of Taste Buds
 Gustatory cells are the receptors
 Possess gustatory hairs (long microvilli)
 Hairs are stimulated by chemicals dissolved in saliva
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Taste Sensations
 Sweet receptors respond to sugars, saccharine,
some amino acids
 Sour receptors respond to H ions or acids
 Bitter receptors respond to alkaloids
 Salty receptors respond to metal ions
 Umami receptors respond to the amino acid
glutamate or the beefy taste of meat
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Developmental Aspects of the Special Senses
 Special sense organs are formed early in embryonic
development
 Maternal infections during the first 5 or 6 weeks of
pregnancy may cause visual abnormalities as well
as sensorineural deafness in the developing child
 Congenital ear problems usually result from missing
pinnas and closed or missing external acoustic
meatuses
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Developmental Aspects of the Special Senses
 Vision requires the most learning
 The infant has poor visual acuity (is farsighted) and
lacks color vision and depth perception at birth
 The eye continues to grow and mature until age
8 or 9
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Developmental Aspects of the Special Senses
 Eye problems
 Strabismus—“crossed eyes”; results from unequal
pulls by the external eye muscles in babies
 Ophthalmia neonatorum—conjunctivitis resulting
from gonorrhea in the mother; baby’s eyelids are
swollen, and pus is produced
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Developmental Aspects of the Special Senses
 Problems of aging associated with vision include
presbyopia, glaucoma, cataracts, and
arteriosclerosis of the eye’s blood vessels
 Presbyopia—“old vision” results from decreasing
lens elasticity that accompanies aging
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Developmental Aspects of the Special Senses
 The newborn infant can hear sounds, but initial
responses are reflexive
 By the toddler stage, the child is listening critically
and beginning to imitate sounds as language
development begins
 Age-related ear problems:
 Presbycusis—type of sensorineural deafness that
may result from otosclerosis
 Otosclerosis—ear ossicles fuse
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Developmental Aspects of the Special Senses
 Taste and smell are most acute at birth and
decrease in sensitivity after age 40 as the number
of olfactory and gustatory receptors decreases
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