Chapter 8
Special Senses
Lecture Presentation by
Patty Bostwick-Taylor
Florence-Darlington Technical College
© 2018 Pearson Education, Ltd.
Special Senses
 Special senses include:
 Smell
 Taste
 Sight
 Hearing
 Equilibrium
 Special sense receptors
 Large, complex sensory organs
 Localized clusters of receptors
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Part I: The Eye and Vision
 70 percent of all sensory receptors are in the
eyes
 Each eye has over 1 million nerve fibers carrying
information to the brain
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Anatomy of the Eye
 Accessory structures include the:
 Extrinsic eye muscles
 Eyelids
 Conjunctiva
 Lacrimal apparatus
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Figure 8.1 Surface anatomy of the eye and accessory structures.
Site where
conjunctiva
merges with
cornea
Eyebrow
Palpebral
fissure
Pupil
Lacrimal
caruncle
Lateral
commissure
(canthus)
Medial
commissure
(canthus)
Iris
Sclera
(covered by
conjunctiva)
Eyelid
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Eyelid
Eyelashes
External and Accessory Structures
 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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External and Accessory Structures
 Conjunctiva
 Membrane that lines the eyelids and eyeball
 Connects with the transparent cornea
 Secretes mucus to lubricate the eye and keep it moist
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External and Accessory Structures
 Lacrimal apparatus = lacrimal gland + ducts
 Lacrimal gland—produces lacrimal fluid (tears);
situated on lateral end of each eye
 Tears drain across the eye into the lacrimal canaliculi,
then the lacrimal sac, and into the nasolacrimal duct,
which empties into the nasal cavity
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External and Accessory Structures
 Tears contain:
 Dilute salt solution
 Mucus
 Antibodies
 Lysozyme (enzyme that destroys bacteria)
 Function of tears
 Cleanse, protect, moisten, lubricate the eye
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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
External and Accessory Structures
 Extrinsic eye muscles
 Six muscles attach to the outer surface of the eye
 Produce gross eye movements
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Figure 8.3a Extrinsic muscles of the eye.
Superior
oblique muscle
Superior
oblique tendon
Superior
rectus muscle
Conjunctiva
Lateral rectus
muscle
(a)
Optic
nerve
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Inferior Inferior
rectus oblique
muscle muscle
Figure 8.3b Extrinsic muscles of the eye.
Trochlea
Superior
oblique muscle
Superior
oblique tendon
Superior
rectus muscle
Axis at
center of
eye
Inferior
rectus muscle
Medial
rectus muscle
Lateral
rectus muscle
(b)
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Common
tendinous ring
Figure 8.3c Extrinsic muscles of the eye.
(c)
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Internal Structures: The Eyeball
 Three layers, or tunics, form 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
 Lens divides the eye into two chambers
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Figure 8.4a Internal anatomy of the eye (sagittal section).
Sclera
Ciliary body
Choroid
Ciliary zonule
Retina
Cornea
Iris
Fovea centralis
Pupil
Aqueous
humor
(in anterior
segment)
Optic nerve
Lens
Scleral venous sinus
(canal of Schlemm)
Vitreous humor
(in posterior segment)
(a)
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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
Vitreous humor
in posterior
segment
Retina
Choroid
Sclera
Aqueous humor
(in anterior
segment)
Lens
Cornea
Ciliary zonule
(b)
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Fovea centralis
Optic disc
Optic nerve
Internal Structures: The Eyeball
 Fibrous layer = sclera + cornea
 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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Internal Structures: The Eyeball
 Vascular layer
 Choroid is a blood-rich nutritive layer that contains a
pigment (prevents light from scattering)
 Choroid is modified anteriorly into two smooth muscle
structures
 Ciliary body
 Iris—regulates amount of light entering eye
 Pigmented layer that gives eye color
 Pupil—rounded opening in the iris
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Internal Structures: The Eyeball
 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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Internal Structures: The Eyeball
 Sensory layer (continued)
 Electrical signals pass from photoreceptors via a twoneuron 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
Pathway
of light
(a)
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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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Sclera
Optic
nerve Choroid
Internal Structures: The Eyeball
 Sensory layer (continued)
 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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Internal Structures: The Eyeball
 Sensory layer (continued)
 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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Internal Structures: The Eyeball
 Sensory layer (continued)
 Cone sensitivity
 Three types of cones
 Each cone type is sensitive to different wavelengths of
visible light
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Figure 8.6 Sensitivities of the three cone types to different wavelengths of visible light.
Light absorption by cone populations
Visible light
420 nm
530 nm
(blue cones) (green cones)
400
560 nm
(red cones)
450
500
550
600
Wavelength (nanometers)
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650
700
Internal Structures: The Eyeball
 Lens
 Flexible, biconvex crystal-like structure
 Held in place by a suspensory ligament attached to the
ciliary body
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Figure 8.4a Internal anatomy of the eye (sagittal section).
Sclera
Ciliary body
Choroid
Ciliary zonule
Retina
Cornea
Iris
Fovea centralis
Pupil
Aqueous
humor
(in anterior
segment)
Optic nerve
Lens
Scleral venous sinus
(canal of Schlemm)
Vitreous humor
(in posterior segment)
(a)
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Central artery
and vein of
the retina
Optic disc
(blind spot)
Internal Structures: The Eyeball
 Lens divides the eye into two chambers
1. Anterior (aqueous) segment
 Anterior to the lens
 Contains aqueous humor, a clear, watery fluid
2. Posterior (vitreous) segment
 Posterior to the lens
 Contains vitreous humor, a gel-like substance
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Figure 8.4a Internal anatomy of the eye (sagittal section).
Sclera
Ciliary body
Choroid
Ciliary zonule
Retina
Cornea
Iris
Fovea centralis
Pupil
Aqueous
humor
(in anterior
segment)
Optic nerve
Lens
Scleral venous sinus
(canal of Schlemm)
Vitreous humor
(in posterior segment)
(a)
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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
Vitreous humor
in posterior
segment
Retina
Choroid
Sclera
Aqueous humor
(in anterior
segment)
Lens
Cornea
Ciliary zonule
(b)
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Fovea centralis
Optic disc
Optic nerve
Internal Structures: The Eyeball
 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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Internal Structures: The Eyeball
 Vitreous humor
 Gel-like substance posterior to the lens
 Prevents the eye from collapsing
 Helps maintain intraocular pressure
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Internal Structures: The Eyeball
 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 8.7 The posterior wall (fundus) of the retina as seen with an ophthalmoscope.
Macula
Fovea
centralis
Lateral
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Blood
vessels
Optic disc Retina
Medial
Physiology of Vision
 Pathway of light through the eye and light
refraction
 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 distant 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 8.8 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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Physiology of Vision
 Pathway of light through the eye and light
refraction (continued)
 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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Figure 8.9 Real image (reversed left to right, and upside down) formed on the retina.
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Physiology of Vision
 Visual fields and visual pathways to the brain
 Optic nerve
 Bundle of axons that exit the back of the eye carrying
impulses from the retina
 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
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Physiology of Vision
 Visual fields and visual pathways to the brain
(continued)
 Optic tracts
 Contain fibers from the lateral side of the eye on the
same side and the medial side of the opposite eye
 Synapse with neurons in the thalamus
 Optic radiation
 Axons from the thalamus run to the occipital lobe
 Synapse with cortical cells, and vision interpretation
(seeing) occurs
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Physiology of Vision
 Summary of the pathway of impulses from the
retina to the point of visual interpretation
1.
2.
3.
4.
5.
6.
Optic nerve
Optic chiasma
Optic tract
Thalamus
Optic radiation
Optic cortex in occipital lobe of brain
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Figure 8.10 Visual fields of the eyes and visual pathway to the brain (inferior view).
Fixation point
Right eye
Left eye
Optic
nerve
Optic
tract
Optic
chiasma
Optic
radiation
Thalamus
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Occipital lobe
(visual cortex)
Physiology of Vision
 Visual fields
 Each eye “sees” a slightly different view
 Field of view overlaps for each eye
 Binocular vision results and provides:
 Depth perception (three-dimensional vision)
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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
 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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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
Physiology of Vision
 Eye reflexes
 Convergence: reflexive movement of the eyes medially
when we focus on a close object
 Photopupillary reflex: bright light causes pupils to
constrict
 Accommodation pupillary reflex: viewing close objects
causes pupils to constrict
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Part II: The Ear: Hearing and Balance
 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
3. Internal (inner) ear
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Figure 8.11 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)
External acoustic
meatus
(auditory canal)
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Hammer Anvil Stirrup
(malleus) (incus) (stapes)
Auditory ossicles
Anatomy of the Ear
 External (outer) ear
 Auricle (pinna)
 External acoustic meatus (auditory canal)
 Narrow chamber in the temporal bone
 Lined with skin and ceruminous (earwax) glands
 Ends at the tympanic membrane (eardrum)
 External ear is involved only in collecting sound waves
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Anatomy of the Ear
 Middle ear cavity (tympanic cavity)
 Air-filled, mucosa-lined 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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Anatomy of the Ear
 Middle ear cavity (tympanic cavity) (continued)
 Pharyngotympanic tube (auditory tube)
 Links middle ear cavity with the throat
 Equalizes pressure in the middle ear cavity so the
eardrum can vibrate
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Anatomy of the Ear
 Middle ear cavity (tympanic cavity) (continued)
 Three bones (ossicles) span the cavity
1. Malleus (hammer)
2. Incus (anvil)
3. Stapes (stirrup)
 Function
 Transmit vibrations from tympanic membrane to the
fluids of the inner ear
 Vibrations travel from the hammer → anvil → stirrup →
oval window of inner ear
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Figure 8.11 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)
External acoustic
meatus
(auditory canal)
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Hammer Anvil Stirrup
(malleus) (incus) (stapes)
Auditory ossicles
Anatomy of the Ear
 Internal (inner) ear
 Includes sense organs for hearing and balance
 Bony labyrinth (osseous labyrinth) consists of:
 Cochlea
 Vestibule
 Semicircular canals
 Bony labyrinth is filled with perilymph
 Membranous labyrinth is suspended in perilymph and
contains endolymph
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Figure 8.11 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)
External acoustic
meatus
(auditory canal)
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Hammer Anvil Stirrup
(malleus) (incus) (stapes)
Auditory ossicles
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 8.13a 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
 Help us keep our head erect
 Send information via the vestibular nerve (division of
cranial nerve VIII) to the cerebellum of the brain
 Anatomy of the maculae
 Hair cells are embedded in the otolithic membrane
 Otoliths (tiny stones) float in a gel around hair cells
 Movements cause otoliths to roll and bend hair cells
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Figure 8.12a 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 8.12b Structure and function of maculae (static equilibrium receptors).
Otolithic
membrane
Head upright
(b)
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Otoliths
Force of
gravity
Hair cell
Head tilted
Dynamic Equilibrium
 Crista ampullaris
 Responds to angular or rotational movements of the
head
 Located in the ampulla of each semicircular canal
 Tuft of hair cells covered with cupula (gelatinous cap)
 If the head moves, the cupula drags against the
endolymph
 Hair cells are stimulated, and the impulse travels the
vestibular nerve to the cerebellum
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Figure 8.13a Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Semicircular
canals
Ampulla
Vestibular
nerve
Vestibule
(a)
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Figure 8.13b Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Ampulla
Endolymph
(b)
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Cupula of crista
ampullaris
Figure 8.13c Structure and function of the crista ampullaris (dynamic equilibrium receptor region).
Flow of
endolymph
Cupula
Direction of body
(c) movement
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Nerve
fibers
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 8.14a Anatomy of the cochlea.
Spiral
organ of
Corti
Temporal
bone
Perilymph in scala vestibuli
Vestibular
membrane
Afferent fibers
of the cochlear
nerve
Temporal
bone
Cochlear
duct (contains
endolymph)
(a)
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Perilymph in
scala tympani
Figure 8.14b Anatomy of the cochlea.
Hair (receptor)
cells of spiral
organ of Corti
(b)
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Basilar
membrane
Tectorial
membrane
Supporting
cells
Vestibular
membrane
Fibers of
the cochlear
nerve
Hearing
 Pathway of vibrations from sound waves
 Move by the ossicles from the eardrum to the oval
window
 Sound is amplified by the ossicles
 Pressure waves cause vibrations in the basilar
membrane in the spiral organ of Corti
 Hair cells of the tectorial membrane are bent when the
basilar membrane vibrates against it
 An action potential starts in the cochlear nerve (cranial
nerve VIII), and the impulse travels to the temporal
lobe
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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 8.15 Route of sound waves through the ear.
EXTERNAL EAR
Auditory
canal
Eardrum
Hammer,
anvil, stirrup
INTERNAL EAR
Oval
window
Fluids in cochlear canals
Upper and middle
lower
Pressure
Pinna
MIDDLE EAR
One
vibration
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Amplitude
Amplification
in middle ear
Spiral organ
of Corti
stimulated
Time
Figure 8.16 Activation of the cochlear hair cells.
Stapes
Fibers of
sensory
Scala
neurons
vestibuli
Oval
Perilymph
window
Round
window
Scala
tympani
Basilar
Cochlear
membrane duct
(a)
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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Part III: Chemical Senses: Smell and Taste
 Chemoreceptors
 Stimulated by chemicals in solution
 Taste has five types of receptors
 Smell can differentiate a wider range of chemicals
 Both senses complement each other and respond
to many of the same stimuli
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Olfactory Receptors and the Sense of Smell
 Olfactory receptors are in roof of nasal cavity
 Olfactory receptor 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 (cranial nerve I)
 Smells are interpreted in the olfactory cortex
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Figure 8.17 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)
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Olfactory receptor
cell
Olfactory hairs
(cilia)
Mucus layer
Route of inhaled air
containing odor molecules
(b)
Taste Buds and the Sense of Taste
 Taste buds house the receptor organs
 Locations of taste buds
 Most are on the tongue
 Soft palate
 Superior part of the pharynx
 Cheeks
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Taste Buds and the Sense of Taste
 The tongue is covered with projections called
papillae that contain taste buds
 Vallate (circumvallate) papillae
 Fungiform papillae
 Filiform papillae
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Figure 8.18a Location and structure of taste buds.
Epiglottis
Palatine tonsil
Lingual tonsil
Foliate
papillae
Fungiform
papillae
(a)
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Figure 8.18b Location and structure of taste buds.
Vallate papilla
Taste buds
(b)
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Taste Buds and the Sense of Taste
 Gustatory cells are the taste receptors
 Possess gustatory hairs (long microvilli)
 Gustatory hairs protrude through a taste pore
 Hairs are stimulated by chemicals dissolved in saliva
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Figure 8.18c Location and structure of taste buds.
Epithelium of tongue
Taste bud
Connective
tissue
Surface of
the tongue
Gustatory
(taste) cell
Basal
cell
Sensory
nerve
fiber
Gustatory hairs
(microvilli) emerging
from a taste pore
(c)
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Taste Buds and the Sense of Taste
 Impulses are carried to the gustatory complex by
several cranial nerves because taste buds are
found in different areas
 Facial nerve (cranial nerve VII)
 Glossopharyngeal nerve (cranial nerve IX)
 Vagus nerve (cranial nerve X)
 Taste buds are replaced frequently by basal cells
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Taste Buds and the Sense of Taste
 Five basic 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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Part IV: 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
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Part IV: 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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Part IV: Developmental Aspects of the
Special Senses
 Age-related eye issues
 Presbyopia—“old vision” results from decreasing lens
elasticity that accompanies aging
 Causes difficulty to focus for close vision
 Lacrimal glands become less active
 Lens becomes discolored
 Dilator muscles of iris become less efficient, causing
pupils to remain constricted
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Part IV: 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
 Congenital ear problems usually result from missing
pinnas and closed or missing external acoustic meatuses
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Part IV: 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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