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Prepared by Patty Bostwick-Taylor,
Florence-Darlington Technical College
CHAPTER
8
Special
Senses
© 2012 Pearson Education, Inc.
The Senses
•Special senses
•Smell
•Taste
•Sight
•Hearing
•Equilibrium
© 2012 Pearson Education, Inc.
The Eye and Vision
•A. 70 percent of all sensory receptors are in
the eyes
•B. Each eye has over a million nerve fibers
•C. Protection for the eye
•Most of the eye is enclosed in a bony orbit
•A cushion of fat surrounds most of the eye
•Which bones make up the eye orbit??
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Site where
conjunctiva
merges with
cornea
Eyebrow
Eyelid
Eyelashes
Pupil
Lateral canthus
Iris
Eyelid
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Medial
canthus
Sclera
(covered by
conjunctiva)
Figure 8.1
Accessory Structures of the Eye
•A. Eyelids
•Meet at the medial and lateral commissure
(canthus)
•B. Eyelashes
•Tarsal glands produce an oily secretion that
lubricates the eye
•Ciliary glands are located between the
eyelashes- modified sweat glands
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Accessory Structures of the Eye
•C. Conjunctiva
•Membrane that lines the eyelids
•Connects to the outer surface of the eye
•Secretes mucus to lubricate the eye and
keep it moist
•What is pink eye?
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Accessory Structures of the Eye
•D. Lacrimal apparatus = lacrimal gland + ducts
•1. Lacrimal gland—produces lacrimal fluid;
situated on lateral aspect of each eye
•2. Lacrimal canaliculi—drain lacrimal fluid
from eyes medially
•3. Lacrimal sac—provides passage of
lacrimal fluid towards nasal cavity
•4. Nasolacrimal duct—empties lacrimal fluid
into the nasal cavity
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Lacrimal
gland
Excretory duct
of lacrimal gland
Conjunctiva
Anterior
aspect
(a)
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Eyelid
Eyelashes
Tarsal
glands
Eyelid
Figure 8.2a
Lacrimal
gland
Excretory ducts
of lacrimal gland
Lacrimal sac
Lacrimal canaliculus
Nasolacrimal duct
Nostril
(b)
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Figure 8.2b
Accessory Structures of the Eye
•5. Function of the lacrimal apparatus
•Protects, moistens, and lubricates the eye
•Empties into the nasal cavity
•6. Lacrimal secretions (tears) contain:
• Dilute salt solution
• Mucus
• Antibodies
• Lysozyme (enzyme that destroys
bacteria)
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Accessory Structures of the Eye
•G. Extrinsic eye muscles
•1. Six muscles attach to the outer surface
of the eye
•2. Produce eye movements
•How is the human eye musculature
different than a cow eye’s?
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Superior
oblique muscle
Superior
oblique tendon
Superior
rectus muscle
Conjunctiva
Lateral rectus
muscle
Optic
nerve
Inferior Inferior
rectus oblique
muscle muscle
(a)
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Figure 8.3a
Trochlea
Superior
oblique muscle
Superior
oblique tendon
Axis at
center of
eye
Superior
rectus muscle
Inferior
rectus muscle
Why are the eye
muscles named
this way?
Medial
rectus muscle
Lateral
rectus muscle
(b)
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Figure 8.3b
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Figure 8.3c
Structure of the Eye
•Layers forming the wall of the eyeball
•1. Fibrous layer
• Outside layer
•2. Vascular layer
• Middle layer
•3. Sensory layer
• Inside layer
© 2012 Pearson Education, Inc.
Why were the layers
of the eye called
“tunics” in the past?
Ciliary body
Ciliary zonule
Cornea
Iris
Pupil
Aqueous
humor
Lens
canal of Schlemm
Vitreous humor
Sclera
Choroid
Retina
Fovea centralis
(macula)
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
(a)
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Figure 8.4a
Ciliary body
Iris
Margin
of pupil
Aqueous humor
(
Lens
Cornea
Ciliary zonule
Vitreous humor
Retina
Choroid
Sclera
Fovea centralis
Optic disc
Optic nerve
(b)
© 2012 Pearson Education, Inc.
Figure 8.4b
Structure of the Eye: The Fibrous Layer
•1. Sclera
•White connective tissue layer
•Seen anteriorly as the “white of the eye”
•2. Cornea
•Transparent, central anterior portion
•Allows for light to pass through
•Repairs itself easily
Why?
•The only human tissue that can be
transplanted without fear of rejection
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Structure of the Eye: Vascular Layer
•1. Choroid is a blood-rich nutritive layer in the
posterior of the eye
•Pigment prevents light from scattering
•2. Modified anteriorly into two structures
•a. Ciliary body—smooth muscle attached to
lens
•b. Iris—regulates light entering eye
•Pigmented layer that gives eye color
•Pupil—rounded opening in the iris
© 2012 Pearson Education, Inc.
Structure of the Eye: Sensory Layer
•1. Retina contains two layers:
•Outer pigmented layer
•Inner neural layer
2. Contains receptor cells (photoreceptors)
• Rods
• Cones
© 2012 Pearson Education, Inc.
1. Do you recognize
anything?
2. What do the axons
of the ganglion cells
form? See next page
if needed.
Pigmented
layer of retina
Rod
Cone
Bipolar
cells
Ganglion
cells
Pathway
of light
(a)
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Figure 8.5a
Pigmented
Neural layer
layer of
of retina
retina
Central
artery
and vein
of retina
Optic
disc
Sclera
Optic
nerve Choroid
© 2012 Pearson Education, Inc.
(b)
Figure 8.5b
Structure of the Eye: Sensory Layer
•Neurons of the retina and vision
•A. Rods
•1. Most are found towards the edges of
the retina
•2. Allow dim light vision and peripheral
vision
•3. All perception is in gray tones
© 2012 Pearson Education, Inc.
Structure of the Eye: Sensory Layer
•Neurons of the retina and vision
•B. Cones
•1. Allow for detailed color vision
•2. Densest in the center of the retina
•3. Fovea centralis–lateral to blind spot
• Area of the retina with only cones
• Visual acuity (sharpest vision) is here
•C. No photoreceptor cells are at the optic disc,
or blind spot
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Structure of the Eye: Sensory Layer
•Cone sensitivity
•A. Three types of cones
•B. Different cones are sensitive to different
wavelengths
•C. Color blindness is the result of the lack of
one cone type
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Light absorption by cone populations
560 nm
(red cones)
530 nm
(green cones)
420 nm
(blue cones)
380
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450
500
550
600
650
700
750
Wavelengths (nanometers)
Figure 8.6
Lens
•A. Biconvex crystal-like structure
•B. Held in place by a suspensory ligament
attached to the ciliary body
•Put your hands in a biconvex shape
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Ciliary body
Ciliary zonule
Cornea
Iris
Pupil
Aqueous
humor
Lens
canal of Schlemm
Vitreous humor
Sclera
Choroid
Retina
Fovea centralis
(macula)
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
(a)
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Figure 8.4a
Lens- no notes!
•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
© 2012 Pearson Education, Inc.
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Figure 8.7
Two Segments, or Chambers, of the Eye
- No Notes
•Anterior (aqueous) segment
•Anterior to the lens
•Contains aqueous humor
•Posterior (vitreous) segment
•Posterior to the lens
•Contains vitreous humor
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Anterior Segment
•A. Aqueous humor
•1. Watery fluid found between lens and
cornea
•2. Similar to blood plasma
•3. Helps maintain intraocular pressure
•4. Provides nutrients for the lens and cornea
•5. Reabsorbed into venous blood through
the scleral venous sinus, or canal of
Schlemm
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Posterior Segment
•B. Vitreous humor
•1. Gel-like substance posterior to the lens
•2. Prevents the eye from collapsing
•3. Helps maintain intraocular pressure
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Macula
Fovea
centralis
Lateral
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Blood
Optic disc Retina
vessels
Medial
Figure 8.8
Pathway of Light Through the Eye
•A. Light must be focused to a point on the
retina for optimal vision
•B. The eye is set for distance vision
(over 20 feet away)
•C. Accommodation—the lens must change
shape to focus on closer objects (less than 20
feet away)
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Retina
Light from distant source
Focal point
(a)
Light from near source
Focal point
Retina
(b)
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Figure 8.9
Pathway of Light Through the Eye
•D. Image formed on the retina is a real image
•E. Real images are:
•Reversed from left to right
•Upside down
•Smaller than the object
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Figure 8.10
Visual Fields and Visual Pathways
•A. 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
•B. 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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Fixation point
Right eye
Left eye
Optic
nerve
Optic
tract
Optic
chiasma
Optic
radiation
Thalamus
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Occipital lobe
(visual cortex)
Figure 8.11
Pathway of vision from light entering pupil
to occipital lobe of brain….
Light →Cornea→ Aqueous humor → Pupil → Lens →
Vitreous Humor → Absorbed by Pigment/Choroid
→ Retina [Rods/Cones Electrical signal → Bipolar
cells → Ganglion cells] →Optic nerve II→ Optic tract
**→ Thalamus → Occipital lobe (visual cortex)
**Axons from both eyes due to crossing over
Slide 8.28
© 2012 Pearson Education, Inc.
A Closer Look
•Emmetropia—eye focuses images correctly on
the retina
•A. Myopia (nearsighted)
•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
•B. Hyperopia (farsighted)
•Near objects are blurry while 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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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
•C. Astigmatism
•Images are blurry
•Results from light focusing as lines, not
points, on the retina due to unequal
curvatures of the cornea or lens
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Homeostatic Imbalances of the Eyes
•D. Night blindness—inhibited rod function that
hinders the ability to see at night
•E.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)
•F.Cataracts—when lens becomes hard and
opaque, our vision becomes hazy and
distorted
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Homeostatic Imbalances of the Eyes
•G. Glaucoma—can cause blindness due to
increasing pressure within the eye
•H. 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
•A. Houses two senses
•Hearing
•Equilibrium (balance)
•B. Receptors are mechanoreceptors
•C. Different organs house receptors for each
sense
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Anatomy of the Ear
•The ear is divided into three areas
•A. External (outer) ear
•B. Middle ear (tympanic cavity)
•C. Inner ear (bony labyrinth)
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External (outer) ear
Middle ear
Internal (inner) ear
Vestibulocochlear
Nerve (auditory)
Pinna
Semicircular
canals
Oval window
(auricle)
Cochlea
Vestibule
Round window
Auditory tube
Tympanic
membrane
malleus
Auditory canal
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incus stapes
Auditory ossicles
Figure 8.12
The External Ear
•A. Involved in hearing only
•B. Structures of the external ear
•1. Pinna (auricle)
•2. Auditory Canal (External acoustic meatus)
•Narrow chamber in the temporal bone
•Lined with skin and ceruminous (wax)
glands
•Ends at the tympanic membrane
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The Middle Ear (Tympanic Cavity)
•A. Air-filled cavity within the temporal bone
•B. Only involved in the sense of hearing
•When does the middle ear fill with fluid? Is that
bad or good??
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The Middle Ear (Tympanic Cavity)
•Two tubes are associated with the middle ear
•The opening from the auditory canal is
covered by the tympanic membrane
•The auditory tube connecting the middle ear
with the throat
•Allows for equalizing pressure during
yawning or swallowing
•This tube is otherwise collapsed
• Why do pacifier-babies get more ear
infections??
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Bones of the Middle Ear (Tympanic Cavity)
•Three bones (ossicles) span the cavity
•Malleus (hammer)
•Incus (anvil)
•Stapes (stirrup)
•Function
•Vibrations from eardrum move the malleus
 incus  stapes  inner ear
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External (outer) ear
Middle ear
Internal (inner) ear
Vestibulocochlear
Nerve (auditory)
Pinna
Semicircular
canals
Oval window
(auricle)
Cochlea
Vestibule
Round window
Auditory tube
Tympanic
membrane
malleus
Auditory canal
© 2012 Pearson Education, Inc.
incus stapes
Auditory ossicles
Figure 8.12
Inner Ear or Bony Labyrinth
•A. Includes sense organs for hearing and
balance
•B. Filled with perilymph
•C. Contains a maze of bony chambers within
the temporal bone
•Cochlea
•Vestibule
•Semicircular canals (balance only!)
© 2012 Pearson Education, Inc.
External (outer) ear
Middle ear
Internal (inner) ear
Vestibulocochlear
Nerve (auditory)
Pinna
Semicircular
canals
Oval window
(auricle)
Cochlea
Vestibule
Round window
Auditory tube
Tympanic
membrane
malleus
Auditory canal
© 2012 Pearson Education, Inc.
incus stapes
Auditory ossicles
Figure 8.12
Organs of Hearing
•Organ of Corti
•A. Located within the cochlea
•B. Receptors = hair cells on the basilar
membrane
•C. Gel-like tectorial membrane is capable of
bending hair cells
•D. Cochlear nerve attached to hair cells
transmits nerve impulses to auditory cortex
on temporal lobe
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Mechanism of Hearing
•A. Vibrations from sound waves move tectorial
membrane
•B. Hair cells are bent by the membrane
•C. An action potential starts in the cochlear
nerve
•Impulse travels to the temporal lobe
•D. Continued stimulation can lead to
adaptation
•Why do you not notice a repetitive sound?
© 2012 Pearson Education, Inc.
MIDDLE EAR
EXTERNAL EAR
Auditory
canal
Eardrum
Hammer,
Oval
anvil, stirrup window
Fluids in cochlear canals
Upper and middle
lower
Pressure
Pinna
INTERNAL EAR
One
Amplitude
vibration
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Amplification
in middle ear
Spiral organ
of Corti
stimulated
Time
Figure 8.16
Spiral
organ of
Corti
Temporal
bone
Perilymph in scala vestibuli
Vestibular
membrane
Afferent fibers
of the cochlear
nerve
Temporal
bone
Cochlear
duct (contains Perilymph in
endolymph)
scala tympani
(a)
© 2012 Pearson Education, Inc.
Figure 8.15a
Hair (receptor)
cells of spiral
organ of Corti
Basilar
membrane
Tectorial
membrane
Supporting
cells
Vestibular
membrane
Fibers of
the cochlear
nerve
(b)
© 2012 Pearson Education, Inc.
Figure 8.15b
Pathway of sound waves from pinna to
temporal lobe of brain…..
•pinna → external auditory canal → tympanic
membrane vibration → (malleus → incus →
stapes) → oval window → fluids of inner ear
movement (perilymph) → Organ of Corti
(basilar/tectorial membranes → hair cells) →
Electrical impulse and neurotransmitter release
→ auditory nerve (VIII) → temporal lobe
Slide 8.40
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Mechanism of Hearing- No notes!
•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
© 2012 Pearson Education, Inc.
Fibers of
Stapes
Scala
sensory
vestibuli
neurons
Perilymph
Oval
window
Round
window
Scala
Basilar
Cochlear
tympani membrane duct
(a)
Fibers of basilar membrane
Apex
(long,
floppy
fibers)
Base (short,
stiff fibers)
20,000
(High notes)
2,000
200
Frequency (Hz)
20
(Low notes)
(b)
© 2012 Pearson Education, Inc.
Figure 8.17
Organs of Equilibrium
•A. Equilibrium receptors of the inner ear are
called the vestibular apparatus
•B. Vestibular apparatus has two functional
parts
•Static equilibrium
•Dynamic equilibrium
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Semicircular
canals
Ampulla
Vestibular
nerve
Vestibule
(a)
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Figure 8.14a
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 8.13a
Otolithic
membrane
Head upright
Otoliths
Force of
gravity
Hair cell
Head tilted
(b)
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Figure 8.13b
Semicircular
canals
Ampulla
Vestibular
nerve
Vestibule
(a)
© 2012 Pearson Education, Inc.
Figure 8.14a
Ampulla
Endolymph
Flow of
endolymph
Cupula of crista
ampullaris
Cupula
Nerve
fibers
(b)
© 2012 Pearson Education, Inc.
Direction of body
movement
(c)
Figure 8.14b-c
Chemical Senses: Taste and Smell
•A. Both senses use chemoreceptors
•1. Stimulated by chemicals in solution
•2. Taste has five types of receptors
•3. Smell can differentiate a large range of
chemicals
•B. Both senses complement each other and
respond to many of the same stimuli
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Olfaction—The Sense of Smell
Receptors: Neurons with long cilia (1000’s)
•Chemicals must be dissolved in mucus for
detection
•Location: Olfactory receptors are in the roof of
the nasal cavity
•Impulses are transmitted via the olfactory
nerve (I)
•Interpretation of smells is made in the olfactory
cortex (tied to limbic system)
© 2012 Pearson Education, Inc.
Olfactory bulb
Cribriform plate
of ethmoid bone
Olfactory tract
Olfactory filaments
of the olfactory
nerve
Supporting cell
Olfactory receptor
cell
Olfactory
mucosa
(a)
Mucus layer
Route of inhaled air
containing odor molecules
Olfactory hairs
(cilia)
(b)
© 2012 Pearson Education, Inc.
Figure 8.18
The Sense of Taste
•Receptors: Taste buds- Only 4 types
(gustatory cells with long microvillisubstances must be dissolved)
•Location of taste buds
•Most are on the tongue
•Soft palate
•Cheeks
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Epiglottis
Palatine tonsil
Lingual tonsil
Fungiform
papillae
(a)
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Figure 8.19a
Circumvallate papilla
Taste buds
(b)
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Figure 8.19b
Structure of Taste Buds
•Nerves (different places = different nerves)
•Facial nerve
•Glossopharyngeal nerve
•Vagus nerve
•Brain Area- gustatory cortex (insular/frontal
lobes)
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Taste Sensations
•Sweet receptors (sugars)
•Saccharine
•Some amino acids
•Sour receptors
•Acids
•Bitter receptors
•Alkaloids
•Salty receptors
•Metal ions
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Umami- Glutamate
**No “spicy” sense
Developmental Aspects of the Special
Senses
•A. Formed early in embryonic development
•B. Eyes are outgrowths of the brain
•C. All special senses are functional at birth
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Developmental Aspects of the Special
Senses
•Eye problems
•A. Strabismus—“crossed eyes” results from
unequal pulls by the external eye muscles in
babies
•B. Ophthalmia neonatorum—conjunctivitis
resulting from mother having gonorrhea.
Baby’s eyelids are swollen and pus is
produced
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Developmental Aspects of the Special
Senses
•Eye problems
•C. Presbyopia—“old vision” results from
decreasing lens elasticity that accompanies
aging
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Developmental Aspects of the Special
Senses
•Ear problems
•A. Presbycusis—type of sensorineural
deafness
•B. Otosclerosis—ear ossicles fuse
© 2012 Pearson Education, Inc.