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Chapter 10
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1
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Chapter 10
The Senses
2

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Introduction
A.
B.
C.
Sensory receptors detect changes in the
environment and stimulate neurons to
send nerve impulses to the brain.
A sensation is formed based on the
sensory input.
Sensory receptors fall into two
categories: Somatic senses (widely
distributed and structurally simple) and
Special senses (complex specialized
sensory organs)
3

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Receptors, Sensations and Perception
A.
B.
Each receptor is more sensitive to a specific
kind of environmental change but is less
sensitive to others.
Types of Receptors
1.
Five general types of receptors are
recognized.
a.
Receptors sensitive to changes
in chemical concentration are
called chemoreceptors.
b.
Pain receptors detect tissue
damage.
4

c.
d.
e.
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Thermoreceptors respond to
temperature differences.
Mechanoreceptors respond
to changes in pressure or
movement.
Photoreceptors in the eyes
respond to light energy.
5

C.
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Sensations and Perception
1.
Sensations are feelings that occur
when the brain interprets sensory
impulses.
2.
At the same time the sensation is
being formed, the brain uses
projection to send the sensation
back to its point of origin so the
person can pinpoint the area of
stimulation.
6

D.
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Sensory Adaptation
1.
During sensory adaptation, sensory
impulses are sent at decreasing
rates until receptors fail to send
impulses unless there is a change in
strength of the stimulus.
7
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General Senses
A.
B.
Receptors associated with the skin,
muscles, joints, and viscera make up the
somatic senses.
Touch and Pressure Senses
1.
Three types of receptors detect
touch and pressure.
2.
Free nerve endings of sensory
nerve fibers in the epithelial tissues
are associated with touch and
pressure.
8
Fig10.01
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Free nerve
endings
Section of
skin
Epithelial
cells
Epidermis
(a)
Sensory
nerve fiber
Epithelial
cells
Dermis
Tactile (Meissner’s)
corpuscle
(touch receptor)
Sensory nerve
fiber
(b)
© Ed Reschke
Lamellated (Pacinian)
corpuscle
(pressure receptor)
Connective tissue
cells
Sensory nerve
fiber
(c)
© Ed Reschke
9
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3.
Tactile (Meissner's) corpuscles are
flattened connective tissue sheaths
surrounding two or more nerve
fibers and are abundant in hairless
areas that are very sensitive to
touch, like the lips.
4.
Lamellated (Pacinian) corpuscles
are large structures of connective
tissue and cells that resemble an
onion. They function to detect deep
pressure.
10
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C.
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Temperature Senses
1.
Temperature receptors include two
groups of free nerve endings: warm
receptors and cold receptors which
both work best within a range of
temperatures.
a.
Both heat and cold receptors
adapt quickly.
b.
Temperatures near 45o C
stimulate pain receptors;
temperatures below 10o C
also stimulate pain receptors
and produce a freezing
sensation.
11

D.
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Sense of Pain
1.
Pain receptors consist of free nerve
endings that are stimulated when
tissues are damaged, and adapt
little, if at all.
2.
Many stimuli affect pain receptors
such as chemicals and oxygen
deprivation.
12
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3.
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Visceral pain receptors are the only
receptors in the viscera that
produce sensations.
a.
Referred pain occurs because
of the common nerve pathways
leading from skin and internal
organs.
13
Fig10.02
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Liver and
gallbladder
Lung and diaphragm
Liver and
gallbladder
Heart
Stomach
Pancreas
Small
intestine
Appendix
Ovary
(female)
Colon
Kidney
Ureter
Urinary bladder
14
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4.
CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pain Nerve Fibers
a.
Fibers conducting pain impulses
away from their source are
either acute pain fibers or
chronic pain fibers.
b.
Acute pain fibers are thin,
myelinated fibers that carry
impulses rapidly and cease
when the stimulus stops.
c.
Chronic pain fibers are thin,
unmyelinated fibers that conduct
impulses slowly and continue
sending impulses after the
stimulus stops.
15
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d.
Pain impulses are processed
in the gray matter of the dorsal
horn of the spinal cord.
e.
Pain impulses are conducted
to the thalamus, hypothalamus,
and cerebral cortex.
16

5.
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Regulation of Pain Impulses
a.
A person becomes aware of
pain when impulses reach the
thalamus, but the cerebral
cortex judges the intensity and
location of the pain.
17
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b.
Other areas of the brain
regulate the flow of pain
impulses from the spinal cord
and can trigger the release of
enkephalins and serotonin,
which inhibit the release of
pain impulses in the spinal
cord.
c.
Endorphins released in the
brain provide natural pain
control.
18
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Special Senses
A.
B.
The special senses are associated
with fairly large and complex structures
located in the head.
These include the senses of smell,
taste, hearing, equilibrium, and sight.
19
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Sense of Smell
A.
Olfactory Receptors
1.
Smell (Olfactory) receptors and taste
receptors are chemoreceptors.
2.
The senses of smell and taste operate
together to aid in food selection.
20
Fig10.04
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nerve fibers within
the olfactory bulb
Olfactory Olfactory
tract
bulb
Cribriform
plate
Olfactory area of
nasal cavity
Superior nasal
concha
Nasal cavity
Cilia
(a)
Olfactory
receptor cells
Columnar
epithelial cells
Cribriform
plate
(b)
21
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B.
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Olfactory Organs
1.
The olfactory organs contain the
olfactory receptors plus epithelial
supporting cells and are located in
the upper nasal cavity.
2.
The olfactory receptor cells are bipolar
neurons with hair-like cilia covering
the dendrites. The cilia project into
the nasal cavity.
3.
To be detected, chemicals that enter
the nasal cavity must first be dissolved
in the watery fluid surrounding the cilia.
22
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C.
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Olfactory Nerve Pathways
1.
When olfactory receptors are
stimulated, their fibers synapse with
neurons in the olfactory bulbs lying
on either side of the crista galli.
2.
Sensory impulses are first analyzed
in the olfactory bulbs, then travel
along olfactory tracts to the limbic
system, and lastly to the olfactory
cortex within the temporal lobes.
23
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D.
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Olfactory Stimulation
1.
Scientists are uncertain of how
olfactory reception operates but
believe that each odor stimulates a
set of specific protein receptors in
cell membranes.
2.
The brain interprets different
receptor combinations as an
olfactory code.
3.
Olfactory receptors adapt quickly
but selectively.
24
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Sense of Taste
A.
Taste buds are the organs of taste and
are located within papillae of the tongue
and are scattered throughout the mouth
and pharynx.
25
Fig10.05
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Papillae
Taste buds
Epithelium
of tongue
Taste cell
(a)
Taste hair
Supporting
cell
Taste
pore
(b)
Connective
tissue
Sensory
nerve fibers
26
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B.
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Taste Receptors
1.
Taste cells (gustatory cells) are
modified epithelial cells that function
as receptors.
2.
Taste cells contain the taste hairs
that are the portions sensitive to
taste. These hairs protrude from
openings called taste pores.
27
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3.
Chemicals must be dissolved in
water (saliva) in order to be tasted.
4.
The sense of taste is not well
understood but probably involves
specific membrane protein
receptors that bind with specific
chemicals in food.
5.
There are five types of taste cells:
sweet, sour, salty, bitter, and
umami.
(others that may be recognized are
alkaline and metallic)
28
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C.
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Taste Sensations
1.
Specific taste receptors are
concentrated in different areas
of the tongue.
a.
Sweet receptors are
plentiful near the tip of
the tongue.
b.
Sour receptors occur
along the lateral edges
of the tongue.
29
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c.
Salt receptors are abundant in
the tip and upper portion of
the tongue.
d.
Bitter receptors are at
the back of the tongue.
e.
Umami receptors responds to
certain amino acid derivatives
such as monosodium glutamate
(MSG).
30
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2.
Taste buds may be responsive to at
least two taste sensations but one is
likely to dominate.
3.
Taste receptors rapidly undergo
adaptation.
31
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D.
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Taste Nerve Pathways
1.
Taste impulses travel on the
facial, glossopharyngeal, and
vagus nerves to the medulla
oblongata and then to the
gustatory cortex of the
cerebrum.
32
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Sense of Hearing
A.
B.
The ear has external, middle, and inner
sections and provides the senses of
hearing and equilibrium.
Outer (External) Ear
1.
The external ear consists of the
auricle which collects the sound
which then travels down the
external acoustic meatus.
33
Fig10.06
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Auricle
Semicircular
canals
Incus
Stapes
Malleus
Cochlea
Vestibulocochlear
nerve
Oval window (under stapes)
Round window
Tympanic cavity
Eardrum
(tympanic
membrane)
External acoustic
meatus
Auditory tube
Nasopharynx
34
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C.
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Middle Ear
1.
The middle ear (tympanic cavity)
begins with the tympanic membrane
(eardrum), and is an air-filled space
(tympanic cavity) housing the
auditory ossicles.
a.
Three auditory ossicles are
the malleus, incus, and
stapes.
35
Fig10.07
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Incus
Malleus
Stapes
© The McGraw-Hill Companies, Inc./Jim Womack, photographer
36
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b.
The tympanic membrane
vibrates the malleus, which
vibrates the incus, then the
stapes.
c.
The stapes vibrates the fluid
inside the oval window of the
inner ear.
2. Auditory ossicles both transmit and
amplify sound waves.
37
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D.
E.
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Auditory Tube
1.
The auditory (eustachian) tube
connects the middle ear to the throat
to help maintain equal air pressure on
both sides of the eardrum.
Inner Ear
1.
The inner ear is made up of a
membranous labyrinth inside an
osseous labyrinth.
a.
Between the two labyrinths is a
fluid called perilymph.
b.
Endolymph is inside the
membranous labyrinth.
38
Fig10.08
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Osseous labyrinth
Perilymph
Membranous labyrinth
Endolymph
Osseous labyrinth
(contains perilymph)
Membranous labyrinth
(contains endolymph)
Semicircular
canals
Utricle
Vestibular nerve
Saccule
Cochlear nerve
Scala
chambers (cut)
containing
perilymph
Cochlear
duct (cut)
containing
endolymph
Ampullae
Oval
window
Round Maculae
window
Cochlea
39
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2.
The cochlea houses the organ of
hearing while the semicircular
canals function in equilibrium.
3.
Within the cochlea, the oval window
leads to the upper compartment,
called the scala vestibuli.
40
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4.
A lower compartment, the scala
tympani, leads to the round window.
5.
The cochlear duct lies between
these two compartments and is
separated from the scala vestibuli
by the vestibular membrane, and
from the scala tympani by the
basilar membrane.
41

6.
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The spiral organ (organ of Corti),
with its receptors called hair cells,
lies on the basilar membrane.
a.
Hair cells possess hairs that
extend into the endolymph of
the cochlear duct.
42

7.
8.
9.
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Above the hair cells lies the tectorial
membrane, which touches the tips
of the stereocilia.
Vibrations in the fluid of the inner
ear cause the hair cells to bend
resulting in an influx of calcium ions.
This causes the release of a
neurotransmitter from vesicles
which stimulate the ends of nearby
sensory neurons.
43
Fig10.09
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Scala vestibuli
(contains perilymph)
Vestibular membrane
Cochlear duct
(contains endolymph)
Spiral organ (organ of Corti)
Branch of
cochlear
nerve
Basilar membrane
Scala tympani
(contains perilymph)
(a)
Tectorial
membrane
(b)
Branch of
cochlear nerve
Hair cells
Nerve
fibers
Supporting
cells
Basilar
membrane
44

F.
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Auditory Nerve Pathways
1.
Nerve fibers carry impulses to
the auditory cortices of the
temporal lobes where they are
interpreted.
45
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Sense of Equilibrium
A.
The sense of equilibrium consists of
two parts: static and dynamic
equilibrium.
1.
The organs of static equilibrium
help to maintain the position of
the head when the head and
body are still.
2.
The organs of dynamic
equilibrium help to maintain
balance when the head and
body suddenly move and
rotate.
46

B.
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Static Equilibrium
1.
The organs of static equilibrium
are located within the bony
vestibule of the inner ear, inside
the utricle and saccule (expansions
of the membranous labyrinth).
2.
A macula, consisting of hair cells
and supporting cells, lies inside the
utricle and saccule.
47
Fig10.11
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Hairs of
hair cells bend
Gelatinous
material sags
Otoliths
Macula
of utricle
Hair cells
Supporting cells
Sensory nerve fiber
(a) Head upright
Gravitational
force
(b) Head bent forward
48

3.
4.
5.
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The hair cells contact gelatinous
material holding otoliths.
Gravity causes the gelatin and
otoliths to shift, bending hair cells
and generating a nervous impulse.
Impulses travel to the brain via
the vestibular branch of the
vestibulocochlear nerve, indicating
the position of the head.
49
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C.
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Dynamic Equilibrium
1.
The three semicircular canals
detect motion of the head, and
they aid in balancing the head
and body during sudden
movement.
2.
The organs of dynamic
equilibrium are called cristae
ampullaris, and are located in
the ampulla of each semicircular
canal of the inner ear. They are
at right angles to each other.
50
Fig10.12
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cupula
Crista
ampullaris
Hairs
Hair cell
Supporting cells
Sensory nerve fibers
51

3.
4.
5.
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Hair cells extend into a domeshaped gelatinous cupula.
Rapid turning of the head or
body generates impulses as
the cupula and hair cells bend.
Mechanoreceptors (called
propriopceptors) associated
with the joints, and the
changes detected by the eyes
also help maintain equilibrium.
52
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Sense of Sight
A.
B.
Accessory organs, namely the lacrimal
apparatus, eyelids, and extrinsic muscles,
aid the eye in its function.
Visual Accessory Organs
1.
The eyelid protects the eye
from foreign objects and is
made up of the thinnest skin
of the body lined with conjunctiva.
53
Fig10.14
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Tendon of levator
palpebrae superioris
Superior
rectus
Orbicularis
oculi
Eyelid
Eyelash
Cornea
Conjunctiva
Inferior
rectus
54

2.
3.
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The lacrimal apparatus produces
tears that lubricate and cleanse the
eye, by way of the lacrimal gland.
a.
Two small ducts drain tears
into the nasal cavity.
b.
Tears also contain an
antibacterial enzyme
(lysozyme).
The extrinsic muscles of the eye
attach to the sclera and move the
eye in all directions.
55
Fig10.16
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Medial
rectus
Superior
rectus
Superior
oblique
Lateral
rectus
(cut)
Inferior rectus
Inferior oblique
56

C.
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Structure of the Eye
1.
The eye is a fluid-filled hollow
sphere with three distinct layers.
57
Fig10.17
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Lateral rectus
Retina
Ciliary body
Suspensory
ligaments
Choroid coat
Sclera
Vitreous humor
Iris
Lens
Fovea centralis
Pupil
Cornea
Aqueous
humor
Anterior
cavity
Anterior
chamber
Posterior
chamber
Optic nerve
Optic disc
Posterior cavity
Medial rectus
58

2.
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Outer Layer
a.
The outer (fibrous) layer is the
transparent cornea at the
front of the eye, and the white
sclera of the anterior eye.
b.
The optic nerve and blood
vessels pierce the sclera at
the posterior of the eye.
59

3.
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Middle Layer
a.
The middle, vascular tunic
includes the choroid coat,
ciliary body, and iris.
b.
The choroid coat is vascular
and darkly pigmented and
performs two functions: to
nourish other tissues of the
eye and to keep the inside of
the eye dark.
60

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c.
The ciliary body forms a ring
around the front of the eye and
contains ciliary processes, ciliary
muscles and suspensory ligaments
that hold the lens in position and
change its shape (focus).
d.
The ability of the lens to adjust
shape to facilitate focusing is called
accommodation.
61

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e.
f.
The iris is a thin, smooth muscle
that adjusts the amount of light
entering the pupil, a hole in its
center.
i.
The iris has a circular set of
and a radial set of muscle
fibers.
The anterior chamber (between
the cornea and iris) and the
posterior chamber (between the
iris and vitreous body and
housing the lens) make up the
anterior cavity, which is filled with
aqueous humor.
62
Fig10.18
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ciliary processes
of ciliary body
Suspensory
ligaments
Lens
Retina
Choroid coat
Sclera
63
Fig10.19
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Ciliary muscle
fibers relaxed
Suspensory
ligaments taut
Lens thin
(a)
Ciliary muscle
fibers contracted
Suspensory
ligaments relaxed
Lens thick
(b)
64

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g.
The aqueous humor
circulates from one
chamber to the other
through the pupil.
65

4.
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Inner Layer
a.
The inner layer consists of the
retina, which contains
photoreceptors; the inner tunic
covers the back side of the eye
to the ciliary body.
b.
In the center of the retina is the
macula lutea with the fovea
centralis in its center, the point
of sharpest vision in the retina.
66

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c.
d.
Medial to the fovea centralis is
the optic disc, where nerve
fibers leave the eye and
where there is a blind spot.
The posterior cavity of the eye
is filled with vitreous humor
and with collagenous fibers
forming the vitreous body.
67
Fig10.23a
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Artery
Veins
Optic
disc
Macula
lutea
Fovea
centralis
(a)
68
Fig10.23b
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
69
(b)
©Mediscan/Corbis

D.
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Light Refraction
1.
Light waves must bend to be
focused, a phenomenon called
refraction.
2.
Both the cornea and lens bend
light waves that are focused on
the retina as due the two humors
to a lesser degree.
70

E.
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Visual Receptors
1.
Two kinds of modified neurons
comprise the visual receptors;
elongated rods and blunt-shaped
cones.
2.
Rods are more sensitive to light
and function in dim light; they
produce colorless vision.
71
Fig10.25ab
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Retinal
pigment
epithelium
Rods
Single sensory
nerve fiber
(a)
Many sensory
nerve fibers
(b)
Cones
72
Fig10.25c
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Rod
Cone
©Frank S.Werblin, Ph.D
73

3.
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Cones provide sharp images in
bright light and enable us to see in
color.
a.
To see something in detail, a
person moves the eyes so the
image falls on the fovea
centralis, which contains the
highest concentration of cones.
b.
The proportion of cones
decreases with distance from
the fovea centralis.
74

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F. Visual Pigments
1.
The light-sensitive pigment in rods
is rhodopsin (visual purple), which
breaks down into a protein, opsin,
and retinal (from vitamin A) in the
presence of light.
75

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a.
Decomposition of rhodopsin
activates an enzyme that
initiates changes in the rod
cell membrane, generating a
nerve impulse.
b.
Nerve impulses travel away
from the retina and are
interpreted as vision.
76

2.
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The light-sensitive pigments in
cones are also proteins; there are
three sets of cones, each containing
a different visual pigment.
77

CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a.
The wavelength of light
determines the color perceived
from it; each of the three
pigments is sensitive to different
wavelengths of light.
b.
The color perceived depends
upon which sets of cones the
light stimulates: if all three sets
are stimulated, the color is
white; if none are stimulated,
the color is black.
78

G.
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Visual Nerve Pathways
1.
The axons of ganglion cells leave
the eyes to form the optic nerves.
2.
Fibers from the medial half of the
retina cross over in the optic
chiasma.
3.
Impulses are transmitted to the
thalamus, nerve pathways called
optic radiations and then to the
visual cortex of the occipital lobe.
79
Fig10.26
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Eye
Optic
nerve
Fibers from
nasal (medial) half
of each retina
crossing over
Optic
chiasma
Optic tract
Lateral
geniculate
body of
thalamus
Optic
radiations
Visual cortex of
occipital lobe
80