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Anatomy and Physiology of Vision
© 2013 Pearson Education, Inc.
Special Senses
• Special sensory receptors
– Distinct, localized receptor cells in head
•
•
•
•
•
Vision
Taste
Smell
Hearing
Equilibrium
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The Eye and Vision
• 70% of body's sensory receptors in eye
• Visual processing by ~ half cerebral cortex
• Most of eye protected by cushion of fat
and bony orbit
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Accessory Structures of the Eye
• Protect the eye and aid eye function
– Eyebrows
– Eyelids (palpebrae)
– Conjunctiva
– Lacrimal apparatus
– Extrinsic eye muscles
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Figure 15.1a The eye and accessory structures.
Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Iris
Eyelid
Pupil
Lacrimal Medial
Sclera
(covered by caruncle commissure
conjunctiva)
Surface anatomy of the right eye
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Eyebrows
• Overlie supraorbital margins
• Function
– Shade eye from sunlight
– Prevent perspiration from reaching eye
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Eyelids
•
•
•
•
Protect eye anteriorly
Separated at palpebral fissure
Meet at medial and lateral commissures
Lacrimal caruncle
– At medial commissure
– Contains oil and sweat glands
• Tarsal plates—supporting connective
tissue
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Eyelid Muscles
• Levator palpebrae superioris
– Gives upper eyelid mobility
• Blink reflexively every 3-7 seconds
– Protection
– Spread secretions to moisten eye
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Eyelids
• Eyelashes
– Nerve endings of follicles initiate reflex
blinking
• Lubricating glands associated with eyelids
– Tarsal (Meibomian) glands
• Modified sebaceous glands
• Oily secretion lubricates lid and eye
– Ciliary glands between hair follicles
• Modified sweat glands
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Figure 15.1b The eye and accessory structures.
Levator palpebrae
superioris muscle
Orbicularis
oculi muscle
Eyebrow
Tarsal plate
Palpebral
conjunctiva
Tarsal glands
Cornea
Palpebral
fissure
Eyelashes
Bulbar conjunctiva
Conjunctival sac
Orbicularis
oculi muscle
Lateral view; some structures shown in sagittal section
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Conjunctiva
• Transparent mucous membrane
– Produces a lubricating mucous secretion
• Palpebral conjunctiva lines eyelids
• Bulbar conjunctiva covers white of eyes
• Conjunctival sac between palpebral and
bulbar conjunctiva
– Where contact lens rests
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Lacrimal Apparatus
• Lacrimal gland and ducts that drain into nasal
cavity
• Lacrimal gland in orbit above lateral end of eye
• Lacrimal secretion (tears)
– Dilute saline solution containing mucus, antibodies,
and lysozyme
– Blinking spreads tears toward medial commissure
– Tears enter paired lacrimal canaliculi via lacrimal
puncta
– Then drain into lacrimal sac and nasolacrimal duct
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Figure 15.2 The lacrimal apparatus.
Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
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Extrinsic Eye Muscles
• Six straplike extrinsic eye muscles
– Originate from bony orbit; insert on eyeball
– Enable eye to follow moving objects; maintain shape
of eyeball; hold in orbit
• Four rectus muscles originate from common
tendinous ring; names indicate movements
– Superior, inferior, lateral, medial rectus muscles
• Two oblique muscles move eye in vertical plane
and rotate eyeball
– Superior and inferior oblique muscles
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Figure 15.3a Extrinsic eye muscles.
Superior oblique muscle
Superior oblique tendon
Superior rectus muscle
Lateral rectus muscle
Inferior
rectus
muscle
Inferior
oblique
muscle
Lateral view of the right eye
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Figure 15.3b Extrinsic eye muscles.
Trochlea
Superior oblique muscle
Superior oblique tendon
Superior rectus muscle
Axis of
rotation
of eye
Inferior
rectus muscle
Medial
rectus muscle
Lateral
rectus muscle
Common
tendinous ring
Superior view of the right eye
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Figure 15.3c Extrinsic eye muscles.
Muscle
Action
Controlling cranial nerve
Lateral rectus
Moves eye laterally
VI (abducens)
Medial rectus
Superior rectus
Inferior rectus
Moves eye medially
III (oculomotor)
Elevates eye and turns it medially
III (oculomotor)
Depresses eye and turns it medially
III (oculomotor)
Elevates eye and turns it laterally
III (oculomotor)
Depresses eye and turns it laterally
IV (trochlear)
Inferior oblique
Superior oblique
Summary of muscle actions and innervating cranial nerves
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Structure of the Eyeball
• Wall of eyeball contains three layers
– Fibrous
– Vascular
– Inner
• Internal cavity filled with fluids called
humors
• Lens separates internal cavity into
anterior and posterior segments
(cavities)
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Figure 15.4a Internal structure of the eye (sagittal section).
Ora serrata
Ciliary body
Sclera
Ciliary zonule
(suspensory
ligament)
Choroid
Cornea
Iris
Pupil
Anterior
pole
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous sinus
Posterior segment
(contains vitreous humor)
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
© 2013 Pearson Education, Inc.
Figure 15.4b Internal structure of the eye (sagittal section).
Ciliary body
Ciliary
processes
Vitreous humor
in posterior
segment
Iris
Margin
of pupil
Anterior
segment
Lens
Cornea
Ciliary zonule
(suspensory
ligament)
Retina
Choroid
Sclera
Fovea centralis
Optic disc
Optic nerve
Photograph of the human eye.
© 2013 Pearson Education, Inc.
Fibrous Layer
• Outermost layer; dense avascular
connective tissue
• Two regions: sclera and cornea
1. Sclera
• Opaque posterior region
• Protects, shapes eyeball; anchors extrinsic eye
muscles
• Continuous with dura mater of brain posteriorly
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Fibrous Layer
2. Cornea
• Transparent anterior 1/6 of fibrous layer
• Bends light as it enters eye
• Sodium pumps of corneal endothelium on inner
face help maintain clarity of cornea
• Numerous pain receptors contribute to blinking and
tearing reflexes
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Vascular Layer (Uvea)
• Middle pigmented layer
• Three regions: choroid, ciliary body, and
iris
1. Choroid region
• Posterior portion of uvea
• Supplies blood to all layers of eyeball
• Brown pigment absorbs light to prevent light
scattering and visual confusion
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Vascular Layer
2. Ciliary body
• Ring of tissue surrounding lens
• Smooth muscle bundles (ciliary muscles) control
lens shape
• Capillaries of ciliary processes secrete fluid
• Ciliary zonule (suspensory ligament) holds lens in
position
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Vascular Layer
3. Iris
• Colored part of eye
• Pupil—central opening that regulates amount of
light entering eye
– Close vision and bright light—sphincter pupillae
(circular muscles) contract; pupils constrict
– Distant vision and dim light—dilator pupillae
(radial muscles) contract; pupils dilate –
sympathetic fibers
– Changes in emotional state—pupils dilate when
subject matter is appealing or requires
problem-solving skills
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Figure 15.5 Pupil constriction and dilation, anterior view.
Sympathetic +
Parasympathetic +
Sphincter pupillae
muscle contracts:
Pupil size decreases.
© 2013 Pearson Education, Inc.
Iris (two muscles)
• Sphincter pupillae
• Dilator pupillae
Dilator pupillae
muscle contracts:
Pupil size increases.
Inner Layer: Retina
• Originates as outpocketing of brain
• Delicate two-layered membrane
– Outer Pigmented layer
•
•
•
•
Single-cell-thick lining
Absorbs light and prevents its scattering
Phagocytize photoreceptor cell fragments
Stores vitamin A
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Inner Layer: Retina
– Inner Neural layer
• Transparent
• Composed of three main types of neurons
– Photoreceptors, bipolar cells, ganglion cells
• Signals spread from photoreceptors to bipolar cells
to ganglion cells
• Ganglion cell axons exit eye as optic nerve
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The Retina
• Optic disc (blind spot)
– Site where optic nerve leaves eye
– Lacks photoreceptors
• Quarter-billion photoreceptors of two types
– Rods
– Cones
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Figure 15.6a Microscopic anatomy of the retina.
Neural layer of retina
Pigmented
layer of
retina
Choroid
Pathway of
light
Sclera
Optic disc
Central artery
and vein of retina
Optic
nerve
Posterior aspect of the eyeball
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Figure 15.6b Microscopic anatomy of the retina.
Ganglion
cells
Axons
of
ganglion
cells
Bipolar
cells
Photoreceptors
• Rod
• Cone
Amacrine cell
Horizontal cell
Pathway of signal output
Pathway of light
Pigmented
layer of retina
Cells of the neural layer of the retina
© 2013 Pearson Education, Inc.
Figure 15.6c Microscopic anatomy of the retina.
Nuclei of
ganglion
cells
Outer segments
of rods and cones
Nuclei of Nuclei of
bipolar rods and
cells
cones
Photomicrograph of retina
Axons of
ganglion cells
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Choroid
Pigmented
layer of retina
Photoreceptors
• Rods
– Dim light, peripheral vision receptors
– More numerous, more sensitive to light than
cones
– No color vision or sharp images
– Numbers greatest at periphery
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Photoreceptors
• Cones
– Vision receptors for bright light
– High-resolution color vision
– Macula lutea exactly at posterior pole
• Mostly cones
• Fovea centralis
– Tiny pit in center of macula with all cones; best vision
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Blood Supply to the Retina
• Two sources of blood supply
– Choroid supplies outer third (photoreceptors)
– Central artery and vein of retina supply inner
two-thirds
• Enter/exit eye in center of optic nerve
• Vessels visible in living person
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Figure 15.7 Part of the posterior wall (fundus) of the right eye as seen with an ophthalmoscope.
Central
artery
and vein
emerging
from the
optic disc
Optic disc
Macula
lutea
Retina
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Internal Chambers and Fluids
• The lens and ciliary zonule separate eye
into two segments
– Anterior and posterior segments
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Internal Chambers and Fluids
• Posterior segment contains vitreous humor that
– Transmits light
– Supports posterior surface of lens
– Holds neural layer of retina firmly against pigmented
layer
– Contributes to intraocular pressure
– Forms in embryo; lasts lifetime
• Anterior segment composed of two chambers
– Anterior chamber—between cornea and iris
– Posterior chamber—between iris and lens
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Internal Chambers and Fluids
• Anterior segment contains aqueous humor
– Plasma like fluid continuously formed by capillaries of
ciliary processes
– Drains via scleral venous sinus (canal of Schlemm) at
sclera-cornea junction
– Supplies nutrients and oxygen mainly to lens and
cornea but also to retina, and removes wastes
• Glaucoma: blocked drainage of aqueous humor
increases pressure and causes compression of
retina and optic nerve  blindness
© 2013 Pearson Education, Inc.
Figure 15.4a Internal structure of the eye (sagittal section).
Ora serrata
Ciliary body
Sclera
Ciliary zonule
(suspensory
ligament)
Choroid
Cornea
Iris
Pupil
Anterior
pole
Anterior
segment
(contains
aqueous humor)
Lens
Scleral venous sinus
Posterior segment
(contains vitreous humor)
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Central artery and
vein of the retina
Optic disc
(blind spot)
Diagrammatic view. The vitreous humor is illustrated only in the bottom part of the eyeball.
© 2013 Pearson Education, Inc.
Figure 15.8 Circulation of aqueous humor.
Cornea
Lens
Posterior
segment
(contains
vitreous
humor)
Iris
Lens epithelium
Lens
Cornea
2
Corneal epithelium
Corneal endothelium
Aqueous humor
1 Aqueous humor
forms by filtration
from the capillaries
in the ciliary
processes.
2 Aqueous humor
flows from the
posterior chamber
through the pupil into
the anterior chamber.
Some also flows
through the vitreous
humor (not shown).
3 Aqueous humor is
reabsorbed into the
venous blood by the
scleral venous sinus.
© 2013 Pearson Education, Inc.
Anterior
segment
(contains
aqueous
humor)
Anterior chamber
Ciliary zonule
(suspensory
ligament)
Posterior chamber
Scleral venous
sinus
Corneoscleral
junction
3
1
Ciliary
processes
Ciliary
muscle
Bulbar
conjunctiva
Sclera
Ciliary
body
Lens
• Biconvex, transparent, flexible, and avascular
• Changes shape to precisely focus light on retina
• Two regions
– Lens epithelium anteriorly; Lens fibers form bulk of
lens
– Lens fibers filled with transparent protein crystallin
– Lens becomes more dense, convex, less elastic with
age
• cataracts (clouding of lens) consequence of aging, diabetes
mellitus, heavy smoking, frequent exposure to intense
sunlight
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Cataracts
• Clouding of lens
– Consequence of aging, diabetes mellitus,
heavy smoking, frequent exposure to intense
sunlight
– Some congenital
– Crystallin proteins clump
– Vitamin C increases cataract formation
– Lens can be replaced surgically with artificial
lens
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Figure 15.9 Photograph of a cataract.
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Light And Optics: Wavelength And Color
• Eyes respond to visible light
– Small portion of electromagnetic spectrum
– Wavelengths of 400-700 nm
• Light
– Packets of energy (photons or quanta) that
travel in wavelike fashion at high speeds
– Color of light objects reflect determines color
eye perceives
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Figure 15.10 The electromagnetic spectrum and photoreceptor sensitivities.
10–5
nm
10–3
Gamma
rays
103
nm 1 nm
X rays
UV
nm
106
Infrared
(109 nm =)
nm
1m
103 m
Micro- Radio waves
waves
Light absorption (percent of maximum)
Visible light
Blue
cones
(420 nm)
100
50
0
400
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Green Red
Rods
cones cones
(500 nm) (530 nm) (560 nm)
450
500
550
600
Wavelength (nm)
650
700
Light And Optics: Refraction And Lenses
• Refraction
– Bending of light rays
• Due to change in speed when light passes from
one transparent medium to another
• Occurs when light meets surface of different
medium at an oblique angle
– Curved lens can refract light
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Figure 15.11 Refraction.
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Refraction and Lenses
• Light passing through convex lens (as in
eye) is bent so that rays converge at focal
point
– Image formed at focal point is upside-down
and reversed right to left
• Concave lenses diverge light
– Prevent light from focusing
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Figure 15.12 Light is focused by a convex lens.
Point sources
Focal points
Focusing of two points of light.
The image is inverted—upside down and reversed.
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Focusing Light on The Retina
• Pathway of light entering eye: cornea, aqueous
humor, lens, vitreous humor, entire neural layer
of retina, photoreceptors
• Light refracted three times along pathway
– Entering cornea
– Entering lens
– Leaving lens
• Majority of refractory power in cornea
• Change in lens curvature allows for fine focusing
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Focusing For Distant Vision
• Eyes best adapted for distant vision
• Far point of vision
– Distance beyond which no change in lens
shape needed for focusing
• 20 feet for emmetropic (normal) eye
• Cornea and lens focus light precisely on retina
• Ciliary muscles relaxed
• Lens stretched flat by tension in ciliary
zonule
© 2013 Pearson Education, Inc.
Figure 15.13a Focusing for distant and close vision.
Nearly parallel rays
from distant object
Sympathetic activation
Lens
Ciliary zonule
Ciliary muscle
Inverted
image
Lens flattens for distant vision. Sympathetic input
relaxes the ciliary muscle, tightening the ciliary zonule,
and flattening the lens.
© 2013 Pearson Education, Inc.
Focusing For Close Vision
• Light from close objects (<6 m) diverges
as approaches eye
– Requires eye to make active adjustments
using three simultaneous processes
• Accommodation of lenses
• Constriction of pupils
• Convergence of eyeballs
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Focusing For Close Vision
• Accommodation
– Changing lens shape to increase refraction
– Near point of vision
• Closest point on which the eye can focus
– Presbyopia—loss of accommodation over age 50
• Constriction
– Accommodation pupillary reflex constricts pupils to
prevent most divergent light rays from entering eye
• Convergence
– Medial rotation of eyeballs toward object being
viewed
© 2013 Pearson Education, Inc.
Figure 15.13b Focusing for distant and close vision.
Parasympathetic activation
Divergent rays
Inverted
from close object
image
Lens bulges for close vision. Parasympathetic input
contracts the ciliary muscle, loosening the ciliary zonule,
allowing the lens to bulge.
© 2013 Pearson Education, Inc.
Figure 15.13c Focusing for distant and close vision.
View
Ciliary muscle
Lens
Ciliary zonule
(suspensory
ligament)
The ciliary muscle and ciliary zonule are arranged
sphincterlike around the lens. As a result, contraction
loosens the ciliary zonule fibers and relaxation tightens
them.
© 2013 Pearson Education, Inc.
Problems Of Refraction
• Myopia (nearsightedness)
– Focal point in front of retina, e.g., eyeball too long
– Corrected with a concave lens
• Hyperopia (farsightedness)
– Focal point behind retina, e.g., eyeball too short
– Corrected with a convex lens
• Astigmatism
– Unequal curvatures in different parts of cornea or lens
– Corrected with cylindrically ground lenses or laser
procedures
© 2013 Pearson Education, Inc.
Figure 15.14 Problems of refraction. (1 of 3)
Emmetropic eye (normal)
Focal
plane
Focal point is
on retina.
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Figure 15.14 Problems of refraction. (2 of 3)
Myopic eye (nearsighted)
Eyeball
too long
Uncorrected
Focal point is in
front of retina.
Corrected
© 2013 Pearson Education, Inc.
Concave lens moves focal
point further back.
Figure 15.14 Problems of refraction. (3 of 3)
Hyperopic eye (farsighted)
Eyeball
too short
Uncorrected
Focal point is
behind retina.
Corrected
© 2013 Pearson Education, Inc.
Convex lens moves focal
point forward.
Functional Anatomy Of Photoreceptors
• Rods and cones
– Modified neurons
– Receptive regions called outer segments
• Contain visual pigments (photopigments)
– Molecules change shape as absorb light
– Inner segment of each joins cell body
© 2013 Pearson Education, Inc.
Figure 15.15a Photoreceptors of the retina.
Process of
bipolar cell
Synaptic terminals
Rod cell body
Inner fibers
Rod cell body
Nuclei
Cone cell body
Mitochondria
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
© 2013 Pearson Education, Inc.
Pigmented layer
Inner
Outer segment segment
Outer fiber
Melanin
granules
Connecting cilia
Apical microvillus
Discs containing
visual pigments
Discs being
phagocytized
Pigment cell nucleus
Basal lamina (border
with choroid)
Photoreceptor Cells
•
•
•
•
Vulnerable to damage
Degenerate if retina detached
Destroyed by intense light
Outer segment renewed every 24 hours
– Tips fragment off and are phagocytized
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Rods
• Functional characteristics
– Very sensitive to light
– Best suited for night vision and peripheral
vision
– Contain single pigment
• Perceived input in gray tones only
– Pathways converge, causing fuzzy, indistinct
images
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Cones
• Functional characteristics
– Need bright light for activation (have low
sensitivity)
– React more quickly
– Have one of three pigments for colored view
– Nonconverging pathways result in detailed,
high-resolution vision
– Color blindness–lack of one or more cone
pigments
© 2013 Pearson Education, Inc.
Table 15.1 Comparison of Rods and Cones
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Chemistry Of Visual Pigments
• Retinal
– Light-absorbing molecule that combines with
one of four proteins (opsins) to form visual
pigments
– Synthesized from vitamin A
– Retinal isomers: 11-cis-retinal (bent form)
and all-trans-retinal (straight form)
• Bent form  straight form when pigment absorbs
light
• Conversion of bent to straight initiates reactions 
electrical impulses along optic nerve
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Phototransduction: Capturing Light
• Deep purple pigment of rods–rhodopsin
– 11-cis-retinal + opsin  rhodopsin
– Three steps of rhodopsin formation and
breakdown
• Pigment synthesis
• Pigment bleaching
• Pigment regeneration
© 2013 Pearson Education, Inc.
Figure 15.15b Photoreceptors of the retina.
Rod discs
Visual
pigment
consists of
• Retinal
• Opsin
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Rhodopsin, the visual pigment in rods,
is embedded in the membrane that forms
discs in the outer segment.
Phototransduction: Capturing Light
• Pigment synthesis
– Rhodopsin forms and accumulates in dark
• Pigment bleaching
– When rhodopsin absorbs light, retinal
changes to all-trans isomer
– Retinal and opsin separate (rhodopsin
breakdown)
• Pigment regeneration
– All-trans retinal converted to 11-cis isomer
– Rhodopsin regenerated in outer segments
© 2013 Pearson Education, Inc.
Figure 15.16 The formation and breakdown of rhodopsin.
11-cis-retinal
2H+
1 Pigment synthesis:
Oxidation
11-cis-retinal, derived
from vitamin A, is
Vitamin A
11-cis-retinal Rhodopsin
combined with opsin
to form rhodopsin.
Reduction
2H+
3 Pigment regeneration:
Enzymes slowly convert
all-trans-retinal to its 11cis form in cells of the
pigmented layer; requires
ATP.
Dark
Light
2 Pigment bleaching:
Light absorption by
rhodopsin triggers a
rapid series of steps in
which retinal changes
shape (11-cis to alltrans) and eventually
releases from opsin.
Opsin and
All-transretinal
O
All-trans-retinal
© 2013 Pearson Education, Inc.
Figure 15.17 Events of phototransduction.
Slide 1
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
cGMP-gated
cation
channel
open in
dark
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
© 2013 Pearson Education, Inc.
3 Transducin
activates
phosphodiesteras
e (PDE).
4 PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
cGMP-gated
cation
channel
closed in
light
5 As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization.
Figure 15.17 Events of phototransduction.
Slide 2
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Visual
pigment
All-trans-retinal
Light
11-cis-retinal
Transducin
(a G protein)
© 2013 Pearson Education, Inc.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
Figure 15.17 Events of phototransduction.
Slide 3
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Visual
pigment
All-trans-retinal
Light
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
© 2013 Pearson Education, Inc.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
Figure 15.17 Events of phototransduction.
Slide 4
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
© 2013 Pearson Education, Inc.
3 Transducin
activates
phosphodiesteras
e (PDE).
Figure 15.17 Events of phototransduction.
Slide 5
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
© 2013 Pearson Education, Inc.
3 Transducin
activates
phosphodiesteras
e (PDE).
4 PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
Figure 15.17 Events of phototransduction.
Slide 6
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race.
2nd
Light Receptor G protein Enzyme
messenger
(1st
messenger)
1 Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
cGMP-gated
cation
channel
open in
dark
11-cis-retinal
Transducin
(a G protein)
2 Visual pigment
activates
transducin
(G protein).
© 2013 Pearson Education, Inc.
3 Transducin
activates
phosphodiesteras
e (PDE).
4 PDE converts
cGMP into GMP,
causing cGMP
levels to fall.
cGMP-gated
cation
channel
closed in
light
5 As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization.
Phototransduction In Cones
• Similar as process in rods
• Cones far less sensitive to light
– Takes higher-intensity light to activate cones
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Light Transduction Reactions
• Light-activated rhodopsin activates G
protein transducin
• Transducin activates PDE, which breaks
down cyclic GMP (cGMP)
• In dark, cGMP holds channels of outer
segment open  Na+ and Ca2+ depolarize
cell
• In light cGMP breaks down, channels
close, cell hyperpolarizes
– Hyperpolarization is signal!
© 2013 Pearson Education, Inc.
Information Processing In The Retina
• Photoreceptors and bipolar cells only
generate graded potentials (EPSPs and
IPSPs)
• When light hyperpolarizes photoreceptor
cells
– Stop releasing inhibitory neurotransmitter
glutamate
– Bipolar cells (no longer inhibited) depolarize,
release neurotransmitter onto ganglion cells
– Ganglion cells generate APs transmitted in
optic nerve to brain
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 1
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
5 Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell
6 No EPSPs occur in
ganglion cell.
7 No action potentials occur
along the optic nerve.
© 2013 Pearson Education, Inc.
Ganglion
cell
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 2
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
Photoreceptor
cell (rod)
−40 mV
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 3
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
Ca2+
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 4
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 5
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 6
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
5 Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 7
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
5 Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell
6 No EPSPs occur in
ganglion cell.
Ganglion
cell
© 2013 Pearson Education, Inc.
Figure 15.18 Signal transmission in the retina (1 of 2).
Slide 8
In the dark
1 cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes.
Na+
Ca2+
2 Voltage-gated Ca2+
channels open in synaptic
terminals.
Photoreceptor
cell (rod)
−40 mV
3 Neurotransmitter is
released continuously.
Ca2+
4 Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results.
5 Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell
6 No EPSPs occur in
ganglion cell.
7 No action potentials occur
along the optic nerve.
© 2013 Pearson Education, Inc.
Ganglion
cell
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Light
Light
Photoreceptor
cell (rod)
−70 mV
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
Bipolar
Cell
Ca2+
Ganglion
cell
© 2013 Pearson Education, Inc.
6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
Slide 1
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
Light
Light
Photoreceptor
cell (rod)
−70 mV
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Slide 2
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
Light
Light
Photoreceptor
cell (rod)
−70 mV
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
Slide 3
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
Light
Light
Photoreceptor
cell (rod)
−70 mV
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Slide 4
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
Light
Light
Photoreceptor
cell (rod)
−70 mV
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
Bipolar
Cell
Ganglion
cell
© 2013 Pearson Education, Inc.
Slide 5
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Light
Light
Photoreceptor
cell (rod)
−70 mV
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
Bipolar
Cell
Ca2+
Ganglion
cell
© 2013 Pearson Education, Inc.
Slide 6
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Light
Light
Photoreceptor
cell (rod)
−70 mV
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
Bipolar
Cell
Ca2+
Ganglion
cell
© 2013 Pearson Education, Inc.
6 EPSPs occur in ganglion
cell.
Slide 7
Figure 15.18 Signal transmission in the retina. (2 of 2).
Below, we look at a tiny column of retina.
The outer segment of the rod, closest to the
back of the eye and farthest from the
incoming light, is at the top.
In the light
1 cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes.
Light
Light
Photoreceptor
cell (rod)
−70 mV
2 Voltage-gated Ca2+
channels close in synaptic
terminals.
3 No neurotransmitter
is released.
4 Lack of IPSPs in bipolar
cell results in depolarization.
5 Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released.
Bipolar
Cell
Ca2+
Ganglion
cell
© 2013 Pearson Education, Inc.
6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
Slide 8
Light Adaptation
• Move from darkness into bright light
– Both rods and cones strongly stimulated
• Pupils constrict
– Large amounts of pigments broken down
instantaneously, producing glare
– Visual acuity improves over 5–10 minutes as:
• Rod system turns off
• Retinal sensitivity decreases
• Cones and neurons rapidly adapt
© 2013 Pearson Education, Inc.
Dark Adaptation
• Move from bright light into darkness
– Cones stop functioning in low-intensity light
– Rod pigments bleached; system turned off
– Rhodopsin accumulates in dark
– Transducin returns to outer segments
– Retinal sensitivity increases within 20–30
minutes
– Pupils dilate
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Night Blindness
• Nyctalopia
• Rod degeneration
– Commonly caused by vitamin A deficiency
– If administered early vitamin A supplements
restore function
– Also caused by retinitis pigmentosa
• Degenerative retinal diseases that destroy rods
© 2013 Pearson Education, Inc.
Visual Pathway To The Brain
• Axons of retinal ganglion cells form optic
nerve
• Medial fibers of optic nerve decussate at
optic chiasma
• Most fibers of optic tracts continue to
lateral geniculate body of thalamus
• Fibers from thalamic neurons form optic
radiation and project to primary visual
cortex in occipital lobes
© 2013 Pearson Education, Inc.
Visual Pathway
• Fibers from thalamic neurons form optic
radiation
• Optic radiation fibers connect to primary
visual cortex in occipital lobes
• Other optic tract fibers send branches to
midbrain, ending in superior colliculi
(initiating visual reflexes)
© 2013 Pearson Education, Inc.
Visual Pathway
• A small subset of ganglion cells in retina
contain melanopsin (circadian pigment),
which projects to:
– Pretectal nuclei (involved with pupillary
reflexes)
– Suprachiasmatic nucleus of hypothalamus,
timer for daily biorhythms
© 2013 Pearson Education, Inc.
Figure 15.19 Visual pathway to the brain and visual fields, inferior view.
Both eyes
Fixation point
Right eye
Suprachiasmatic
nucleus
Pretectal
nucleus
Lateral
geniculate
nucleus of
thalamus
Superior
colliculus
Left eye
Optic nerve
Optic chiasma
Optic tract
Lateral
geniculate
nucleus
Superior
colliculus
(sectioned)
Uncrossed
(ipsilateral) fiber
Crossed
(contralateral) fiber
Optic
radiation
Occipital
lobe
(primary visual
cortex)
The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field
do not cross at the optic chiasma.
© 2013 Pearson Education, Inc.
Corpus callosum
Photograph of human brain, with the right side
dissected to reveal internal structures.
Depth Perception
• Both eyes view same image from slightly
different angles
• Depth perception (three-dimensional
vision) results from cortical fusion of
slightly different images
• Requires input from both eyes
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Visual Processing
• Retinal cells split input into channels
– Color, brightness, angle, direction, speed of
movement of edges (sudden changes of
brightness or color)
• Lateral inhibition decodes "edge" information
– Job of amacrine and horizontal cells
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Visual Processing
• Lateral geniculate nuclei of thalamus
– Process for depth perception, cone input
emphasized, contrast sharpened
• Primary visual cortex (striate cortex)
– Neurons respond to dark and bright edges,
and object orientation
– Provide form, color, motion inputs to visual
association areas (prestriate cortices)
© 2013 Pearson Education, Inc.
Cortical Processing
• Occipital lobe centers (anterior prestriate
cortices) continue processing of form, color, and
movement
• Complex visual processing extends to other
regions
– "What" processing identifies objects in visual field
• Ventral temporal lobe
– "Where" processing assesses spatial location of
objects
• Parietal cortex to postcentral gyrus
– Output from both passes to frontal cortex
• Directs movements
© 2013 Pearson Education, Inc.