Download Vision B

Ch. 15
Special Senses: Vision
Slides mostly © Marieb & Hoehn 9th ed.
Other slides by WCR
Light And Optics: Wavelength And Color
• Light
– Packets of electromagnetic radiation (energy)
– Light waves have different wavelengths
• Visible light
– The (small) range of electromagnetic wavelengths
which our eyes can detect: 400-700 nm
– Different objects reflect different wavelengths, which
we perceive as different colors
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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
• Convexly-curved lens refract light to bring it to a
focus
• Image formed at focal point is upside-down and
left-right reversed
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Figure 15.11 Refraction.
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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 at boundaries along pathway
– Air to cornea/aqueous humor
– Aqueous humor to lens
– Lens to vitreous humor
• Most bending happens at air-cornea boundary
• Lens curvature is the “fine adjustment”
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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
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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.
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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
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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.
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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
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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
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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
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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
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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.
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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
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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
• Isomers: cis- (bent) and trans- (straight)
– Absorbing a photon causes bent-to-straight (cis –totrans) shape change
– Change from bent-to-straight initiates reactions 
electrical impulses along optic nerve
Rhodopsin = cis-retinal (bent retinal) + opsin
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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
– Light absorption causes retinal to change to
trans isomer
– Retinal and opsin separate (rhodopsin
breakdown)
• Pigment regeneration
– trans retinal converted to cis
– Cis-retinal rejoins opsin to form rhodopsin
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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
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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).
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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!
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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
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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.
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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
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6 EPSPs occur in ganglion
cell.
7 Action potentials
propagate along the
optic nerve.
Slide 8
Visual Pathway To The Brain
• Axons of retinal ganglion cells form optic nerve
• Half of the fibers (medial half) of each optic nerve cross
over at optic chiasm; optic tracts exit
• Most optic tract fibers go to lateral geniculate nucleus of
thalamus
• Fibers from thalamic (LGN) neurons form optic radiation
and project to primary visual cortex in occipital lobes
• Other optic tract fibers go to superior colliculi in midbrain
(initiating visual reflexes)
• A few ganglion cells contain melanopsin and project to
other brain areas
– Regulate pupil diameter, daily rhythms
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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.
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Corpus callosum
Photograph of human brain, with the right side
dissected to reveal internal structures.
Depth Perception
• Eyes see world from slightly different angles
• Depth perception (three-dimensional vision)
results from detection of the small differences
between R & L eye images
• Requires input from both eyes
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Visual Processing
• Retinal cells
– Color, brightness, edge detection (by amacrine and
horizontal cells)
• Lateral geniculate nuclei of thalamus
– Process for depth perception, cone input emphasized,
contrast sharpened
• Primary visual cortex (striate cortex)
– Neurons detect edges, object orientation, movement
– Provide form, color, motion inputs to visual
association areas (prestriate cortex)
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Cortical Processing
• Occipital lobe centers (prestriate cortex) continues
processing form, color, movement
• Complex visual processing extends to other regions
– "What" processing identifies objects in visual field
– "Where" processing assesses spatial location of
objects
– Output from both passes to frontal cortex
© 2013 Pearson Education, Inc.