1. Special Senses: Vision Slides mostly © Marieb & Hoehn 9th ed.
Ch. 15
Special Senses: Vision
Slides mostly © Marieb & Hoehn 9th ed.
Other slides by WCR

2. Light And Optics: Wavelength And Color
Packets of electromagnetic radiation (energy)
Light waves have different wavelengths
Visible light
The (small) range of electromagnetic wavelengths which our eyes can detect: nm
Different objects reflect different wavelengths, which we perceive as different colors
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3. Light absorption (percent of maximum)
Figure The electromagnetic spectrum and photoreceptor sensitivities.
(109 nm =)
10–5 nm
10–3 nm
1 nm
103 nm
106 nm
1 m
103 m
Gamma
rays
X rays UV
Infrared
Micro-
waves
Radio waves
Visible light
Blue
cones
(420 nm)
Green
cones
(530 nm)
Red
cones
(560 nm)
Rods
(500 nm)
100
Light absorption (percent of maximum) 50
400
450
500
550
600
650
700
Wavelength (nm)
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4. Light And Optics: Refraction And Lenses
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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5. Figure Refraction.
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6. Point sources Focal points
Figure 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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7. 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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8. 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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9. Sympathetic activation
Figure 15.13a Focusing for distant and close vision.
Sympathetic activation
Nearly parallel rays
from distant object
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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10. 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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11. 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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12. Parasympathetic activation
Figure 15.13b Focusing for distant and close vision.
Parasympathetic activation
Divergent rays
from close object
Inverted
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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13. 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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14. Emmetropic eye (normal)
Figure Problems of refraction. (1 of 3)
Emmetropic eye (normal)
Focal
plane
Focal point is
on retina.
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15. Myopic eye (nearsighted)
Figure Problems of refraction. (2 of 3)
Myopic eye (nearsighted)
Eyeball
too long
Uncorrected
Focal point is in
front of retina.
Concave lens moves focal
point further back.
Corrected
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16. Hyperopic eye (farsighted)
Figure Problems of refraction. (3 of 3)
Hyperopic eye (farsighted)
Eyeball
too short
Uncorrected
Focal point is
behind retina.
Convex lens moves focal
point forward.
Corrected
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17. 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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18. Process of bipolar cell
Figure 15.15a Photoreceptors of the retina.
Process of bipolar cell
Light
Light
Light
Synaptic terminals
Inner fibers
Rod cell body
Rod cell body
Nuclei
Cone cell body
Mitochondria
Outer fiber
Connecting cilia
segment
Inner
Apical microvillus
Outer segment
Discs containing
visual pigments
Pigmented layer
Discs being phagocytized
The outer segments
of rods and cones
are embedded in the
pigmented layer of
the retina.
Melanin granules
Pigment cell nucleus
Basal lamina (border
with choroid)
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19. Degenerate if retina detached Destroyed by intense light
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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20. Functional characteristics
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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21. Functional characteristics
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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22. Table 15.1 Comparison of Rods and Cones
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23. 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 –to-trans) shape change
Change from bent-to-straight initiates reactions  electrical impulses along optic nerve
Rhodopsin = cis-retinal (bent retinal) + opsin
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24. Rhodopsin, the visual pigment in rods,
Figure 15.15b Photoreceptors of the retina.
Rod discs
Visual
pigment
consists of
• Retinal
• Opsin
Rhodopsin, the visual pigment in rods,
is embedded in the membrane that forms
discs in the outer segment.
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25. 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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26. Pigment regeneration:
Figure The formation and breakdown of rhodopsin.
11-cis-retinal
2H+ 1
Pigment synthesis:
Oxidation
11-cis-retinal, derived
from vitamin A, is
combined with opsin
to form rhodopsin.
Vitamin A
11-cis-retinal
Rhodopsin
Reduction 2
Pigment bleaching:
Light absorption by
rhodopsin triggers a
rapid series of steps in
which retinal changes
shape (11-cis to all-trans) and eventually releases from opsin.
Dark
2H+
Light 3
Pigment regeneration:
Enzymes slowly convert
all-trans-retinal to its 11-cis form in cells of the
pigmented layer; requires
ATP.
Opsin and
All-trans-
retinal O
All-trans-retinal
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27. Phosphodiesterase (PDE)
Figure Events of phototransduction.
Slide 1
Recall from Chapter 3 that
G protein signaling mechanisms
are like a molecular relay race. 1
Light
(1st
messenger)
Receptor
G protein
Enzyme
2nd
messenger
Retinal absorbs light
and changes shape.
Visual pigment activates.
Phosphodiesterase (PDE)
Visual
pigment
All-trans-retinal
Light
cGMP-gated
cation channel
open in dark
cGMP-gated
cation channel
closed in light
11-cis-retinal
Transducin
(a G protein)
Visual pigment
activates
transducin
(G protein). 2
Transducin
activates
phosphodiesterase (PDE). 3
PDE converts
cGMP into GMP,
causing cGMP
levels to fall. 4
As cGMP levels fall,
cGMP-gated cation
channels close, resulting
in hyperpolarization. 5
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28. 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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29. 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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30. 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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31. released continuously.
Figure Signal transmission in the retina (1 of 2).
Slide 1
In the dark
cGMP-gated channels
open, allowing cation influx.
Photoreceptor depolarizes. 1
Na+
Ca2+
Voltage-gated Ca2+
channels open in synaptic
terminals. 2
Photoreceptor
cell (rod)
−40 mV
−40 mV
Neurotransmitter is
released continuously. 3
Ca2+
Neurotransmitter causes
IPSPs in bipolar cell.
Hyperpolarization results. 4
Hyperpolarization closes
voltage-gated Ca2+ channels,
inhibiting neurotransmitter
release.
Bipolar
Cell 5
No EPSPs occur in
ganglion cell. 6
Ganglion
cell
No action potentials occur
along the optic nerve. 7
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32. Below, we look at a tiny column of retina.
Figure Signal transmission in the retina. (2 of 2).
Slide 8
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
cGMP-gated channels
close, so cation influx
stops. Photoreceptor
hyperpolarizes. 1
Light
Photoreceptor
cell (rod)
Voltage-gated Ca2+
channels close in synaptic
terminals. 2
−70 mV
−70 mV
No neurotransmitter
is released. 3
Lack of IPSPs in bipolar
cell results in depolarization. 4
Depolarization opens
voltage-gated Ca2+ channels;
neurotransmitter is released. 5
Bipolar
Cell
Ca2+
EPSPs occur in ganglion
cell. 6
Action potentials
propagate along the
optic nerve. 7
Ganglion
cell
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33. 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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34. Figure 15.19 Visual pathway to the brain and visual fields, inferior view.
Both eyes
Fixation point
Right eye only
Left eye only
Right eye
Left eye
Optic nerve
Supra-
chiasmatic
nucleus
Pretectal
nucleus
Optic chiasma
Optic tract
Lateral
geniculate
nucleus
Superior
colliculus
(sectioned)
Uncrossed
(ipsilateral) fiber
Crossed
(contralateral) fiber
Lateral
geniculate
nucleus of
thalamus
Optic
radiation
Superior
colliculus
Occipital
lobe
(primary visual
cortex)
Corpus callosum
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.
Photograph of human brain, with the right side
dissected to reveal internal structures.
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35. 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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36. Visual Processing Retinal cells Lateral geniculate nuclei of thalamus
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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37. 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
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