1. Physiology Of Vision Dr.Spandana Charles MD
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2. Light And Optics: Wavelength And Color
Eyes respond to visible light
Small portion of electromagnetic spectrum
Wavelengths of 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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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
Curved lens can refract light
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5. Figure Refraction.
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6. Concave lenses diverge light
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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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 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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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. 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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13. Emmetropic eye (normal)
Figure Problems of refraction. (1 of 3)
Emmetropic eye (normal)
Focal
plane
Focal point is
on retina.
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14. 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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15. 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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16. 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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17. 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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18. 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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19. 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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20. Table 15.1 Comparison of Rods and Cones
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21. 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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22. 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
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23. 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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24. 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
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25. 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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26. 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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27. 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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28. 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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29. Below, we look at a tiny column of retina.
Figure Signal transmission in the retina. (2 of 2).
Slide 1
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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30. Move from darkness into bright light
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
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31. Move from bright light into darkness
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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32. 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
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33. 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
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34. Fibers from thalamic neurons form optic radiation
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)
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35. 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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36. Both eyes view same image from slightly different angles
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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37. Lateral geniculate nuclei of thalamus
Visual Processing
Lateral geniculate nuclei of thalamus
Process for depth perception, cone input emphasized, contrast sharpened
Primary visual cortex
Neurons respond to dark and bright edges, and object orientation
Provide form, color, motion inputs to visual association areas (prestriate cortices)
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