Chap. 9. The eye

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Chap. 9. The eye
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- Blind spot and seamless world
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-Scuba diver
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Eye disorders Strabismus Glaucoma Detached
retina Retinitis pigmentosa, Macular
degeneration Cataract (clouding of the eye's
lens)
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Strabismus (??)
Esotropia
Exotropia
Def A lack of eye muscle coordination Cause
extraocular muscles, neural control muscles,
neurotoxins, blindness, mechanical problems in
the eye With delayed treatment, vision loss in
one eye may be permanent.
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Detached Retina
Cause Degenerative changes in the retina, most
common in nearsighted people, or from a blow to
the eye. Detachment may also be caused by a tumor
or disease in the eye. Treatment laser beam
sealing
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In most cases of glaucoma (???), the fluid that
bathes the eye does not drain properly and leads
to increased pressure. Some scientists believe
that a defective protein produced by one of the
mutated glaucoma genes can trigger this
heightened pressure by clogging an area, known as
the trabecular meshwork, that guards the fluid's
escape route. From Society for Neurosci. Brain
Briefing.
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Retinitis pigmentosa (RP) start to see in blurs,
as if a piece of wax paper is permanently
strapped over their eyes. Degeneration of rod
cells first, and then cone cells Macular
degeneration loss of central vision
Tiny syringe to administer gene therapy, inject
survival factors and transplant healthy cells
into the back of the eye.
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Blocking apoptosis prevents blindness in
Drosophila retinal degeneration mutants Florence
F. Davidson Hermann Steller Nature 391 587 -
591 (1998) - Abnormally high levels of apoptosis
are linked to many degenerative diseases
including RP - Two Drosophila mutant strains
that exhibit age-related retinal degeneration
(rhodopsin and rhodopsin effector) - Apoptosis
and was blocked by eye-specific expression of
the baculoviral cell survival protein p35
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Vision correction with lenses and
surgery Photorefractive keratoectomy (PRK) - or
Laser Eye Surgery - Laser reshapes the outer
surface of the cornea to change the amount of
refraction. - 2,500 an eye.
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Visual field
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-Visual angle -Visual acuity
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Rod more discs more pigments thus 1000 times
more sensitive Pigments 1 in rod 3 in cones
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- Peripheral retina is more sensitive to
light! - Good visual acuity by low ratio
of photoreceptor and ganglion cells
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- No color perception at night. Why? Require
more energy to become bleached - Color of
dashboard blue-green (500 nm) or red (700
nm)?
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Genes and vision
Nathans J, Hogness DS. Isolation, sequence
analysis, and intron-exon arrangement of the gene
encoding bovine rhodopsin. Cell. 1983
Oct34(3)807-14. Isolation and nucleotide
sequence of the gene encoding human
rhodopsin. Proc Natl Acad Sci U S A. 1984
Aug81(15)4851-5. Molecular genetics of human
color vision the genes encoding blue, green, and
red pigments. Science. 1986 Apr
11232(4747)193-202. Nathans J, Piantanida TP,
Eddy RL, Shows TB, Hogness DS. Molecular genetics
of inherited variation in human color
vision. Science. 1986 Apr 11232(4747)203-10.
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Dark adaptation - Dilation of the pupil -
Regeneration of bleached rhodopsin - Adjustment
of functional circuitry of the retina
(delivery of information from more rods to each
ganglion cell) Light adaptation
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Genetic mistakes and the colors we see
- Three cone pigments Red and green on X chr.,
blue in 7 - Problems common in man (one X chr.
recessive) - Trichromat normal - Anomalous
Spectrum change (6) - Dichromat Loss of one
pigment (2) cf. anomalous dichromat in
women (1) - Monochromat rare - Tetrachromat in
women?
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Fig. 9.22.
Receptive field The area of retina that, when
stimulated with light, changes the membrane
potential of a neuron. Dimension a fraction of
a degree in the center to several degrees in
the periphery (1 mm3.5 o) Center-surround
receptive field Direct and indirect
coupling Depol vs hyperpol (antagonizing
effect) On-center vs off-center
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On-center cell Depolarized when a light in the
center Off-center cell Depolarized when a shadow
in the middle
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- Mainly responsive to differences within the
receptive field - Why the center-surround
organization of the receptive field? To
exaggerate the contrast.
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Parvo Magno
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Number 5 90 Receptive field
Large Small Conduction rate Fast Slow AP
pattern Burst Sustained Detection
of Movement Color
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- Color-opponent cells/center-surround receptive
field - RG- (parvo), BY- (non-P and non-M) -
What if a white light is illuminated to the
entire Rc field?
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Perceived color is based on the relative activity
of ganglion cells
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Parallel processing 1. Depth perception 2.
Independent streams of information from on- and
off-center ganglion cells from each retina 3. M
and P cells