Eye and Vision
Fox, a substantial part of Chapter 10
Vision is usually covered everywhere, and Freeman has the topic
book) eye and retina structure
book) spectrum and spectra for the 3 cones that mediate color vision
the eye picture of an ophthalmologist's office
cornea, iris, pupil, conjunctiva, sclera, extraocular muscles
lens, aqueous (anterior chamber), vitreous (vitreous chamber), retina, fovea,
there is a blind spot where the optic nerve exits
Here is a picture showing focus of an arrow up-side-down on the retina,
trivial except that is shows that most of the bending is at the cornea where
the change of index of refraction at the air-cornea interface is very large.
diopters - reciprocal of focal distance in m
cornea is 0.024 m, 42 diopters
Hyperopia-far-sighted, need convex lens,
Myopia-near-sighted, need concave lens, involves abnormal elongation of
visual angle, acuity - Snellen eye chart - 20/20 is seeing letter 5 min
Accomodation and presbyopia
loss of accomodation with age explains Presbyopia
Benjamin Franklin developed bifocals
This is a difficult concept and the best figure I've seen to explain it:
If ciliary muscle is relaxed, ligaments are tight and lens is stretched
If ciliary muscle contracts, ligaments have slack and lens relaxes to greater
bulge for near vision.
Here, in an
albino rabbit eye dissection, you can see the suspensory ligaments of the
Glaucoma - pressure is too high because aqueous humor does not drain well,
ganglion cells die, treated with drops or surgery
Floaters in vitreous especially in myopia
Diabetic retinopathy blood vessels overgrow, leak, blast holes in retina
with laser decreases angiogenesis
Cataract - lens becomes opaque, remove and often replace with intraocular
lens, made of polymethyl methacrylate, known to be tolerated since pieces
from airplane visors would nlodge in pilots under fire (and since about
1988, these have been doped with UV blockers)
Retinitis pigmentosa is tragic, people can see when young, lose rod vision
(tunnel vision [ring scotoma] because rods are in mid-periphery).
Rods go first and eventually cones which is strange if rod molecules are
There are autosomal and X-linked types, dominant and recessive.
There are other genetic degenerations and stationary (not progressive) blindnesses
are in molecules of transduction cascade (book only mentions missense mutations
in opsin) as well as in other rod and cone molecules.
There is a web site where information relevant to the retina, especially
genetic causes of blindness, accumulates (site)
Age related macular degeneration may have an genetic basis too
An interesting and related story has to do with dilation of the pupil.
Recall that atropine, a muscarinic antagonist, dilates the pupil.
That means that the parasympathetic nervous system constricts the pupil.
Recall that parasympathetic = cranio-sacral, and here the cranial nerve
is the occulomotor nerve (#III)
By contrast, the sympathetic n.s. dilates (in dim light), and the nerve
has to come up from the superior cervical ganglion (of the thoraco-lumbar
A bright light in one eye causes the other pupil to constrict too. (Try
this in front of a mirror with a flashlight.)
Neurologists can make use of information based on defects in the pupillary
"the eye is the window to the brain" -- physician can actually
look at CNS
For instance, increased crainial pressure (like from tumor) shows up as
Optic disc is where optic nerve exits and blood supply enters and exits.
Fovea is high acuity cone vision.
Macula lutea is area where carotenoids (lutein and zeaxanthin) form blue-blocking
Here is a picture
a friend (Lynette Feeney-Burns) gave me before she retired (in about 1990).
It is labeled "normal macular pigment - chow diet," and it demonstrates
the density of yellow pigment around the fovea in (presumably) monkeys fed
a diet adequate in carotenoids. Currently, it is known that the carotenoids
lutein and zeaxanthin are in nerve layers in the light path to the receptors
of the fovea (cones). We get these yellow-appearing caroteinois in our diet
(e.g. from spinach and corn). It is thought that they help to protect cones
from damage that may be induced by blue light. It was found that the concentration
is increased with dietary increases, and now lutein is included in multi-vitamins.
Here, in a sheep
eye dissection, you can see the optic disc. Retina is white, pigment epithelium
and choroid are black.
Rods and Cones
Figure (here is a pdf
of this figure)
Hecht, Schlaar and Pirenne (1942) published a study that a human subject
can see a light so dim that 6-14 quanta were absorbed over a 500 rod area;
that means one rod can "see" one quantum.
Here are some calculations showing how to determine the energy of a photon
using Planck's constant (obviously very low).
I also roughly calculate to show that the threshold for audition is comparably
Photoreceptors- 125 million receptors 20/1 rods to cones
(converge on 1 million ganglion cells)
Adds to sensitivity of rods and to acuity for cones.
Retina is mounted backwards relative to the path of light
expands on the above figure with retinal wiring diagram:
Straight through: Photoreceptor -> bipolar -> "ganglion"
cells (whose axons form the optic nerve)
Horizontal interactions: Horizontal cells and amacrine cells
Pigment epithelium - melanin, vitamin A conversions, and phagocytosis of
spent photoreceptor membrane
rods concentrated off-fovea, cones on-fovea
Rod, peripheral vision, dim black and white, sensitive - "scotopic"
Very sensitive - 1 photon
Cone, fovea, color, acuity - "photopic"
Shown in rats, rods are supported by retinal
RPE: (1) melanin that blocks light reflection
(2) metabolism to provide 11-cis retinal (chromophore ofvisual pigment,
(3) phagocytosis and recycling of shed rod tips
Cells are postmitotic and the indigestible residue of the phagolysosomal
system is lipofuscin,
a fluorescent aging pigment, a topic on which I've done research.
spectral sensitivity of rods and 3 cone types
confirms Young -Helmholtz trichromatic theory
3 kinds of cone 420 530 560
3 kinds of cone opsins which are evolutionarily related in humans and OW
green and yellow (middle and long wavelength) cone opsins are near each
other on X
(blue cone opsin is on human chromosome 7, rod on chromosome 3)
evolutionary bottleneck hypothesis color vision re-evolves after nocturnal
life (where adaptive pressure for cone vision is relaxed) early in mammalian
Red or green color blindness - on X, thus preferentially in males.
Blindnesses were thought to be from altered genes, but numbr of copies in
human population is variable, and cross-over accidents can even make chimeric
Female "carriers" should actually be mosaics of color blind vs
normal retina because of Mary Lyon X-inactivation hypothesis
superfamily of G-protein-coupled receptors (7 transmembrane domain receptors)
light causes 11-cis retinal (aldehyde of vitamin A, retinol) to turn to
("retinene" - term used in book - is old fashioned)
George Wald 1967 Nobel
The alpha subunit of transducin (the name for the G protein) activates cGMP
Less cGMP (ligand) and channel closes so cell hyperpolarizes since sodium
This figure repeats the above point.
Because there are Na+ channels in the outer segment and a Na+-K+ ATPase
in the inner segment (where there are lots of mitochondria manufacturing
ATP), there is a dark current turned off in the light.
(Also shows how rod cell is a stack of disks with rhodopsin and other transduction
Transmitter is released in dark - but less when light is on
(and processing - my coverage here will be minimal)
Temporal retina does not cross at optic chiasm but goes to ipsilateral lateral
geniculate nucleus (part of the thalamus)
Nasal retina goes to contralateral LGN
At LGN, inputs from 2 eyes does not mix.
Projection to cortex where eye inputs mix for stereopsis, and processing
for contrast, moving lines, angles, etc takes place.
Superior colliculus (important for eye movement control)
My interests center around vision, so a visit to the
research interests of my home page will offer various topics about vitamin
A, ultraviolet light, and Drosophila mutants. The Biology Department
has a vertebrate vision specialist, Judith Ogilvie.
Ariel in SLU's Pharmacology Physiology Department is one of my fellow
wizards in visual science, also Dr.
Kisselev in Ophthalmology. Dr.
Kolar in Pathology has intrests in eye pathology.
Exam questions from 2004 - 2011 relating to this outline
Blind spot, answer either (1) In terms of anatomy, why is this found on
the temporal VISUAL field? or (2) Why is that area blind?
because the optic nerve exits the eye on the nasal RETINAL field, there
can be no receptors where the optic nerve exits the eye
Why would you have tunnel vision in retinitis pigmentosa?
loss or rods in the mid-periphery
Glaucoma: answer either (1) What compartment does not drain properly in
this disorder? or (2) What is the mechanism of loss of vision (blindness)
[e.g. media lose their transparency, refractory error, etc].
aqueous humor (anterior chamber) ganglion cells (the ones that form the
optic nerve) degenerate
"A laser is used to destroy patches of the retina." Answer either
(1) Who might actually benefit from this procedure? or (2) Why would the
ophthalmologist avoid blasting the fovea with the laser?
a diabetic, fovea is too essential for high acuity color vision at the point
What kind of people would have the focussed image in front of the retina
instead of on the retina?
If the ciliary muscle is contracted, what happens to... (answer one of these)
(1) the suspensory ligaments, or (2) the shape of the lens, or (3) your
they become slack, it gets rounder, for near vision
"Eye care professionals dilate the pupil with belladona alkaloids."
Pretend this is all you know and then explain, on that basis, what output
from the parasympathetic nervous system does to the pupil.
drug blocks parasympathetic neuro-effector junction, so parasympathetic
must do opposite, constrict
"A single rod can 'see' one photon of light." How is it that such
a cell physiology piece of knowledge was first established on the basis
of ordinary people like you and me indicating whether or not they could
see various light stimuli?
psychophysics, careful calibrations, careful measurements, showed only 6-14
quanta absorbed over a 500 rod area
A cis to trans isomerization of what chromophore, (name that chromophore),
a component of a G protein coupled receptor, is the only thing light actually
does in visual transduction?
11-cis retinal (retinene) the aldehyde of vitamin A
I suggested that a gene duplication on the X chromosome could be used to
explain how the superfamily of G protein coupled receptors evolved. Which
two proteins are made by the two duplicated X chromosomal genes I am referring
the genes for red- and green-absorbing rhodopsins
Rhodopsin signals to a G protein and the G protein signals to what enzyme?
Answer for either the vertebrate or for the fruit fly. Hints: cGMP is involved
in the vertebrate rod and the fly utilizes the phosphoinositide signal transduction
a phosphodiesterase that converts cGMP
Coated pits and multivesicular bodies are involved in recycling of photoreceptive
membrane in fruit flies. By contrast, for vertebrate rods, an additional
type of cell is needed in the phagolysosomal system. What is this cell called?
retinal pigmen epithelium
Why are you blind in your blind spot?
No receptor cells can be presen t where the optic nerve exits and the blood
supply enters and exits
What happens to your vision when the ciliary muscle is contracted?
When ligaments become flaccid, the lens gets rounder, accommodating for
"Night blindness," "ring scotoma," and "tunnel
vision" are symptoms of what disorder?
Which portion of the nervous system connects to the iris to mediate the
constriction of the pupil elicited by a light stimulus?
Parasympathetic, occulomotor nerve (#3), from ciliary ganglion
Why is there lutein in your vitamin pill?
This is one of the carotenoids in the macular pigments that protect foveal
cones from blue light
Young and Helmholtz proposed a widely accepted theory of trichromatic color
vision in humans. Answer either (1) what kind of cell, or (2) what kind
of molecule has these three specific peak wavelengths of sensitivity?
Why is it more accurate to refer to women heterozygous for red or green
blindness as "mosaics" instead of "carriers?"
since it is on the X, they have some cells with one X and the rest of their
cells have the other X
In Prof. Stark's research seminar, the rhodopsin promoter or the heat shock
promoter was used to drive expression of rhodopsin labeled with green fluorescent
protein into the rhodopsin-containing organelle that mediates vision. On
what kind of molecule is a promoter?
The promoter is the part of the gene upstream of the coding sequence (thus
Prof. Stark showed transmission electron micrographs from his own work and
work from other researchers of big vesicles in the visual cell that he believed
carried rhodopsin into the rhodopsin containing organelle that mediates
vision. What cell structure would have produced these vesicles?
Rough endoplasmic reticulum via Golgi apparatus
State a major difference between retinitis pigmentosa and age related macular
Rods (as young adult) ve cones (in elderly)
State one of the three fundamental functions of the retinal pigment epithelium.
Black "paint," retinoid metabolism, phagocytosis of distal tips
of rods that are shed daily
The aldehyde of vitamin A (retinal, formerly called retinene) is hit by
light. What happens to it?
It bends [isomerizes] (it relaxes from 11-cis to all trans)
Under what circumstances is there lots of cGMP to open the channels in the
rod outer segment?
In the dark
In Prof. Stark's research seminar, a recent suggestion was that clathrin
was associated in an "import" pathway (rough endoplasmic reticulum
to Golgi apparatus to the organelle where rhodopsin mediates vision). More
traditionally, for many decades, clathrin was seen (in what structure?)
in the transmission electron microscope.
Coated pits, coated vesicles, endocytitic structures, clearance
In what context is vitamin A relevant to a G protein coupled receptor?
The aldehyde of vitamin A is the chromophore that attaches to the protein
rhodopsin (which is a G protein coupled receptor)
What intracellular ligand, whose function is to open channels, is decreased
when light stimulates a rod?
Energy equals Planck's constant times the frequency. Energy of what?
of one photon
For what population of people is macular degeneration most common?
What must be bound to the G-protein-coupled-receptor protein to make the
fully-functional rhodopsin molecule that absorbs light?
What happens to a rod's neurotransmitter release when light hits the rod?
The ciliary muscle contracts, in accomodation, to let you do what?
see up close
How does an eye care professional test for glaucoma?
poke the eye for a pressure check
In the dark, a current of Na+ ions flows from the sodium pump in the inner
segment through what in the outer segment?
When an axon from a ganglion cell goes toward the brain in the optic nerve,
where does it first synapse?
What layer at the back of the eye is black?
retinal pigment epithelium, also choroid
Some men taking Viagra (sildenafil) report impaired color vision. Why might
Viagra and phototransduction both involve cGMP
Where are the genes of the long- and middle-wavelength cone rhodopsins located?
Our gaze seems more relaxed for far vision even though suspensory ligaments
are relaxed for the lens to accomodate for near work. What does contract
to change the lens shape to see up close?
A current of Na+ in the dark along the rod is from the Na+/K+ pump in the
inner segment and what in the outer segment?
Na+ channel (or transduction machinery)
What specific cellular defect explains the tunnel vision (ring scotoma)
of retinitis pigmentosa?
What is the only direct effect of light in initiating phototransduction?
cis -> trans isomerization of retinal
What kind of lens corrects for myopia (near-sightedness)?
Which two rhodopsins used for human color vision are coded by adjacent genes
on the X chromosome?
middle and long wavelength (green and red [yellow])
Ganglion cells are killed from high eye pressure. What is this disorder
What layer of black cells supports rod outer segments by phagocytosis and
conversions of vitamin A?
retinal pigment epithelium
Epinephrine binds one G protein-coupled receptor (GPCR). What other GPCR,
used for vision, is a pigment that contains a form of vitamin A?
As it applies to diabetic retinopathy, what is angiogenesis?
formation of new, fragile, blood vessels
Laser photocoagulation decreases angiogenesis. Here we are treating to prevent
further blindness from what disorder?
Which side of the retina projects to the ipsilateral lateral geniculate
body at the optic chiasm?
Macular degeneration (MD) aflicts a certain population as reflected by the
"A" in the name (AMD). What population?
Under what conditions are cGMP levels in the rod lowered?
when stimulated by light
Why is it inaccurate to consider a chromosomally normal woman to be be a
carrier for a recessive X-linked color blindness if the other chromosome
is normal (wild-type, dominant)?
mosaic of cells with one vs the other X functioning (Mary Lyon X inactivation
Why does each eye have a blind spot?
there can be no receptors where the optic nerve exits
Why is cataract usually a less severe form of blindness than retinitis pigmentosa?
there is straightforward surgery for cataract, whereas rods die and cannot
be recovered for rp
Scientific information has led to the decades-old conventional wisdom that
vitamin A is good for vision. Other than being the pigmented portion of
rhodopsin, where else do carotenoids come in to play, explaining why lutein
is a dietary supplement or a component in a multi-vitamin pill?
macular pigments protect fovea from blue light
Presbyopia is a defect in what process that affects most people over 40
Why is the cornea actually a stronger lens in the eye than the lens?
because of the big change in index of refraction from the air-cornea interface
Why is the rod depolarized in the dark (but not in the light)?
because cGMP opens sodium channels
The genes for the yellow- and green-light absorbing rhodopsins are near
each other on which chromosome?
The tip of the rod outer segment is sloughed off on a daily basis and "recycled."
Where does this cellular fragment go?
into retinal pigment epithelium
What does activation of the parasympathetic portion of the occulomotor nerve
do to the pupil?
If your eye pressure is high, answer ONE of the following: (1) name of disorder,
(2) what is not draining appropriately, or (3) loss of what cells mediates
glaucoma, aqueous humor, ganglion cells
How can rhodopsin be mutant yet the patient still can see until retinitis
pigmentosa first presents in the teens or 20's?
in autosomal dominant, a normal recessive gene still expresses rhodopsin
Suppose you could stimulate a nerve to achieve the same effect as applying
belladonna alkaloids to the eye. What type of nerve would you stimulate?
A whole lot of rods converge onto each ganglion cell. What does this wiring
What is lost in age related macular degeneration (AMD)? (Your answer can
be cellular or functional.)
cones, fovea; vision
If you stare straight at a mark (X), while a stimulus is presented to both
eyes to the right of the X, describe how the stimulus projects to the thalamus
(lateral beniculate nucleus).
temporal retina of left eye - ipsilateral projection; and nasal retina of
right (contralateral) to left
In the molecular mechanism from light absorption to hyperpolarization of
the rod, exactly what does 11-cis retinal do?
absorb light, convert to trans
Why would you see much better under water with vs without goggles?
keep the normal air-cornea interface (with the normal indices of refraction)
"The blind spot is about 15o off fovea in your nasal retinal field
(temporal visual field)." Translate.
location off axis is measured in angle, since the optic nerve exits 15o
on the side toward the nose, the inversion of the image would make it seem
15o off to the side
"Visual experience can influence the progression of myopia." On
what basis can that statement be made with scientific authority?
research where the vision was distorted with goggles and the eye changed
"If the ciliary muscle is relaxed, the ligaments are tight and"
Finish this thought with respect to the shape of the lens and what that
does for vision.
lens flattens for distance vision
In the course a lifetime, a retinal pigment epithelial cell's ability to
carry out it's function might deteriorate. Why?
it fills with the indigestibloe residue of the phagolysosomal system
If you were convinced that blue light damages foveal cones, how might you
alter your diet starting now, while you are young, to delay age related
macular degeneration (AMD)?
eat veggies or vitamin pills with lutein or zeaxanthin
With what sort of methodology were scientists as early as 1942 able to come
to the conclusion that a rod can respond to one photon?
psychophysics, human subjects report if they can see calibrated lights
"A derivative of vitamin A is the chromophore of rhodopsin." Translate.
vitamin A aldehyde is the light absorbing portion attached to the protein
Why would the evolution of red-green color vision on the X chromosome relate
to the richness of olfactory sensation?
it is a simple example of the evolution of G protein-coupled receptors
"Color normal is dominant, color blind is recessive, hence women can
be heterozygous carriers for green blindness." Why is this an oversimplification?
they would be mosaics of cells with one or the other X functioning
Why is there an extracellular current from the rod outer segment to its
inner segment in the dark?
The channel letting Na+ in is in the OS, the sodium pump is in the IS
In terms of the channel gating, why does a rod hyperpolarize in response
the ligand (cGMP) that holds the Na+ channel open is broken down
You stare at an X with your right eye. Answer either (1) Where would the
blind spot be? (state this using the sort of terminology that might be used
in optometry) Or (2) How would you determine where it is while you are staring
at the X?
a certain angle (about 15 degrees) off axis (away from the fovea) in the
temporal visual field (nasal retinal field)
A muscle that mediates accommodation contracts. Answer either (1) What happens
for your vision? Or (2) What happens to the shape of the lens?
(1) better for near vision (2) becomes rounder
"The loss of vision is referred to as a 'ring scotoma.'" Answer
(1) What disorder? Or (2) Why is it a ring?
1-retinitis pigmentosa, rods (in mid-periphery) are lost
Which portions of the nervous system mediate constriction and dilation of
parasympathetic (constriction) and sympathetic (dilation)
One photon absorbed by rhodopsin elicits an electrophysiological response
in a rod. That plus what retinal wiring consideration makes scotopic vision
convergence of multiple rods to one ganglion cell
Why does the retinal pigment epithelial cell have so much to phagocytose?
tips of outer segments are shed daily
How (between New World monkeys and Old World monkeys) did there get to be
(at least) two color vision genes on the X chromosome?
unequal crossing over duplicated the gene
What is the only thing that light does directly to excite rhodopsin?
cause 11-cis to all trans isomerization of retinal
A light is switched on. What happens to the transmitter release from the
The eye pressure is high. Answer either (1) What isn't draining properly?
(2) Death of what cell type would mediate vision loss? Or (3) What is this
Aqueous humor, ganglion cell, glaucoma
Why would a person with my interests need to use Planck's constant?
Since it gives energy of a photon, it can assist in calculations of light
When an ophthalmologist looks at your retina, what is the second most conspicuous
landmark after the optic disc?
What does the Mary Lyon X inactivation hypothesis have to say about heterozygous
carriers of red or green colorblindness (both of which are on the X)?
It should not be a matter of dominant vs recessive, but her retina must
be a mosaic
What is it that allows the sodium channel in the rod outer segment close?
Removal of cGMP by phosphodiesterase
In the visual projection, where is the first place where inputs from both
eyes can connect to the same cell?
Not until the cortex
What is lost making people with age related macular degeneration (AMD) blind?
Cone (foveal) vision
How is vision changed when suspensory ligaments are relaxed?
lens rounder for near vision
What would output from the occulomotor nerve (cranial nerve #3), connecting
through the ciliary ganglion, do to vision in bright light?
parasympathetic constrict pupil to decrease that bright light to retina
Who was short-changed by the bottleneck proposed for the evolution of color
Why might you expect that medications like Viagra for erectile dysfunction
might affect vision?
It inhibits cGMP phosphodiesterase
Fish, birds and mice can see ultraviolet light but you cannot. What is the
difference between their eyes and yours in this regard?
obviously, their lenses must transmit UV, also their retinas have UV receptors
What happens to the vision of a fruit fly in a mutant that has degeneration
of the predominate receptor type, R1-6?
R1-6's contribution to response is abolished, leaving R7 (with its UV sensitivity)
and R8 contributing to physiological and behavioral response
How was it first shown, a few decades ago, that there were environmental
(not just genetic) influences in progressive myopia?
vision changed in chicks pecking for seeds if they were fit with goggles
What part of the eye is at fault when the subject experiences floaters?
"Ring scotoma." Answer either (1) Why a ring? Or (2) What is the
name of this disorder?
since rods are off fovea, ring of rod blindness is around fovea
A pigment that looks yellow and absorbs blue light is situated in the light
path on the way to the foveal cones. Answer either (1) What is it made of?
Or (2) How might it be useful in terms of visual health?
carotenoids (zeaxanthin and lutein), might prevent or delay age-related
Either remember or figure out from what I remind you. Planck's constant
(h) is 6.63 x 10­p;34 (what units?) E is energy, frequency (n)= speed
of light divided by wavelength of light. E=hn.
Phagocytosis by the retinal pigment epithelium (RPE). Answer either (1)
What is phagocytosed? Or (2) Describe, in some way, the burden this imposes
on the RPE cell.
distal tips of rods, postmitotic cells last a lifetime and such high traffic
leads to accumulation of age pigment
What is the next molecule in the cascade directly after the G protein (transducin)
I can see 350 nm light but you cannot. Why can't you see UV (ultraviolet)
your lens blocks it
Give the approximate wavelength for the peak sensitivity of human rods?
When it was first introduced by Harris, Stark and Walker in 1976, what was
the sev (sevenless) mutant used for?
to show that R7 was a UV receptor, to show the contribution of R7 to the
(spectral) responsivity of the eye
What happens to the vision of a vitamin A deprived mouse after it has had
vitamin A replacement therapy?
since it was very low, it comes up, it comes up in the UV especially
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