Physiology of Vision- Graf Flashcards
The eye requires coordination of many things.
If you add all the components of outline: color, color contrast, #d then you have a total picture of everything. ?????
Eye disease prevalence
Eye disease:
Myopia, hyperopia, cataracts, retinal
Almost 25% of the population suffer from eye diseases.
Examples of Eye disease
myopia: blurred vision
age related macular degeneration: area of sharpest vision is degraded
cataract: cloudy lens, blurred vision; is preventable
glaucoma: high intracranial pressure and retina gets destroyed from the periphery
diabetic retinopathy: occurs when high blood sugar levels cause damage to blood vessels in the retina; degeneration of BVs
The eye can only take in a narrow range of frequency. Why?
-narrow range of frequency we can see because we evolved from water dwelling animals
16 Hz – 20 kHz
400-700 nm (visible light)
Eye structure
Sclera is tough you need a razor blade to go through it.
Vitreous humour (flow out and eye will collapse) and aqueous humor produced by the ciliary body. It is evacuated by the Canal of Schlemm.
Pupil is defined by its surrounding (iris).
Iris means rainbow and is responsible for the color of our eyes.
Cornea are the strongest refractive surface. Lens has some refractory surface.
Choroid surrounds the eye and is abundant with BVs responsible for providing nutrients to the eye.
Retina have the photoreceptors.
Eye embryology
- eye comes from the optic vesicle which invaginates back on itself
- optic nerve is part of the brain (not a real nerve)
- eye is formed from the neural ectoderm (makes the retina) and the surface ectoderm (makes the cornea and lens)
all eyes have photoreceptors and lens to focus the images (no matter what species we’re talking about)
Ciliary muscle vs Iris
Ciliary muscle is a circular muscle for far/near objects. It is antagonist to the elasticity of the lens. Under normal conditions the ciliary muscle is relaxed and the lens are pulled flat. When you want to accomodate or look at something near, the ciliary muscle contract and pulls against the lens elasticity allowing focus. When you get older your eyes lose elasticity, so you need glasses to see things.
Iris is for constriction/dilation of pupil. Dilator pupillae is by sympathetic system and sphincter pupillae is by the parasympathetic system.
Atropine eye drops is a parasympathetic antagonist so parasympathetic muscle cannot work (sphincter pupillae cannot constrict and ciliary muscle cannot contract to make lens thicker for focus).
What are the external eye muscle?
- lateral rectus
- medial rectus
- superior rectus
- inferior rectus
- superior oblique
- inferior oblique
What is emmetropia, hyperopia, and myopia?
Emmetropia: normal sighted: image hits directly on the retina
Hyperopia: the image gets formed behind the retina getting a blurred image; to shorten the optical power; far-sighted; correct with convex lenses
Myopia: to long the optical power; the image gets formed in front of the retina and the image is blurred; near sighted; correct with concave lenses
What is astigmatism and presbyopia?
Astigmatism: non-spherical aberration of the cornea
Presbyopia: old sighted; need reading glasses; a gradual, age-related loss of the eyes’ ability to focus actively on NEARBY objects
Snellen Chart
- measures visual functions
- 20/20 vision is at the 8th line (a normal sighted person)
- the visual system works on two things: a linear system that works on spatial frequency (how much you can discern the minimum resolution you get which depends on your optical powers and density of photoreceptors) and contrast
high contrast means low spatial frequency AND
low contrast means high spatial frequency means you can’t see the letters
Snellen chart has sloan letters. They work on the spatial frequency detection of the visual system. One letter element 1 minute of arc. Period- 2 minutes of arc.
60 minutes an arch?????
Retina
100,000,000 photoreceptors in each eye (100 million)
- we have rods and cones
- 15 types of bipolar, horizontal, 30 types of amacrine, and 45 types of ganglion cells)
-retinal ganglion gives rise to the optic nerve (optic nerve are the axons of retinal ganglion)
lateral information flow: horizontal and amacrine cells
information throughput: bipolar and ganglion ??????
Rods and cone
rods and cones distributed preferentially
on the fovea you only have cones
parafovea you have rod peak. At the fovea you have cone peak???????
macula which contains the fovea; has 50% of input to brain
1:0.5 ??????
fovea: one cone for two retinal ganglion cells; receptor fields are very big and images are sharp
periphery: one cone for 50-100 retinal ganglion cells
We have 3 cone types, what are they?
We are tri-chromatic. Three cone photopigments.
red
green
blue
with different peaks in the light spectrum
photopic vision is color vision (cones)
scotopic vision is night vision (rods) ?????
One rod photopigment
- Rhodopsin
- colorblindness
can you read the number 8 image
Dogs only have 2 cone types. Butterfly and fish have many more. Our color vision was lost because originally mammals were night animals but color was reinvented during industrialization????
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What chromosomes are the cones encoded from?
IMPORTANT SLIDE TO KNOW
- vision is encoded on the genes.
- blue color is encoded on chromosome 7
- red and green is encoded on the long arm of the X chromosome
- As a consequence blue colorblindness affects male and females equally
- red/green affects mainly males (female can get it if they inherit two bad X chromosomes)
Pathiophysiology of Color vision
noram color vision
loss of one cone mechanism
loss of long-wavelength cone mechanism
loss of medium-wavelength cone mechanism:
loss of short-wavelength cone mechanism: tritanopia
How do we see?
- the language of the brain is APs , so everything has to be translated to AP
- We have photopigments (rhodopsin of cone pigments) that help with phototransduction.
Vitamin A deficiency fist showed with night blindness. It takes time to reduce the bleaching????
Photoelectric transduction
resting membrane potential= RMP
- RMP of photoreceptors is -40
- shine light on photoreceptors the RMP becomes -70
activation of the photoreceptors results in hyperpolarization and release of less glutamate; under darkness the photoreceptors are depolarized
uses cGMP to open Na+ channels NOT cAMP
Describe the process of the phototransduction.
?????
How do we see light and dark?
it depends on the bipolar cells (throughput)
off-type hyperpolarize to light (turn off)
on type depolarize to light (turn on)
They have different postsynaptic glutamate ?????
- Darkness is simply NOT the absence of light, the brain needs antagonism.
- In one case you shine light in another case you shine darkness.
?????
glutamate is inhibitory for the on-type receptor
On -type/center: glutamate is inhibitory. When photoreceptor is stimulated hyper -polarization less glutamate release less inhibition EPSP in bipolar cell: sign -inverting: metabotropic glutamate receptor (mGluR6) ??????
Receptive field
in the visual system we have a round
IMPORTANT TO KNOW
Horizontal cells are inhibitory. In darkness, horizontal cells are
depolarized by the release of
glutamate from the depolarized photoreceptors. Thus, they release their inhibitory transmitter and inhibit their target neurons (photoreceptors). Horizontal cells do not have action potentials, but receptor
potentials.
Horizontal cells go to photoreceptors NOT bipolar cells.
???????
Glutamate in one case acts excitatory but in another case is inhibitory.
Amacrine cells
are digitized ?????
retinal ganglion cells
have proper APs ?????
How do things get to the brain?
we two eyes and the eyes look at the world
in visual terms
nasal retina goes to the contralateral lateral geniculate nucleus
temporal retina goes to the ipsilateral lateral geniculate nucleus
right nasal retina / left
temporal retina —>
left visual hemifield
left nasal retina / right
temporal retina —>
right visual hemifield
Law of Weber Muller-Gudden
the left world is seen by the left eye but projected to the right side of brain VICE VERSA
Several Parallel Visual Paths: originate in the retina
superior colliculi used in humans as a template for spatial orientation
hypothalamus is for circadian rhythm
V1 has nothing to do with trigeminal …what does it mean
????
hotdogs and hamburgers
complete perception of a horse we discussed in the beginning
look at 2020 notes
