Vision Flashcards
Compound eyes
Multiple ommatidia units -> focus on sheet of receptors
- wide field, great depth focus
- range of wavelengths (UV), can detect polarized light
- adapted for movement
- less resolution -> compensate with bigger eyes
vs Refractive eye - lens create image on retina
- higher resolution
Overview of vision
Light collection and focusing (eye) ->
Transduction (photon -> electrical signal) ->
Processing - retina -> optic nerve -> lateral geniculate nucleus ->
-> further processing in visual cortex
Aqueous humor
In anterior chamber of eye (between lens and cornea)
Produced by ciliary epithelium
- 2 uL/min -> replace all 10-20x/d
Absorbed through trabecular meshwork -> canal of Schlemm
- blockage -> pressure (Glaucoma) -> impedes bloodflow -> nerve and peripheral damage -> blindness
Eye anatomy
Sclera = connective tissue (white) - cornea = clear, most refractive power (vs lens is adjustable) - limbus = junction Choroid - - vessels -> O2, nutrients to retina - pigmented epithelium -> absorbs photons - lens - specialized choroid Retina = neural element
Function of iris
“aperture” for light - range 1.5-8 mm
Circular muscles = sphincter
Radial muscles = dilator
Constriction
- less light (30x change - protective)
- less distortion (center of lens) -> fine detail work
- depth of field increased
Lens function
Variable refraction = accomodation
- lens wants to be round, ligaments hold flat
- less elasticity with age -> no accomodation by age 65
Parasympathetic -> ciliary muscles contract -> ligaments relax -> lens round -> more refraction -> close vision
(also constrict pupil so less distortion through round lens)
Sympathetic -> muscles relax -> flat lens -> less refraction -> far vision
Refraction abnormalities
Emmetropia = normal - refraction exactly onto retina
Myopia = near-sighted
- too much refraction (cornea curve, eyeball elongated) -> refraction in front of retina
- can’t see far objects (lens is already flat)
- correct with convex lenses
- more common, uneven growth of eyeball
Hyperopia - far-sighted
- not enough refracting power -> behind retina
- can’t see close objects
- correct with concave lenses (add refractive power)
Cataracts
Responsible for 1/2 of blindness
Crystallins (proteins) in lens fibers breakdown
-> opacities
Anatomy of posterior eye
Vitreous humor - fills posterior chamber
Pigmented epithelium - underneath retina
- absorbs photons that pass through retina (prevents distortion)
Fovea = highest resolution
- more cones vs rods
- higher density of receptors
- overlying neural layers pulled to the side
Optic disc - ganglion cell axons -> optic nerve
- small “blind spot” - no photoreceptors
Animal eyes
Compound eyes (separate slide)
Fovea
- dogs and cats have area centralis - higher density but still more rods
- rodents - concentration in upper retina
- raptors - second fovea in upper retina (can see ground with high acuity while flying!)
- whales - two foveas
Octopus - ganglion cells in back -> no optic disc/blind spot
Tapetum - guanine crystals in choroid
- reflect light -> improves night sensitivity, decreases acuity
Ex cats: wide peripheral, high sensitivity/night (lots of rods, tapetum, wide pupil, large cornea), lower acuity, dichromats
Choroid disorders
Retinal detachment
- separation of retina (inner optic cup) and pigmented epithelium (outer optic cup) -> displaces retinal fields, loss of nutrition
Macular degeneration - loss of epithelium -> loss of retina
- most common deficit in elderly
- wet: tissue degeneration and vessel proliferation
- dry: deposition of “drusen” = yellow proteins, lipids
Anatomy of retina
Part of CNS (neuro-ectoderm origin) -> layers, glutamate
Neurons:
- photoreceptors - rods, cones
- bipolar cells
- horizontal cells - center vs surround inhibition
- amacrine cells - complex, 20 varieties, 8 neurotransmitters
- ganglion cells - output of APs via optic nerve
Also have supporting glial/Muller cells -> K+ levels
Light goes through other layers (unmyelinated) before hitting receptors
Photoreceptor anatomy
Synaptic terminal (external) -> glutamate vesicles
Inner segment - nucleus, synthetic organelles
Outer segment - modified cilium (microtubules)
- 1000 membranous discs (rods free floating, cones attached)
- regenerated 3 discs/hour -> phagocytosis by epithelium
Types of photoreceptors
Rods - more sensitive, lower acuity (-> night vision)
- longer, more photopigment
- also integrate over longer time (100 msec) -> can’t detect change faster than 12 Hz
- convergence -> bipolar cell (also 100 million/eye vs 5 million cones)
- saturated during daylight - threshold near starlight
Cones - better at everything except dim light
- better resolution - concentrated in fovea
- detect axial vs diffuse rays
- 1:1 to bipolar vs convergence
- better temporal resolution (up to 55 Hz)
- color via different photopigments
Photoreceptor function
Light -> hyperpolarization -> less glutamate release
Baseline:
- cGMP -> Na (Ca) channels open -> “dark current” -> depolarized (-40 mV) -> voltage-gated Ca channels open -> continuous release of glutamate
Light -> activates rhodopsin -> phosphodiesterase -> cleaves cGMP -> Na channels close -> hyperpolarize -> less Ca influx (NaCa exchanger still active) -> less Ca -> less release of glutamate
Rhodopsin
Visual pigment, in outer segment discs
Opsin = transmembrane protein (7 segments), covalently binds
11-cis-retinal - derivative of Vitamin A
- photon changes conformation to all-trans-retinal -> “activated”
“Activated rhodopsin” -> transducin = G protein in disc membrane
Transducin -> alpha subunit binds GTP -> activates phosphodiesterase
Termination of photoreceptor response
Retinal recycling
low Ca, cGMP -> activates rhodopsin kinase (normally inhibits) -> phos-rhodopsin -> binds with arrestin
-> blocks interaction with transducin
-> promotes dissociation and recycling of trans-retinal -> 11-cis
cGMP metabolism
- Transducin = GTPase -> phosphodiesterase inactivated
- less Ca ->releases inhibition of guanylate cyclase -> cGMP
-> baseline dark current
(also important for adaptation - cyclase activity compensates for continuous PDE activation -> dark current)
Recycling of retinal
Occurs via pigmented epithelium
11-cis retinal -photon> all-trans retinal (active) ->
arrestin promotes dissociation ->
special retinol binding proteins ->
pigmented epithelium
all-trans retinal -reduction> all-trans retinol (Vitamin A) -> 11-cis retinal -> recycled to photoreceptor
- Vitamin A deficiency decreases substrate for final step -> night blindness
Color vision
Different opsin proteins -> different optimal wavelengths
Humans = trichromats (blue, green, red)
- vs fish and shrimp more, most mammals dichromats
Color disorders
Rod pigment (blue, dusk/dawn) - 3rd chromosome
Blue - 7th
- missing -> tritanopia
Green and red - X chromosome (-> 8% male vs 0.5% female)
- evolved via duplication -> red + 1-3 greens
- variations due to unequal homologous recombination
-> colorblind if disfunctional hybrid or lack of gene
(no green = deuteropia, no red = protanopia)
Can have polymorphisms -> variation in normal sensitivity
- ex Ser180Ala = different red pigment -> heterozygous women have enhanced spectral sensitivity
Test with Ishahara plates
Ganglion cell function
Output cells: fire action potentials (receptor -> bipolar -> ganglion)
- some spontaneous/baseline activity
Receptive field = circular response area
- either ON-center or OFF-center response (separate slide)
-> essential for contrast detection, edges
(more efficient, fewer axons needed, less distortion of key info)
- equal numbers of ON and OFF -> parallel inputs
Development depends on input
Types of ganglion cells
M - magnocellular - large field, movement detection
P - parvocellular - small field, color detection
W type = weird - no center-surround organization
- photosensitive: melanopsin detects blue light -> G proteins
- circadian rhythms?
Receptive field physiology
Opposite responses to center vs surround
- peripheral/surround receptors -> excite horizontal cells -> inhibit center receptors (surround light -> hyperpolarize -> less glutamate from periphery -> less inhibition -> more release from center)
Response depends on glutamate receptors of bipolar cells
- ON-center: metabotropic G-protein -> K+ channels open -> hyperpolarize -> fewer APs if more glutamate, more APs if less glutamate
- OFF-center: ionotropic NMDA/AMPA -> Na/Ca channels -> depolarize -> more APs if more glutamate (less light in center)
Strategies for treating vision loss
Viral gene -> photoreceptor proteins
- ex add red rhodopsin -> monkeys become trichromatic
Add photoreceptor precursor cells (inject -> incorporate)
New photoreceptor molecules (associate with neurons, conformational change)
Artificial lenses (blocking blue light dec macular degeneration but does it also disrupt circadian cycles?)
Artificial retinas - microchip converts to electrical signal
- use for macular degeneration, retinitis pigmentosa
