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
Projections of ganglion cells
All via optic nerve (myelinated)
Half decussate at optic chiasm
- generally R retina (L visual field) -> R hemisphere
Suprachiasmatic nucleus (SCN) of hypothalamus
- circadian, mostly from W ganglia
Superior colliculus -> topographic map
- combines with auditory, somatic -> coordinates head/neck/eye movements via midbrain, cerebellum
Pretectal -> bilateral Edinger-Westphal nucleus -> CN3 -> ciliary ganglion -> pupillary reflexes (direct and consensual)
Lateral geniculate nucleus (separate slide)
Lateral geniculate nucleus
Receives most outflow from retinal ganglion cells
Point-to-point topography -> primary visual cortex
Layers (6) - alternate ipsi and contralateral
- Outer (3-6) = parvocellular - color and fine detail
- Inner (1,2) = magnocellular - movement, depth, contrast
Visual input is only 10-20% - lots of feedback from cortex (ex sleep)
Optic chiasm
R retinal field (L visual field) -> R hemisphere
Temporal retinal fibers (contralateral field)
- express EphB1 - can’t cross optic chiasm cells that express Ephrin B2 -> stay on same side
Nasal fibers (temporal ipsi field)
- don’t express EphB1 -> cross
Overview of cortical vision pathways
LGN -> two distinct pathways = “optic radiation”
- superior fibers (lower visual field) -> parietal -> above calcarine
- inferior fibers (upper field) -> Meyer’s/temporal loop -> below calcarine
Primary cortex = V1, area 17 -> parallel streams
- ventral stream = object recognition, color
- V1 -> V4 -> temporal
- dorsal stream = depth, motion, spatial
- V1 -> V2 -> MT -> parietal
V2/V2 = area 18 - occipital, depth V4 = area 19 - occipital, color V5 = area 19 = middle temporal = MT - motion, spatial
Network of visual cortex
Input: all LGN to Layer 4 -> projects to layers 2,3 -> layer 5 -> layer 6 Output: - layers 2,3 -> cortical - layer 6 -> LGN feedback
Organization of visual cortex
Point-point organization (more area to fovea)
- inverted - vertical and bilateral
Vertical columns -> orientation of simple cells
Ocular dominance columns - predominant input from one segment of one retina (not clear-cut)
- usually paired -> hypercolumn
- “blobs” - interspersed, color specificity
- radioactive transneuronal tracing - tritiated proline -> eye -> LGN synapse -> cortical synapse
Visual field lesions
Monocular blindness - eye, optic nerve
Anopsia - large visual field deficit
- bitemporal - missing both temporal fields (nasal receptors) - chiasm/pituitary
- homonomous - missing one sided field
- past chiasm - tract, LGN, radiations, primary cortex
- can have quadrants - ex L Meyer’s loop -> missing upper R
Cortical cells
All specific for location (via region of cortex)
Simple cell - combination of ganglion input -> orientation
Complex - combination of simple cell inputs
-> orientation and “end” ie length of line (inhibition from neighboring segments)
Hypercomplex - combination of complex -> movement
Increasing convergence and ability to extract information
Motion detection
Two mechanisms
- image movement vs eye
- eye movement (if following object)
V5/MT area important
Akinestopia = motion blindness
Can exaggerate with video -> see baby breathing, pulse in face
Depth perception
Stereoscopic clues - combining eyes - used cortical coincidence cells -> detect in front or behind plane of focus
Monocular clues - >100 ft (retinal images same)
- previous familiarity (car)
- relative size (two people)
- interposition (hidden behind)
- linear perspectives (lines converge)
- shadows, illumination
- motion parallax (closer things move faster across field)
Uses V2, V3, V5/MT
Color perception
Color = property of object (color constancy), wavelength can change based on source
- hue - proportion of green, red, blue
- saturation - amount of combined
- brightness - total stimulation
Retina: single-color center-surround (parvocellular) ->
LGN -> cortical “blobs” ->
double-opponent (ex red middle, green surround)
-> V4, area 19
Higher level cortical processing
“Binding problem” - how to pick out relevant information
- aperceptive agnosia - can’t recognize familiar objects
Scanning (focus on details) -> feature maps (color, texture, depth) -> master maps
- selective attention to different features in progression
Development of cortical organization
Chemical signals -> initial repulsion and attraction
-> proliferation of projections
Coordinated electrical activity/input -> sorting and segregating
- uneven input (lazy eye) -> thinner columns
- must be coincident
- strabismus = misalignment due to muscle weakness or coordination -> more divergent projections or loss of input from one eye (amblyopia/lazy eye)
- esotropia = convergent, cross
- exotropia = divergent, wall-eyed
- critical plasticity period (0-6y in humans)
-> permanent “cortical blindness” or lazy eye
- also compensation - ex remove hemisphere -> full function
