Retinal Phototransduction and Signal Processing Flashcards
sclera
- relatively spherical and avascular
- white dense CT that covers the globe posterior to the cornea
- strong tough external framework to protect the delicate optic and neural structures
- maintains the shape of the globe so that the retinal image is undisturbed and provides attachment for the extraocular muscles to rotate the globe and the ciliary muscle to accommodate the lends
cornea
- the window of the eye
- mechanically strong and transparent CT that covers anterior 1/6 surface of the eye
- most powerful focusing element of the eye, twice as powerful as the lens
- 80% of the refraction
lens
- specialized epithelial tissue that is responsible for fine tuning the image that is projected on the retina
- inside the eye surrounded by aqueous humor
- transparent, has high refractive power
- elastin based zonular fibers stabilize the lens and allow accommodation to occur
uveal tract
- consist s of three structures
- choroid
- ciliary body
- iris
choroid
-capillary bed nourishing the photoreceptors and outer retina
ciliary body
- two parts
- muscle controlling refractive power of the lens and vascular component that produces aq humor
iris
- colored portion of the eye seen through the cornea
- contains two sets of muscles with opposing actions that allow the size of the pupil to be adjusted by neural control
anterior chamber
- volume behind the cornea and in front of the lens
- filled with aq humor
posterior chamber
-region between the vitreous and the lens
vitreous humor
-thick gelatinous substance filling the space between the back of the lens and the surface of the retina
retina
-contains neurons that absorb light and process visual information in the images and send it to the brain
macula
- oval spot containing a yellowish pigment
- supports high acuity
fovea
-small depression at the center of the macula-highest spatial acuity
optic disk
- whitish circular area where retinal axons leave the eye and travel through the optic nerve to targets in the midbrain and thalamus
- where blood vessels enter
ophthalmic artery
- anterior segment to iris and ciliary body
- retinal systems
vascular for anterior segment
from anterior ciliary arteries and long posterior ciliary arteries
-penetrating vessels through the sclera vascularize the iris and ciliary body
cataracts
- clouding of the lens that affects vision
- related to aging
- leading cause of blindness worldwide
- by 80 more than half of americans have one or have had surgery
- risk factors-aging, diabetes, sunlight, smoking
- hazy vision, poor night vision, glare and faded colors
- surgical removal of the cloudy lens and replacement with an artifical lens
- aggregates of proteins
glaucoma
- group of diseases that damage the eye’s optic nerve and can result in vision loss
- normal tension, open angle, closed angle
- risks- elevated eye pressure, thin cornea, abnormal optic nerve anatomy, high blood pressure
- not all ppl with high IOP develop glaucoma
- no symptoms until too late, loss of peripheral visual fields
- eye drops to increase aq production or increase drainage
- surgery
production and flow of aqeuous humor
- circulating aq humor nourishes the cornea and lens structures that must be transparent and therefore devoid of blood vessels
1. secreted by ciliary epithelium lining the ciliary processes
2. flows around the lens and through the pupil into the anterior chamber
3. leave the eye by passive flow at the anterior chamber angle - open angle slow development caused by obstruction of drainage canals
- closed angle is sudden increase in IOP, closed or narrow angle between iris and cornea
photoreceptors
- point toward back of the eye
- retina is window to brain
- ganglion layer, inner retina, the photoreceptor nuclei and inner segments before PR outer segments, where phototransduction occurs
layers of the retina
3 nuclear, 2 plexiform, 1 fiber
- nerve fiber layer
- ganglion cell layer
- inner plexiform layer
- inner nuclear layer
- outer plexiform layer
- outer nuclear layer
- PR outer segments
- pigment epithelium
outer nuclear layer
-photoreceptor somas
outer plexiform layers
-photoreceptor/ bipolar/ horizontal cell synapses
inner nuclear layer
-horizontal, bipolar, amacrine cell somas
inner plexiform layer
-bipolar/amacrine/ganglion cell synapses
ganglion cell layer
-ganglion cell somas
pigmented epithelium
- melanin containing cells behind PR
- backstop for light
- maintains phototransduction machinery of PR by recycling discs, pigment regeneration, and PR nourishment
cell types in retina
- PR
- horizontals
- bipolar
- amacrine
- ganglion
glia in retina
- muller glial cells
- microglia (immune system)
- astrocytes-neurovascular
vertical information flow
-PR–>bipolar cells–>ganglion cells
lateral information flow
-mediated by horizontal cells and amacrine cells
neurotransmission
- cells along vertical pathways release glutamate
- cells mediating lateral information transmission mostly GABA or glycineric
rods and cones
- ciliated cells
- outer segment connected to inner via connecting cilium
- outer segment has phototransduction machinery
- inner is housekeeping
- synaptic terminal connects bipolar and horizontal cells
- nt is glutamate
PR response to light 1
- respond to light with graded hyperpolarizations
- in darkness cells are depolarized and nt released continuously
- when in light- respond with graded hyperpolarizations
- spreads passively to the synapse where is reduces amount of glutamate
PR in dark
- Na and Ca cations flow inward through cGMP gated channels at the outer segment
- K ions flow outward through K selective channels at the inner segments
- Na and Ca in depolarizes the cell
- K out hyperpolarlizes the cell
- combined action result in resting potential of -40
PR in light
- absorption of light reduces cGMP levels in the outer segment
- cGMP gated channels close, reducing the inflow of Na and Ca
- K continues to flow out of the inner segment reducing the amount of positive charger
- PR hyperpolarized and nt decreases
- reduction of inward current results in a reduction of intracellular Ca
phototransduction
- rhodopsin is pigment opsin plus chromophore retinal in disk membranes (opsin is G protein coupled receptor)
- retinal absorbs photon and rhodopsin becomes activated
- goes from 11-cis to all trans and activates transducin and decreases cGMP, which increases hyperpol
- all trans retinal breaks covalent bond and exits pocket
- high amplification
- 1 R* closes 200 cGMP channels
dark adaptation
- restoration of sensitivity after exposure to illumination
- after retinol is used, it jumps out and is transferred to the pigment epithelium on IRBP and changed back into 11-cis to be put back in with an opsin molecule
fovea
- the ganglion cells, inner plexiform layer and inner nuclear layer are pushed away for high acuity
- fovea has lots of cones, periphery has rods
one bipolar cell, many rods
- high sensitivity
- low resolution
one bipolar cell, one cone
- each cone projects onto 2 bipolar cells, but each bipolar cell only has one cone
- low sensitivity, high resolution
- midget BC
functional specialization of rod and cone systems
- rods in periphery, very sensitive, respond to single photon, high convergence onto bipolar cells, pool signals from 15-30 rods
- high sens low spatial, most numerous
- cones in fovea, less sensitive, need 100 photons to elicit response, high acuity, high spatial resolution, low sens, color
scotropic
rod only -high sens -low acuity no color -dark
photopic
- cone only
- low sens, high acuity
- color
mesopic
cone and rod together
retinitis pigmentosa
- a group of genetic eye conditions that lead to incurable blindness 1:4000
- night blindness, tunnel vision, legally blind by 40, loss of ERG
- mutation of genes for rhodopsin and other rod proteins (PDE, cGMP gated channels, RDS/peripherin) leading to degeneration of rods and eventually cones
- over 100 gene defecs
- no treatment
age related macular degeneration
- leading cause of vision loss, 10% of people over 50, 33% of people over 75
- wet- abnomal blood vessels behind the retina grow under the macula, leaking and rapidly damaging the retina
- dry-RPE and PR of the macula degenerate, accumulation of drusen, 85%
- loss of central vision and acuity
- disease encroaches on freedom of elderly and mobility
- risk- aging, smoking, inheritence
- local IF and complement
- treatment- wet- laser coag of vessels and intravitreal injection of anti-neovascular agents
- dry- gradual atrophy of central retina, antioxidants slow
diabetic retinopathy
- retinopathy from complications of DM
- risk- up to 80% of all patients who have had diabetes for 10 years or more
- no early warning signs
- blurry vision with macular edema
- new vessels bleed into retina and block vision
- non proliferative-hyperglycemia induced pericyte death leads to incompetence of the vascular walls, micro aneurysms and dot and blot hemorrhages
- vascular beading and cotton wool spots
- proliferative stage-new, fragile vessels grow, which leak blood
- laser surgery to reduce edema and injections with anti-neovascular factors
spatial contrast
-visual system is good at differentiating light/dark
receptive field
- area of the retina that when stimulated elicits a change in the response of a neuron
- on center and off center
on center/off surround receptive field
- light in center- excitation (depol)
- light in surround- inhibition (hyperpol)
- an optimal excitatory stimulus is a spot of light against a dark background
- an optimal inhibitory stimulus is a dark spot against a light background
- APs increase when light in center, decrease when in surround
off center/ on surround
- light in center- inhibition (hyperpol)
- light in surround- excitation (depol)
- optimal excitatory- dark spot against light
- optimal inhibitory- light spot against dark
on center moving from dark to light
-first goes down because more inhibitory, then increases when center in light, then back down
on/off ganglion and bipolar cells
- on BCs depolarize in response to light, hyperpolarize in dark, off hyperpol in response to light, depol in dark
- glutamate receptors on bipolar cells determine the on off center properties of the BCs and those of the ganglion cells they innervate
- graded depolarizations of BCs lead to increased glutamate release at their synapses and depolarization of the GC they contact
- if cone hyperpolarizes (response to light), off center bipolar cell gets same sign signal and hyperpolarizes, then off center ganglion doesn’t fire AP
- if cone hyperpolarizes, on center bipolar cell gets opposite sign signal and BC depolarizes, ganglion cell fires APs
receptive field construction
- horizontal cells are post-synaptic to photoreceptors
- if cone hyperpolarizes, horizontal cells get signal and send opposite back
- graded potentials
- lateral inhibition to bipolar cells
- OPL by horizontal cells, in IPL by amacrine
- horizontal cells form the surround
- see slide
color
-short, medium, long wavelength cones are basis for color (blue, green, red)
cone opsins
- G protein coupled receptor superfamily of signal transducing receptors
- 7 transmembrane domains
- tunes absorption of light to a particular region of the spectrum
- color opsins defined by different proteins
color blindness
-absence of one or more cone classes are responsible for common color blindness
anatomical parallel pathways
- M cells and P cells different kinds of cones for different types of light
- parvocellular and magnocellular
- koniocellular pathway
- P cells red/green off on, M blue/yellow
intrinsically photosensitive retinal ganglion cells
- contain primitive opsin
- autonomous response to bright lights
- vast dendritic trees
- pupillary control
- other roles include synchronization of circadian rhythms