Week 2 - Retinal Phototransduction and Signal Processing Flashcards
sclera
- what is it?
- what does it do?
relatively spherical and avascular, white dense connective tissue that covers globe posterior to cornea
- provides strong tough external framework to protect delicate optic and neural structures
- maintains shape of globe so retinal image is undisturbed and provides attachment for extraocular muscles to rotate globe and ciliary muscle to accommodate lens
cornea
- what is it?
- what does it do?
“window of the eye”
- mechanically strong and transparent connective tissue that covers anterior 1/6 of eye
- most powerful focusing element of the eye, roughly twice as powerful as lens
- responsible for 80% of refraction in eye
lens
- what is it?
- what does it do?
- what does it contain?
specialized epithelial tissue that is responsible for fine-tuning image that is projected on retina
- lies inside eye surrounded by aqueous humor
- transparent and has high refractive power
- elastin-based zonular fibrils stabilize lens and allow accommodation to occur
what is the uveal tract made of?
consists of 3 structures
- choroid
- ciliary body
- iris
choroid
capillary bed nourishing photoreceptors and outer retina
ciliary body
has two parts:
- ciliary muscle: controls refractive power of lens
- vascular component: produces aqueous humor filling anterior chamber
iris
colored part of eye seen through cornea
-has 2 sets of muscles with opposing actions that allow size of pupil (opening at center) to be adjusted by neural control
anterior chamber
volume behind cornea and in front of lens
-filled with aqueous humor
posterior chamber
region between vitreous and lens
aqueous humor
-production and flow
clear watery liquid that nourishes cornea and lens
- produced by vascular component of ciliary body/epithelium lining ciliary processes
- flows around lens and through pupil into anterior chamber
- leaves eye by passive flow at anterior chamber angle
vitreous humor
thick gelatinous substance filling space between back of lens and surface of retina
retina
contains neurons that absorb light and process visual info in images and send that info to the brain
macula
oval spot containing a yellow pigment (xantophyl)
-supports high acuity
fovea
- what makes it special?
- what is the visual angle?
small depression at center of macula
- has highest spatial acuity by pushing away ganglion cells, IPL, and INL
- visual angle subtended is 0.5 degrees (full moon or thumb nail at arm distance)
- involves 0.01% of retinal area, but 10-50% of optic nerve
- no S cones or rods
optic disk
whitish circular area where retinal axons leave eye and travel through optic nerve to targets in midbrain and thalamus
-site where blood vessels supply inner retina enter eye
blood supply of eye
ocular vessels are derived from ophthalmic artery (from internal carotid) divides into 2 vascular systems
- anterior segment = iris and ciliary body
- retinal systems
vascularization of anterior segment
originates from anterior ciliary arteries and long posterior ciliary arteries
-penetrating vessels through sclera vascularize iris and ciliary body
retinal blood supply
delivery of metabolic substrates and O2 to retina is accomplished by two separate vascular systems: inner retinal and choroidal
-retinal and choroidal vessels differ morphologically and functionally from each other
cataracts
- what it is
- risk factors
- symptoms
- treatment
clouding of lens that affects vision, mostly related to aging, and leading cause of blindness worldwide (50% Americans have/had cataracts by 80 yo.)
- RF: aging, diabetes, sunlight, smoking
- symptoms: hazy vision, poor night vision, glare, faded colors
- treatment: surgical removal of cloudy lens, replacement with artificial lens; very little recovery time required after surgery
how are cataracts formed?
disruption in order of organization of lens cell fibers, or aggregation of PRO within them, can destroy transparency of cell
glaucoma
- what it is
- risk factors
- symptoms
- treatment
group of diseases that damage eye’s optic nerve and result in loss of peripheral vision fields
- RF: elevated eye pressure (from poor drainage of aqueous humor), thin cornea, abnormal optic nerve anatomy, HTN
- symptoms: none until too late (loss of peripheral visual fields)
- TRT: eye drops to decrease aqueous production and/or increase drainage
types of glaucoma
- normal tension
- open angle
- closed angle
difference between open and close angle glaucoma
open: slow development of pathology
- caused by obstruction of drainage canals
close: sudden increase in intraocular pressure due to collapse of wall blocking drainage (more severe)
- closed or narrow angle between iris and cornea
where to photoreceptors point?
towards the back of the eye
how does light enter the eye?
light comes in first hitting the ganglion cell layer, the rest of the inner retina, then photoreceptor (PR) nuclei and inner segments before PR outer segments where phototransduction occurs
- thus light must travel through thickness of retina before striking and activating rods and cones, so absorption of photons by visual pigment of photoreceptors is translated into first biochemical message into electrical message that can stimulate all succeeding neurons of retina
- -retinal message concerning photic input is transmitted to brain by spiking discharge pattern of ganglion cells via optic nerve
what are the 7 layers of the retina? what kind of cells do they have?
3 nuclear (cell soma), 2 plexiform (synaptic connections), 1 nerve fiber layer
- photoreceptor outer segments (OS)
- outer nuclear layer (ONL): photoreceptor somas
- outer plexiform layer (OPL): photoreceptor / bipolar / horizontal cell synapse
- inner nuclear layer (INL): horizontal, bipolar, and amacrine cell somas
- inner plexiform layer (IPL): bipolar / amacrine / ganglion cell synapses
- ganglion cell layer (GCL): ganglion cell somas
- nerve fiber layer (NFL)
what is the pigmented epithelium made of? what do they do?
melanin-containing cells behind photoreceptors
- acts as backstop for light
- maintains phototransduction machinery of photoreceptors by recycling of PR discs
- pigment regeneration
- photoreceptor nnourishment
what are examples of retinal glia cells and where are they?
astrocytes - neurovascular
microglia - immune system
Mueller cells - radial glia-like in ionic milieu, guidance during development
how does vertical information flow?
photoreceptors –> bipolar cells –> ganglion cells
how does lateral information flow?
horizontal cells and amacrine cells
how does vertical neurotransmission occur?
cells along vertical path (photoreceptors, bipolar, and ganglion cells) release glutamate
how does horizontal neurotransmission occur?
cells mediating lateral information transmission (horizontal and amacrine cells) release mostly GABA and glycineric
how many rods or cones do one bipolar cell contact?
1 bipolar cell contacts many rods, but only 1 cone
-but one cone will target 2 types of bipolar cells (one inverse and one regular)