L3 pt 2 Vision Flashcards
Retina
innermost layer, contains neurons sensitive to light and transmit via central targets
Uveal tract
compoased of choroid, ciliary body, and iris
Choroid
rich capillary bed nourishes photoreceptors, high concentration of light absorbing pigment melanin
Ciliary body
ring of tissue encircling the lens
has a muscular and vascular portion
Iris
colored portion of the eye seen through the cornea
controls pupillary constriction and dilation
Sclera
outermost tissue layer and comprised of white fibrous tissue
Cornea
outer layer in front of eye with specialized transparent tissue that permits light rays to enter the eye
Humors of eye
Aqueous = front of eye
Vitreous = back of lens and retina
Accommodation
dynamic changes in the shape of the lens
Distance = lens is thin and flat, has least refractive power. high tension in zonule fibers
Near = lens is thick and round, most refractive power, relaxed zonule fibers, contracted ciliary muscle
How does accommodation occur?
tension of the ciliary muscle that surrounds the lens and zonule fibers
elasticity of the lens and tension of zonule fibers create counteracting forces
Adjustments of the pupil
help to clarify images and adjust how much light enters the eye
controlled by sympathetic and parasympathetic NS
Zonule fibers
radially arranged connective tissues bands that attach to the ciliary muscle
Which nervous system is the retina a part of?
central
forms from the diencephalon
Retinal Circuitry
retina converts graded electrical activity of photoreceptors (rods and cones) into action potentials . possible b/c of short distances
pathway goes from photoreceptor to bipolar to ganglion (projection neuron) cell located in optic nerve
Phototransduction
absorption of light by photopigment in photoreceptors initiates a cascade of events that changes the membrane potential of the receptor, causing NT release
Horizontal cells
allow for lateral interactions between photoreceptors and bipolar cells
help with sensitivity to light/contrast
Pigment Epithelium
light rays must pass through various non-light-sensitive elements of the retina and vasculature before reaching photoreceptors.
reason for it: provides vasculature for photoreceptors, helps to generate new cells and remove old ones
Hyperpolarization and photoreceptors
light stimulus causes hyperpolarization and not depolarization
causes membrane to go from -40 to -65
hyperpolarization causes the number of Ca2+ channels opened to be reduced, and rate of NT is reduced.
Rods
low spatial resolution
sensitive to light
good for low light
multiple inputs to one ganglion, making it better at detecting light
more rods than cones
greater density in retina
less photons cause a reaction vs cones
Cones
very high spatial resolution
insensitive to light
good for color
only one cone per ganglion, making it better at spatial resolution
greater density in fovea
responses to light are more quickly adapted in cones vs rods because it does not saturate at high levels of steady illumination–> membrane potential of individual cones still vary
Cones and colors
-three types of cones, each defined by the photopigment they contain
-each is differentially sensitive to different wavelengths
-cones are colorblind, just reactive to different amounts of photons
Blue: short
Green: medium
Red: long
On/Off Retinal Cells
On center = burst of action potentials with light directly on it
Off center = burst of action potential with light off
help to detect edges in vision
On-center/Off-center Bipolar cells
help with the perception of contrast
both types of cells get the same input, more glutamate in the dark, less glutamate in light
Off center bipolar cells
Are excited by baseline glutamate release; light/decrease in glutamate causes these to become more hyperpolarized
have ionotropic receptors that cause bipolar cells to depolarize in response to INCREASED OR BASELINE glutamate release
On center bipolar cells
express a G-protein that when bound to glutamate, close sodium channels and hyperpolarize the cell
decreased levels of glutamate cause depolarization
- Light enters IN THE CENTER, causing decreased glutamate
- Reduction of inhibitory signals on on-center bipolar cells
- Allows for transmission of excitatory signals to on-center ganglion cells
Luminance contrast
differences in level of illumination that falls on the receptive field center and the level of illumination that falls on the surround
most ganglion cells respond better to small spots of light confined to the receptive field vs uniform illumination
Horizontal cells and glutamate
these cells influence the amount of glutamate the photoreceptors will release onto bipolar cell dendrites
glutamate released by photoreceptors depolarizes horizontal cells, Horizontal cells have a hyperpolarizing effect on photoreceptors
overall effect is to oppose changes in the membrane potential of photoreceptors that are induced in phototransduction events in outer segment
Decreased glutamate and horizontal cells
decreased glutamate from on-center light = reduces inhibitory feedback from horizontal cells onto bipolar cells
provide feedback inhibition to on-center receptors that can be changed with decreased glutamate
