PSL300H1 Study Guide - Final Guide: Retinal Pigment Epithelium, Visual Acuity, Visual Phototransduction
ProfessorHae- Young Kee
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phototransduction → light
cones and rods
For humans, visible light between 400-700 nm.
phototransduction = conversion of light energy into electrical engergy, occurs on retina in
photoreceptors. → densely packed on area of retina called macula. resulting in most visual acuity.
optic disk is medial to the macula, optic nerve and retinal veins exit the eyeball, and artery enters. no
photoreceptors here → eyeball's blind spot.
photoceptors are on the outer later on the retina. other neurons and their axons are in between the
photoreceptors and light.
melanin in pigment epithelium absorbs rays that escape photoreceptors, preventing reflected rays from
ruining retinal image. choroid layer → contains blood vessels.
2 main types of photoreceptors:
cones : high acuity and colour vision in bright light. most densely packed in fovea, pit in middle of macula
with only cones. and no other cells to block the light. used for visual 2-point discrimination. (side note:
accessory cells are pulled away from fovea to maximize light).
rods : more sensitive. function in low light. out number comes 20:1, discriminate shape.
both have same basic structure: outer segment → membrane folds like disk-like layers (in rods, outermost
disks detached from cell). disk contain visual pigments that transduce light into changes in membrane
in inner segment: nucleus and organelles for ATP and protein synthesis, and in a basal layer, synapse that
releases glutamate ( the only NT at this point ).
visual pigments are bound to the disk membranes: rods visual pigment = rhodopsin → 2 molecules :
opsin (protein in membrane) and retinal ( vit. A derivative, light absorbing part)
in darkness → retinal binds opsin. single photon can make retinal shape shapes and release opsin →
bleaching, altering MP.
disks are packed with rhodopsin, and can be separated into opsin. retinal can change conformation,
whether absorbs light or not, can undergo a conformational change.
three types of cones have 3 pigments, all similar to rhodopsin.
phototransduction involves 3 cations channels in the photoreceptor membrane: K+, CNG and Ca2+ :
in darkness → cGMP (binds to CNG) levels are high in the rod cytosol.
K+ and Cyclic Nucleotide-Gated channels are open, rod slightly depolarized to -40 mV.
results in Ca2+ channels opening in the synaptic terminal, so glutamate released onto bipolar cells.
light cleaves rhodopsin, starting a 2nd messenger cascade via transducin:
light hits rhodopsin → conf change → transducin activated (G-protein) → activates phosphodiesterase →
[cGMP] falls, CNG channels close, slows/stops influx of K+ cations, K+ efflux continues. membrane
potential falls to -70mV, less glutamate released. activate retinal diffuses out of rod, transported into
in recovery phase, retinal combines with opsin:
retinal goes back to inactive form in pigment epithelium, returns to rod and recombines with opsin →
forming inactive rhodopsin.
recovery can take a while, which is why eyes adapt slowly when we move from light to dark.
VISION - PHOTOTRANSDUCTION
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