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Phototransduction - Coggle Diagram
Phototransduction
Adaptation
Dark adaptation
getting used to the dark :sunny: :arrow_right: :new_moon_with_face:
increased (millionfold) sensitivity to light
dilation of the pupils (small effect)
regeneration of unbleached rhodopsin and adjustment of the functional circuitry of the retina
Light adaptation
getting used to light :new_moon_with_face: :arrow_right: :sunny:
Rods
light bleaches photopigment
activates G-proteins
activate an effector enzyme
changes the cytoplasmic concentration of a second messenger molecule (decrease)
membrane ion channel
closes
altered MP
less Na+
more negative MP
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Na+
usually keeps photoreceptor MP at –30mV
cGMP
PDE
transducin
rhodopsin
opsin (GPCR) + retinal
Cones
light bleaches photopigment
require more energy to become bleached :sunny:
In bright light, cGMP levels in rods fall to the point where the response to light becomes
saturated
can't hyperpolarize any further
vision during the day depends entirely on the cones
initially cones are hyperpolarized as much as possible
gradual
depolarization
of the membrane back to about –35 mV
cGMP-gated Na+ channels also admit calcium
Ca2+ has an
inhibitory
effect on guanylyl cyclase in the dark (when Na+ channels open)
when channels close :bulb:, less flow of Ca2+
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after adjusting, we are able to register
relative
changes in light level
photoreceptor bleaching and other adaptational mechanisms, such as calcium’s influence on cGMP, can occur on a
cone-by-cone basis.
contains one of three opsins that give the photopigments different spectral sensitivities
short-wavelength
medium-wavelength
long-wavelength
ratios of activations determine color
Young–Helmholtz trichromacy theory
rest of mechanism the same as rods
How do the different photoreceptors convert, or
transduce
, light energy into changes in membrane potential?