Phototransduction Flashcards
How is the dark current generated? (Unusually positive RMP)
The presence of cyclic nucleotide gated ion channels that are held open by high resting levels of cGMP.
This creates a continuous inward current carried by Na and Ca, holding the membrane at a relatively depolarized membrane potential.
What happens when a photoreceptor is stimulated with light?
The membrane hyperpolarizes and the rate of neurotransmitter release declines in proporiton to the degree of hyperpolarization
Describe the pathway of hyperpolarization of a photoreceptor in response to light.
Retinal absorbs light and changes conformation
Change in retinal, induces confromational change in opsin
Opsin activates transducin, which activates phosphodiesterase
PDE degenerates cGMP and closes CNG channels
What two parts compose the photopigment in rods and what are their functions?
Retinal - light absorbing chromophore
Opsin - Retinal binding protein that tunes retinal’s absorbance to a particular region of the light specturm
What is the main advantage to signal cascade in phototransduction?
Signal amplification
Absorbance of one photon can close 2% of CNG channels
What are the three different classes of cones?
Short (Blue) wavelength sensitive
Intermediate (Green) wavelength sensitive
Long (red) wavelength sensitive
What differentiates the different types of cones?
The type of opsin molecule that it expresses
What is the only way to determine the wavelength of light that a cone is sensitive to?
Compare the output of different classes of cone
There is considerable overlap, so the response of a single cone cannot provide information about the wavelength of light
What is different between rods and cones in regards to light sensitivity?
Cones are much less sensitive to light, making them best suited for vision under bright light
What is dyschromatopsia?
Color blindness
What is the most common cause of dyschromatopsia?
Inherited sex-linked alteration in one of the genes coding for the red or green cone photopigments
What is a less common type of dyschromatopsia?
Alteration affecting the blue cone photopigment
What is light adaptation and how is it controlled?
As illumination level is steadily increased, light sensitivity declines
Controlled by intracellular calcium concentration - low calcium = low sensitivity
What is scotopic vision?
Completely rod-mediated
Low levels of illumination
What is mesopic vision?
Level of illumination where rods and cones are both activated
