Light Adaption/Transduction Pathway Flashcards
Conditions of testing Absolute Threshold
Young, Healthy Trained subjects Method of Constant Stimuli Monocular Viewing Complete Dark adaption Stimulus presented on region of highest rod density
What color is used for a fixated target during Absolute threshold testing and why?
Red, because neither rods or cones are very sensitive to red so it maintains dark adaption.
Why is the method of constant stimuli used for absolute threshold testing?
It has maximum reliability and validity
Under photopic conditions, what wavelength is most easily absorbed? What color is that?
555 nm, green
Under scotopic conditions, what is the most easily absorbed wavelength? What color is that?
507 nm, bluish
Why does red light not bleach the photoreceptors?
Red light is not absorbed well under photopic or scotopic conditions so it does not change the photoreceptors much.
What are the test stimuli characteristics when performing Absolute threshold testing?
- 510 nm wavelength (near max scotopic sensitivity)
- 10 arc minute diameter (smaller than scotopic critical diameter)
- 1 millisecond duration (shorter than scotopic critical duration)
- 20 degree temporal retinal position (maximum rod density location)
- Red fixation light (minimize light adaption/bleaching)
- Maxwellian View Presentation (retinal illuminance of stimulus not affected by pupil diameter)
What must the eye be presented with in order to detect light with 60% reliability in a fully dark-adapted environment?
The eye must be presented with 90 quanta (photons) on the cornea and 8-10 quanta (photons) within the retinal critical duration for temporal summation and critical area for spatial summation.
What does the Poisson Distribution refer to?
Poisson Distribution referes to the quantal fluctuations of a flash. There is no way of getting the exact number of quanta that you want each time. The distribution shows the probability of the the number of photons that will be present per flash.
How many photons are needed to produce a statistically significant response in a rod?
1
Where is the highest density of rods located?
18 degree or 4.5 mm from fovea center
Where is the highest density of L and M cones?
In fovea
Where is highest density of S cones?
1-2 degrees from fovea
Are cones low or high gain?
Low gain
What is rhodopsin?
It is a seven-transmembrane receptor that is the visual pigment of rods (visual purple). It is embedded in the discs’ membrane. Composed of two parts, opsin (membrane bound G protein-coupled receptor) and a chromophore, retinal.
What determines the spectral absorption of a molecule?
differences in the opsin portion of a molecule
What 2 things make up a visual pigment?
Visual pigment = opsin + chromophore
What gives a molecule color?
Chromophore. It absorbs certain wavelengths of light.
Explain the role of Vitamin A in vision?
You get vitamin A from your diet. It gets broken down in the liver from beta carotenes to form All-trans retinol. This then travels to the RPE by choriocapillaries. Once there, it undergoes many reactions to form 11-cis retinal. If there is too much 11-cis retinal, it can be stored in the RPE. IRBP (interstitial retinal binding protein) then helps 11-cis retinal get to the rod’s outer segment by passing through the interphotoreceptor space. It is then bound with opsin. Once light is present, it changes conformation to all-trans retinal. Opsin and all-trans retinal dissociate from each other. All-trans retinal is then converted back to all-trans retinol by retinol dehydrogenase (photopigment isomerization) and gets carried back through interphotorecptor space in to the RPE by IRBP. The process is repeated.
During photo-activation of rhodopsin, what wavelength and state is rhodopsin in that activates G-protein in the photoreceptor disc membrane? (activated rhodopsin, R*)
It is in the Metarhodopsin II state at 380 nm. (ultraviolet light and bleached)
What is the membrane potential of a photoreceptor in the dark?
About -40 to -50 mV
What occurs in the pathway when rhodopsin is activated? (light on)
When rhodopsin is activated in the disc membrane, it will bind to a G-protein that has an alpha, beta, and gamma subunit (Transducin) attached to a GDP molecule. The G-protein then becomes activated by attaching a GTP on its alpha subunit. Once this happens, the alpha subunit breaks away from the activated rhodopsin, beta subunit, and gamma subunit. The aplha G-protein then interacts with a Phosphodiesterase molecule (PDE). This molecule has an alpha, beta, and two gamma subunits. The activated G-protein removes a gamma subunit from the PDE exposing a catalytic site. This catalytic site can then bind to a cGMP molecule and convert it into GMP. The decrease in cGMP will result in a closure of cGMP gated channels and a hyperpolarization of the photoreceptor.
How long does it take for G-protein (Transducin) to interact with rhodopsin and dissociate?
0.1 ms
By activating one rhodopsin molecule, what is created?
700 activated Transducin molecules (G-protein)
A reduction of 1400 cGMP in the outer segment
Transmembrane potential hyper polarizes and decreases by 1 mv.
A net closure of about 230 cGMP channels (2% of the channels in the entire outer segment that are open in the dark)
