Light Adaption/Transduction Pathway

Question 1

Conditions of testing Absolute Threshold

Answer 1

Young, Healthy Trained subjects
Method of Constant Stimuli
Monocular Viewing
Complete Dark adaption
Stimulus presented on region of highest rod density

Question 2

What color is used for a fixated target during Absolute threshold testing and why?

Answer 2

Red, because neither rods or cones are very sensitive to red so it maintains dark adaption.

Question 3

Why is the method of constant stimuli used for absolute threshold testing?

Answer 3

It has maximum reliability and validity

Question 4

Under photopic conditions, what wavelength is most easily absorbed? What color is that?

Answer 4

555 nm, green

Question 5

Under scotopic conditions, what is the most easily absorbed wavelength? What color is that?

Answer 5

507 nm, bluish

Question 6

Why does red light not bleach the photoreceptors?

Answer 6

Red light is not absorbed well under photopic or scotopic conditions so it does not change the photoreceptors much.

Question 7

What are the test stimuli characteristics when performing Absolute threshold testing?

Answer 7

• 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)

Question 8

What must the eye be presented with in order to detect light with 60% reliability in a fully dark-adapted environment?

Answer 8

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.

Question 9

What does the Poisson Distribution refer to?

Answer 9

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.

Question 10

How many photons are needed to produce a statistically significant response in a rod?

Answer 10

1

Question 11

Where is the highest density of rods located?

Answer 11

18 degree or 4.5 mm from fovea center

Question 12

Where is the highest density of L and M cones?

Answer 12

In fovea

Question 13

Where is highest density of S cones?

Answer 13

1-2 degrees from fovea

Question 14

Are cones low or high gain?

Answer 14

Low gain

Question 15

What is rhodopsin?

Answer 15

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.

Question 16

What determines the spectral absorption of a molecule?

Answer 16

differences in the opsin portion of a molecule

Question 17

What 2 things make up a visual pigment?

Answer 17

Visual pigment = opsin + chromophore

Question 18

What gives a molecule color?

Answer 18

Chromophore. It absorbs certain wavelengths of light.

Question 19

Explain the role of Vitamin A in vision?

Answer 19

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.

Question 20

During photo-activation of rhodopsin, what wavelength and state is rhodopsin in that activates G-protein in the photoreceptor disc membrane? (activated rhodopsin, R*)

Answer 20

It is in the Metarhodopsin II state at 380 nm. (ultraviolet light and bleached)

Question 21

What is the membrane potential of a photoreceptor in the dark?

Answer 21

About -40 to -50 mV

Question 22

What occurs in the pathway when rhodopsin is activated? (light on)

Answer 22

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.

Question 23

How long does it take for G-protein (Transducin) to interact with rhodopsin and dissociate?

Answer 23

0.1 ms

Question 24

By activating one rhodopsin molecule, what is created?

Answer 24

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)

Question 25

What kind of channels are activated by cGMP and allow the influx of Ca++ and Na+?

Answer 25

Cyclic nucleotide-gated cation channels

Question 26

What ions determine the polarization of a photoreceptor? Which ions come into the photoreceptor? Which ions flow out of the photoreceptor?

Answer 26

Caa+, Na+ flow in

K+ flows out

Question 27

Referring to ions, what happens in the photoreceptor membrane when the lights are on?

Answer 27

The cGMP-gated channels close which inhibits the influx of Ca++ and Na+ into the photoreceptor. K+ continues to get pumped out of the cell which causes the interior to become more negative resulting in hyperpolarization. The reduction of Ca++ inside the cell inhibits the release of Glutamate (by inhibiting synaptic vesicle exocytosis) into the synaptic cleft.

Question 28

Referring to ions, what happens in the photoreceptor membrane when the lights are off?

Answer 28

The cGMP-gated channels are open which allows the influx of Ca++ and Na+ into the photoreceptor. K+ gets pumped out of the cell but at a slower rate than Ca++ and Na+ coming in, which causes the interior to become more positive resulting in depolarization. The increase of Ca++ inside the cell influences the release of more Glutamate into the synaptic cleft.

Question 29

General steps that occur within an activated photoreceptor (5)

Answer 29

1) Photoactivation- photon is absorbed by a visual pigment mole-cule on a disc in the outer segment.
2) Biochemical cascade- activated photopigment initiates reactions that close 
 some ion channels on the cell membrane.

3 )Electronic spread- closure of ionic channels decreases inward current, 
 creating net outward current, resulting in a hyperpolarization of the cell membrane (more negative charge).

4) Synaptic deactivation- at synaptic terminal, calcium channels CLOSE when 
 cell hyperpolarizes (and open when cell depolarizes). Thus, hyperpolarization causes a decrease in the internal concentration of free calcium ion.
5) Decrease in glutamate release- In the presence of calcium, synaptic vesicles 
 continuously release glutamate into cleft. A decrease in internal concentration of 
 calcium leads to a decrease in the rate of release of glutamate at the terminal.

Question 30

What are inter-photoreceptor gap junctions? Who has them? What do they do?

Answer 30

Inter-photoreceptor gap junctions are electrical connections among neighboring receptors. They are found in rods, L and M cones but not S cones. They function to increase sensitivity by signal averaging. Signal averaging reduces the random noise by the inverse of the square root of the number of signals averaged.

Example: How much would 16 random signals be reduced to by using signal averaging?

√16 = 4. 1/4= 25%

Question 31

Dark-adapted cones are about ____ times less sensitive than dark-adapted rods.

Answer 31

100 times

Question 32

The retina can operate over about a _____ unit change
in ambient environmental illumination.

Answer 32

14 log

Question 33

Cones respond over a ______ range than rods do.

Answer 33

Broader

Question 34

What percent of rods are bleached when rod responses are saturated?

Answer 34

10%

Question 35

Describe rod and cone saturation. What lighting conditions would make rods saturated? Why?

Answer 35

When light is on because more rhodopsin would interact with photons thus closing the cGMP gated ion channels. When the environmental luminance keeps increasing, the rods can’t continue their response and would plateau and would have little change. The cones response would then take over as they are not fully saturated.