Dark adaptometry and electrophysiology

Question 1

What happens during rod activation?

Answer 1

When a photoreceptor is activated by light there is a rapid hyperpolarisation of the cell membrane which stops the release of glutamate from the synapse, thereby activating the visual pathway.

Question 2

What happens during the phototransduction cascade?

Answer 2

• On absorption of a photon of light, a receptor protein R is activated to R*
• Causing activation of the G-protein to G*
• Which in turn activates, the effector protein E to E*
• Its enzymatic activity causes hydrolysis of cGMP which leads to a decrease in cGMP concentration

Question 3

What is the final element of the phototransduction cascade?

Answer 3

The final element of the cascade is the closure of the cGMP-gated channel, which results in decrease in the influx of ions, and reduction in circulating circuit

Question 4

What happens during recovery?

Answer 4

• Under extremely bright conditions, rhodopsin is bleached and for the bleached rhodopsin molecule to absorb another photon, the pigment must be regenerated.

Question 5

What is the regeneration process?

Answer 5

It is a slow process and takes around 30-60 mins when a significant percentage of the pigment has been bleached in rods

Question 6

What is the process of recycling photopigments?

Answer 6

Rods shed their discs containing bleached Rh and these discs are phagocytes by the RPE and the pigment recycled.
- Rods shed at dawn
- Cones shed at dusk

Question 7

What happens during dark adaption?

Answer 7

1. There is a immediately a loss of inhibition of cones and rods by horizontal cells
2. Next the intracellular calcium, cGMP return to dark levels and photoreceptors rest their threshold ( 5 mins for cones, 30 mins for rods)
3. Over time photopigment is renewed in cones and rods they reach threshold sensitivity (Hours overnight)

Question 8

How does intensity of pre adapting light affect dark adaption?

Answer 8

The brighter the light the more pronounced the rod cone break and longer the delay of the break

Question 9

How does the length of the pre-adapting light affect DA?

Answer 9

The longer the light exposure the longer the time to threshold and the more pronounced the rod cone break

Question 10

How does retinal eccentricity affect DA?

Answer 10

• Break occurs earlier with greater eccentricity
• At the fovea there is no break

Question 11

Why are wavelength stimulus’ important for DA?

Answer 11

For long wavelengths there is no rod cone break

Question 12

What happens with DA and diseases?

Answer 12

Any disease that involves RPE dysfunction, photoreceptor alterations will result in delayed or reduced dark adaption

Question 13

Retinal Pigmentosa and DA?

Answer 13

• Hereditary
• Night blindness early onset (childhood)
• Characterised by disc pallor, bone spicule pigmentation, arteriolar narrowing
• Constriction of VF to central + small peripheral island
• DA delayed rod coe break and decreased absolute sensitivity

Question 14

What happens with diabetes and DA?

Answer 14

The underlying mechanism of DR is thought to be metabolic load placed on retinal cells lack of insulin and the increased level of blood glucose.

Rods have a uniquely high metabolic rate, demanded by the processes which enable them to signal the absorption of single quanta

Question 15

CSNB Abnormal funds
Oguchi’s disease and DA?

Answer 15

• Yellow/green shape appearance of the retina which disappears after dark adaption
• Cone function is normal
• There is delayed DA but normal threshold reached after 4 hours in the dark

Question 16

CSNB Fundus
Albipunctans and DA?

Answer 16

• White dots all over the fundus, but sparing macula
• Delayed cone and rod adaption
• Cause is 11-cis retinol dehydrogenase which converts 11-cis retinol to 11-cis retinal

Question 17

How to measure the dipole at the eye?

Answer 17

Place an electrode at the front of the eye (cornea) and on a part of the eye that is insulated from the cornea (sclera, fornix) then you can get a signal. If you use a differential signal to a reference (head, finger, nose) then you can get a cleaner signal

Question 18

What are the differences in electrodes?

Answer 18

• One electrode measures signal of interest + noise
• The other just measures noise
  when you subtract one from the other an you reduce noise and end up with clean signal

Question 19

What happens during a flicker ERG?

Answer 19

• If the stimulus flickers then the ERG response comes to a steady state oscillation.
• The speed at which the retina can continue to respond to flicker is important