Visual cycle,Dark adaptation, light adaptation

Hello learners, I hope all of you are having a good time and have a good eye to read my article. Good eye !!!!!!! That may be seems funny but soon you will understand why I have said that. Actually today I want to discuss about how vitamin A helps in vision and how deficiency of vitamin A in diet can cause night blindness. Lets proceed-

( do you want to learn more about vitamin A
Have a look to my another 2 articles about vitamin A
1. Overview of vitamin A

https://read.cash/@Saleh123/overview-of-vitamin-a-967d40dd

2. Functions of vitamin A

https://read.cash/@Saleh123/functions-of-vitamin-a-8534939b)


***Visual cycle (rhodopsin cycle):How vitamin A helps in vision and prevent night blindness***

1. Visual process involves vitamin A containing visual pigments present in photoreceptor cells( rods and cons ) of retina.

2. Visual pigments are basically conjugated proteins consisting of opsin(protein) and 11-cis retinal (Prosthetic group).

3. Vsiual pigments present in rods Is called rhodopsin which is most light sensitive and helps in dim light vision.
*(Mnemonic: remember ro(d) =(d)im light )

4. Visual pigments present in cons is called Iodopsin which is responsible for bright light vision and color vision.
*(Mnemonic: remember (c)ons =(c)olor vision )


5. In response to light waves visual pigments(rhodopsin, iodopsin ) belaches to opsin and all trans retinal through a series of photochemical isomerizations.
This photolysis of visual pigments initiates visual signals (nerve impulse) in photoreceptor cells that finally helps in vision.

6. Most of the all trans retinal produced by bleaching of visual pigment is isomerized back to 11 cis retinal directly or indirectly via retinol to resynthesize the visual pigment. Part of the all trans retinal is lost and not available for resynthesis of the visual pigments, so it needs to be make up from dietary vitamin A in the form of all trans retinol which is isomerized to 11-cis retinol and then dehydrogenated to 11- cis retinal to form visual pigments.

7. Therefore, continual supply of retinol is essential for normal visual function.
Retinol deficiency depresses the resynthesis of visual pigment and interferes with the function of photoreceptors particularly rods causing night blindness.

Now lets discuss about another important topic(light adaptation / dark adaptation)


Dark adaptation/ light adaptation:
1. If a person spends a considerable length of time in bright light and then moves to a dim light environment, gradually visual pigments(rhodopsin) concentration in rods and cons increases. As a result visual threshold decreses and the retina slowly become more sensitive to light so that it can respond to the light of low intensity to allow vision. This is called dark adaptation.

2. In bright light,visual pigments(rhodopsin) in rods and cons undergo photolysis(bleaching) to retinal and opsin reducing its concentration. This causes reduction of the sensitivity of eye to light that raises visual threshold so that eye can respond to the light of high intensity. This is known as light adaptation.

3. When individual goes to dim light the retinal and opsin recombine to build up rhodopsin store necessary for the optimal function of rods and cones at low intensity light. The time needed to build up the necessary rhodopsin store is the time required for dark adaptation.