How We See 2 - Transduction Flashcards
What is phototransduction?
The conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
What is a lamellae?
They are cell membranes that contain the visual pigments of rods and cones
What are the visual pigments in the lamellae of rods and cones?
Why are there different pigments?
Rhodopsin in rods
Cone opsins - S, M and L
Different pigments for different light wavelengths
What happens when light hits rhodopsin/any opsin?
The conformation of the vitamin A chromophore resting in the opsin changes from cis to trans - the trans conformation does not fit into the opsin and so the opsin splits - releasing the vitamin A compound
What does release of vitamin A compound from rhodopsin cause?
Causes a phototransduction cascade that results in Na gates of the photoreceptor cell closing
This causes hyperpolarization of the cell, transmitted by flux of Ca ions at the synapse - results in stimulation of retinal cell (action potential)
How does visual pigment regeneration occur?
ATP used to reverse the steps of the phototransduction cascade
Pigment epithelial cell under the rods and cone layer then reverses the cis-trans reaction and the trans compound goes back into the lamellae
Why is sufficient intake of vitamin A necessary to see?
It is the basis of the vitamin A trans chromophore, when the cis-trans reversal reaction in the pigment epithelium occurs some of the chromophore is lost as retinyl esters (wastage)
If the vitamin A is not replenished the rods and cones cannot be regenerated
How long does it take vitamin A deficiency to show? Why?
About 6 months
Because the liver keeps about a 6 month store of vitamin A during times of sufficient nutrition
First vision related signs of vitamin A deficiency?
Night blindness
More rods than cones so they’re affected first
Signs of vitamin A deficiency?
- Blindness
- Bitot’s spots in conjunctiva (keratin deposits in conjuctiva - dark spots on white of eyes)
- Corneal ulceration
- Corneal melting
