ANN 111: Principles of Animal Nutrition and Feed Technology (2+1)

FUNCTIONS VITAMIN A AND VISION

Rhodopsin synthesis – Visual cycle

• The retina is located at the back of the eye. When light passes through the lens, it is sensed by the retina and converted to a nerve impulse for interpretation by the brain.
• Retinol is transported to the retina via the circulation, where it moves into retinal pigment epithelial cells.
• There, retinol is esterified to form a retinyl ester, which can be stored. When needed, retinyl esters are broken apart to form 11-cis retinol, which can be oxidized to form 11-cis retinal.
• 11-cis Retinal can be shuttled to the rod cell, where it binds to a protein called opsin to form the visual pigment, rhodopsin (visual purple).
• Rod cells with rhodopsin can detect very small amounts of light, making them important for night vision.
• Absorption of a photon of light catalyzes the isomerization of 11-cis retinal to all-trans retinal and results in its release.
• This isomerization triggers the generation of an electrical signal to the optic nerve.
• The nerve impulse generated by the optic nerve is conveyed to the brain where it can be interpreted as vision.
• Once released all-trans retinal is converted to all-trans retinol, which can be transported to the retinal epithelial cell to complete the visual cycle.
• Inadequate retinol available to the retina results in impaired dark adaptation, known as "night blindness."