Visual cycle – Simplified

11-cis-retinal combines with opsin protein to form rhodopsin. Light stimulates conversion of rhodopsin bound 11-cis-retinal to all-trans-retinol (all-trans-retinol is transported from photoreceptor cells to retinal pigment epithelium), which subsequently dissociates from opsin (bleaching), leading to membrane depolarization and initiation of action potential in photoreceptors in the neural retina.

From: https://www.d.umn.edu/~jfitzake/Lectures/DMED/Vision/Retina/VisualCycle.html

Once in the RPE cell, the all-trans-retinal is either stored or converted back to the 11-cis-retinal form for transport back to the photoreceptors, completing the visual cycle. Recycling of rhodopsin is a key component ofย dark adaptation.

11-cis-retinal cannot be synthesized de novo by humans, but must be converted from vitamin A (deficiency leads to night blindness).

Study details of 10 layers of retina and process of phototransduction.

Mnemonics:

LighT phase: aLL Trans-retinal with opsin
dark phase: 11 cis-retinal with opsin
Dark: Depolarization of photoreceptor (Displays inhibitory neurotransmitters and hyperpolarizes bipolar cells)
ligHt: Hyperpolarization of photoreceptor (Hides inhibitory neurotransmitters and depolarizes bipolar cells)

Mnemonics adapted from Ophthalmonics by Dhaval Patel, MD.

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