The visual pigment of rods is a deep purple pigment called rhodopsin.  Rhodopsin is the a seven trans membrane protein and is from the family of G protein coupled receptors (GPCRs). These trans membrane proteins switch from an inactive to an active form on the binding of a ligand. The activated receptor can trigger an intracellular signal cascade. 

Rhodopsin buried in a POPC membrane.  Retinal is shown in orange.  Image taken from /rhodopsin/ without permission.


Rhodopsin is arranged in a single layer in the membranes of each of the thousands of discs in the rods outer segment.  Rhodopsin forms light through the entire spectrum but it preferentially absorbs green light at a wavelength of 497nm.

Image shows the structure of bacteriorhodopsin.  Its structure and function is similar to the protein in the human eye.  Image taken from /courses/biomi290/MEMBRANE/RHODOPSIN.HTML



Rhodopsin is formed in the dark from vitamin A.  Vitamin A is oxidised to 11-cis retinal and combined with scotopsin (the opsin found in rods) to form rhodopsin.  When rhodopsin absorbs light, retinal changes from 11-cis to all-trans retinal.  The retinal-scotopsin complex breaks down allowing them to separate.  This breakdown is known as the bleaching of the pigment.  The breakdown of rhodopsin triggers a transduction process involving a rapid cascade of intermediates.  Once the all trans-retinal detaches from the opsin it is transported to the epithelium by a protein carrier.  Within the epithelial cells, retinal is converted to its 11-cis isomer in an ATP requiring process.  Retinal then heads back to the photoreceptor cells outer segments to begin the process again.

Image taken from without permission.

Electronic effects in the rods

In the dark sodium ions leak into the outer segments of the rods.  This produces depolarizing potentials that keep calcium channels open in their synaptic endings.  This results in continuous neurotransmitter release by the rods at their synapses.  When light triggers rhodopsin breakdown the sodium permeability of the outer segment membrane decreases rapidly.  This mechanism involves destruction of cyclic GMP (intracellular second messenger which keeps sodium channels open in the dark).  The effect of light turns off the sodium entry causing the rod cells to develop a hyperpolarizing receptor potential that inhibits their release of neurotransmitter.


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