1. Illustrated by: Carrie Wade & Esther Torres
Vision
Illustrated by:
Carrie Wade
& Esther Torres

2. Introduction
When light goes through your eye it passes your pupil focused on highly organized collection of light sensitive neurons. Two types are known as rods and cones
Rods sense low levels of light but cannot discriminate color
Cones are less sensitive to light but can discriminate color.

3. How does signaling work?
Signaling of the rod or cone cell is a light induced decrease in the cGMP (cyclic guanosine monophosphate) causing the cGMP-gated ion channel to close.
The plasma membrane becomes hyperpolarized by the Na+K+ ATPase

4. Rods and cones synapse with interconnecting neurons that carry information to the ganglion neurons near the outer surface of the retina which send the signal through the optic nerve to the visual cortex of the brain

5. Visual transduction process
Visual transduction begins when light touches rhodopsin which is found in the outer segments of rod and cone cells.
Rhodopsin is a protein with seven membrane spanning alpha helices.
The light absorbing chromophore is covalently to opsin a protein component of rhodopsin through a Schiff base to a Lys residue

6. Several steps of transduction result in huge amplification of signal
In the dark GDP binds to all three subunits of the transducin protein and no signal is sent
When rhodopsin is excited it interacts with transducin replacing GDP with GTP from the cytosol
Pg. 464 Figure 12-38
Several steps of transduction result in huge amplification of signal

7. Mechanism of Transducin Translocation
The mechanism is based on the difference in membrane affinities between the αβγ-heterotrimer of transducin and individual Gαt and Gβ1γ1 subunits .
In dark adapted rods the heterodimer is associated with the outer discs of the membrane due to two lipid modifications. Because each subunit has a separate lipid modification, their membrane affinities become reduced 2.

9. Turning the signal off
Shortly after illumination of rod or cone cells the photo sensory system shuts off.
The alpha subunit reconnects with the beta subunit and GTP is hydrolyzed

10. Rhodopsin undergoes a conformational change induced by light absorption exposing Thr and Ser residues in the carboxyl-terminal
Residues get phosphorylated by rhodopsin kinase and is bound by protein arrestin 1 stopping any further interaction between rhodopsin and transducin

11. Color Vision
Color vision in cone cells work essentially the same way, but have different light receptors
Each cone cell expresses one kind of photoreceptor protein that are different enough that the chromophore in different places resulting in the photoreceptors having different absorption spectra

12. Color Blindness
The different types of color blindness come from different opsin mutations
One form is the loss of red photoreceptor, these are red-dichromats
Others lack green pigment, they are green-dichromats
John Dalton was color blind, his eyes were tested when he died and found that he was a green-dichromat

13. Literature Cited
Nelson, David L., David L. Nelson, Albert L. Lehninger, and Michael M. Cox. Lehninger Principles of Biochemistry. New York: W.H. Freeman, Print.
Arshavsky, V. Y., and M. E. Burns. "Photoreceptor Signaling: Supporting Vision across a Wide Range of Light Intensities." Journal of Biological Chemistry (2011): Web.