Sandbox 173: Difference between revisions

The excited rhodopsin interacts with a large number of transducin molecules, found in the cytoplasic face of the disk membrane. Transducin is a member of the heterotrimeric GTP-binding proteins family, and it binds to GDP in the dark. This interaction generates a signaling cascade where transducin molecules are activated through the trigger of GDP-GTP nucleotide exchange in the α subunit<ref name="Article6"/>. Each activated transducin dissociates into Tα-GTP and Tβγ subunits, and Tα-GTP activates [http://en.wikipedia.org/wiki/CGMP-specific_phosphodiesterase_type_5 cGMP-specific phosphodiesterase] by binding and removing its inhibitory subunit<ref>Textbook</ref>.  

The cGMP phosphodiesterase is an integral protein of the retina with its active site on the cytoplasmic side of the disk. Its inhibitory subunit tightly binds to it in the dark and suppresses its activity.  The now activated phosphodiesterase degrades many molecules of cGMP, efficiently decreasing the concentration of cGMP <ref>Textbook</ref>. This results in the closing of the cGMP-gated cation channels in the plasma membrane of the outer segment. The cell hyperpolarizes due to the decrease in the influx of sodium and calcium ions, which results in the decrease of the release of glutamate into the synaptic cleft. This electric signal of this hyperpolarization is sent to the brain through ranks of interconnecting neurons and then through the optic nerve<ref name="Article6"/>.

In the event of a decrease in light intensity, GTP is hydrolyzed and the α-subunit of transducin reassociates with the βγ subunits, releasing the inhibitory subunit of phosphodiesterase. This subunit reassociates with phosphodiesterase and inhibits its activity<ref>Textbook</ref>.  

The concentration of cGMP is returned to the “dark” state by the conversion of GTP to cGMP by [http://en.wikipedia.org/wiki/Guanylate_cyclase guanylyl cyclase], activated through the efflux of calcium ions through the sodium/calcium ion exchanger. The reduction in the concentration of calcium ions also inhibits phosphodiesterase activity. Both actions reopen the cation channels and restore the system to pre-stimulus state<ref>Textbook</ref>.

Opsins are also photoreceptor proteins and are concentrated in cone cells, cells that are less sensitive to light but can discriminate colours. Opsins are slightly different light receptors than rhodopsin in that they can detect light from different spectrums and distinguish between their wavelengths. The ability to differentiate between colours is related to the three types of cone cells, each using one of the three related opsin photoreceptors<ref>Textbook</ref>.