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The biochemistry of eyes and vision is primary signal transduction, the process of converting information from receptors into chemical change in the cell.  Light is focused by the lens onto the retina.  Rods and cone are the photosensory neurons that form synapses with interconnecting neurons in the eyes.  Then the signals from many neurons are integrated by the ganglion neurons and later transmitted to the optic nerve. 

Light causes a cascade of chemical reactions that result in the hyperpolarization of the cell.  When light interacts with the rods and cones, 11-cis-retinal, a component of the integral protein rhodopsin, is converted into all trans retinal.  This induces a change in the conformation of rhodopsin and results in rhodopsin catalyzing the replacement of GDP to GTP bound to the protein transducin.  The protein transducin dissociates into two subunits, one bound to GTP, the other bound to two molecules of GDP.  The complex of transducin bound to GTP activates the cGMP phosphodiesterase enzyme (PDE).  PDE changes biologically active cGMP to biologically inactive 5’-GMP; this causes a massive decrease in the concentration of cGMP.  As a result of the low concentration of cGMP, the cGMP-gated ion channels close and the cell becomes hyperpolarized. 

The process by which the cell returns to the resting potential is also interesting.  Remember the transducin protein? It hydrolyzes its bound GTP to GDP and reassociates with its other subunit.  This causes the inhibitory subunit of PDE to bind and deactivate it.  The concentration of calcium ions in the cell is low enough to allow guanylyl cyclase to synthesize cGMP.  When the concentration of cGMP is high enough, the ion channels will open, sodium ions will enter, and the cell will return to its resting potential. 

There are several properties of this process that are relevant to our study of the sense of sight.  Signal transduction significantly amplifies the original signal.  According to my biochemistry textbook (Lehninger Principles of Biochemistry), one photon of light closes about 1000 ion channels and changes the cell’s membrane potential by 1 mV.  It is also important to recognize the time scale for this process, it takes time on the order of seconds to minutes to prepare rhodopsin for excitation again.  All the trans-retinal in the rhodopsin molecule must be replaced with 11-cis-retinal.  While there are a lot of biochemical details to get lost in, it is important to remember the end result; information is transferred from photosensory neurons to other neurons to the brain by electrochemical changes in cell membrane polarity.