As a transducer, the eye must detect and respond to the physical phenomenon of electromagnetic radiation; and it must then transform that electromagnetic energy into a signal the nervous system can perceive and transmit.
Detection and conversion of the light into chemically-mediated nervous impulses takes place in the highly-specialized outer segments of the rod and cone cells that constitute our retina. Rods and cones contain visual pigments called rhodopsin which is an organic compound (retinal) bound to a protein (opsin). The light level striking the cells is translated first into a cascade of chemical changes via the rhodopsin cycle (figure 1), and ultimately into an electrical one through the following sequence of steps:.
a) In darkness, rhodopsin (opsin + retinal) is inactive (or stable), cGMP is high and ion channels are open.
b) Light bleaches rhodopsin, thereby splitting it into its constituents. Opsin decreases cGMP level, activates transducin, closes sodium channels and hyperpolarizes the cell to -70 mV. This changes the rate at which neurons of the visual system fire action potentials. The brain uses the pattern of these action potentials to interpret what we are seeing.
c) In the recovery phase, retinal recombines with opsin to regenerate the photopigment.
The cycle can be represented as in figure 2.