Photoreceptor cells

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Photoreceptor cells are found in the retina of the eye and are responsible for transducing, or converting, light into nerve signals that can be ultimately transmitted to the brain via the optic nerve. In vertebrates, there are two types of photoreceptor cells called rods and cones. The human retina contains about 125 million rod cells and 6 million cone cells. Rods and cones serve different functions in vision; therefore, the ratio or mix of rods and cones may vary among different animals.

Histology
A photoreceptor cell contains a membranous photoreceptor protein called an opsin which contains a pigment molecule called retinal. In rod cells these together are called rhodopsin. In cone cells there are different types of opsins that combine with retinal to form pigments called photopsins. Different classes of photopsins react to different ranges of light frequency to allow the eye to distinguish colors. The function of the photoreceptor cell is to convert the light energy into a form of energy more readily usable or functional to the organism: this conversion is called signal transduction.

Signal-transduction pathway
The signal-transduction pathway in this case is the mechanism by which the energy of a photon signals a mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to either the transmittance or inhibition of a neural signal that will be fed to the brain via the optic nerve. The pathway for photoreceptor cells follows these basic steps:


 * 1) Light is absorbed by rhodopsin or by one of the various photopsins, causing the opsin to change shape.
 * 2) The shape change in the opsin activates a G protein called transducin.
 * 3) Transducin, in turn, activates the enzyme phosphodiesterase.
 * 4) The enzyme hydrolyzes the second messenger cGMP to GMP
 * 5) Because cGMP acts to keep Na+ ion channels open, the conversion of cGMP to GMP closes the channels.
 * 6) The closing of Na+ channels hyperpolarizes the cell.
 * 7) The hyperpolarization of the cell slows the release of the neurotransmitter glutamate, which can either excite or inhibit the postsynaptic bipolar cells.