Cone Receptors

Intro | Amacrine Cells | Back of the Eyeball | Bipolar Cells | Blind Spot & Optic Nerve | Cone Receptors | Retinal Ganglion Cells | Horizontal Cells | Rod Receptors

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Cone Receptors are named for their cone-like shape. These receptor cells are sensitive to bright light and are specialized for color vision. Cones are found in greatest density at the fovea. In fact, this region, located at the center of the retina, consists exclusively of cones. Fovea means "pit" (approximately 0.33 centimeters in diameter), and is named for its appearance relative other regions of the retina. The relative absence of both blood vessels and axons in this region contribute to the appearance of a pit in the retina and the reduction of light scatter. The increased density of cone receptor cells in the fovea and reduced interference of entering light by other structures both contribute to the enhanced acuity of information processed in the fovea. Because cones are particularly sensitive in bright light and illumination enhances visual acuity, they provide the sharpest and most detailed images of the outside world.

As is the case for rods, when a cone is activated by light it is in a hyperpolarized state (as opposed to depolarized state). While at rest, cone cells transmit a steady inhibitory input to the bipolar cells. The transduction process, as it occurs in the rods of the retina, occurs in a similar manner in the cone cells. The transduction process of cone cells, however, is mediated via three cell types that differ based on the wavelength of light absorbed by the photopigments (the opsins). Each type of cone cell responds selectively to short, medium, and long wavelengths. Although less is known about the opsins than the rod pigment, rhodopsin, their structure and function appear to be quite similar to that of rhodopsin. When excited, cones release the excitatory neurotransmitter glutamate.

Two models have been offered in explanation of how the cone cells create color vision, the trichromatic (Young-Helmholtz) theory and the opponent-process theory. The trichromatic theory speculates that we perceive color based on the ratio of activity across all three types of cone cells. The opponent-process theory may occur at the level of bipolar cells and beyond, where the cellular characteristics support the encoding of paired opposites such as red versus green, blue versus yellow, and white versus black. For example, many visual cells are excited by green light at the retina and inhibited by red light. Color-blindness may be caused by absent or deficient cones containing any one or all of the photopigments. This defect is passed along genetically as a recessive gene on the X (female) chromosome. For this reason, color-blindness occurs far more frequently in males (8% incidence) than females (1% incidence).