Cones (eye)



Cone cells, or cones, are cells in the retina of the eye which only function in relatively bright light. There are about 6 million in the human eye, concentrated at the fovea. They gradually become more sparse towards the outside of the retina.

Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of color. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods (Kandel et al., 2000). Because humans (normally) have three kinds of cones, with different photopsins, which have different response curves (that is, they respond to variation in color in different ways), we have trichromatic vision.

Types
The three kinds of cones typically respond most to yellowish-green (long wavelength or L), bluish-green (medium or M), and blue-violetish (short or S) light (peak wavelengths of 564 nm, 534 nm, and 420 nm respectively).

The difference in the signals received from the three kinds allows the brain to perceive a wide range (gamut) of different colors.

The color yellow, for example, is perceived when the yellowish-green receptor is stimulated slightly more than the bluish-green receptor, and the color red is perceived when the yellowish-green receptor is stimulated significantly more than the bluish-green receptor. Similarly, blue and bluish hues are perceived when the blue-violetish receptor is stimulated more than the other two.

The S (bluish-violet) cones are most sensitive to light at wavelengths around than 420 nm. However, the lens and cornea of the human eye are increasingly absorbative to smaller wavelengths, and this sets the lower wavelength limit of human-visible light to approximately 380 nm (which is therefore called 'ultraviolet' light). The eye is more sensitive to green light than other colors because this stimulates two of the three kinds of cones almost equally.

Structure
Cone cells are larger and less numerous than rods. Structurally, cone cells have a cone-like shape at one end where the pigment that filters incoming light, giving them their different response curves. They are typically 50 µm long, and their diameter varies from 1.0 to 4.0 µm, being smallest and most tightly packed at the center of the eye (the fovea). The blue-sensitive cells are a little larger than the others, and an order of magnitude less common.

Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with a neuron such as a bipolar cell. The inner and outer segments are connected by a cilium (Kandel et al., 2000). The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials (Kandel et al., 2000).

Like rods, the outer segments of cones have invaginations of their cell membranes that create stacks of membranous disks. Photopigments exist as transmembrane proteins within these disks, which provide more surface area for light to affect the pigments. In cones, these disks are attached to the outer membrane, whereas they are pinched off and exist separately in rods. Neither rods nor cones divide, but their membranous disks wear out and are sloughed off at the end of the outer segment, to be consumed and recycled by phagocytic cells.

Table
Comparison of rod and cone cells, from Kandel et al., 2000.