Cone cells, or cones, are
photoreceptor cells in the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
s of vertebrate
eyes including the
human eye
The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm.
...
. They respond differently to
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
of different
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...
s, and the combination of their responses is responsible for
color vision. Cones function best in relatively bright light, called the
photopic region, as opposed to
rod cells, which work better in dim light, or the
scotopic region. Cone cells are densely packed in the
fovea centralis
The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.
The fovea is responsible for sharp central vision (also called foveal vision), which is ...
, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the
optic disc
The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye.
The ganglion cell axons form ...
, contributing to the
blind spot. There are about six to seven million cones in a human eye (vs ~92 million rods), with the highest concentration being towards the
macula
The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal av ...
.
Cones are less sensitive to light than the
rod cells in the retina (which support vision at low light levels), but allow the
perception
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
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.
Cones are normally one of three types: S-cones, M-cones and L-cones. Each type expresses a different
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...
:
OPN1SW,
OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2 is a similar opsin.
See also
* Opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, ...
,
OPN1LW, respectively. These cones are sensitive to visible wavelengths of light that correspond to short-wavelength, medium-wavelength and longer-wavelength light respectively. Because humans usually have three kinds of cones with different
photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
s, which have different response curves and thus respond to variation in color in different ways, humans have
trichromatic vision
Trichromacy or trichromatism is the possessing of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats.
The normal expl ...
. Being
color blind
Color blindness or color vision deficiency (CVD) is the decreased ability to see color or differences in color. It can impair tasks such as selecting ripe fruit, choosing clothing, and reading traffic lights. Color blindness may make some aca ...
can change this, and there have been some verified reports of people with four types of cones, giving them
tetrachromat
Tetrachromacy (from Greek ''tetra'', meaning "four" and ''chromo'', meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with te ...
ic vision.
[
][
]
The three pigments responsible for detecting light have been shown to vary in their exact chemical composition due to
genetic mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...
; different individuals will have cones with different color sensitivity.
Structure
Types
Humans normally have three types of cones, usually designated L, M and S for long, medium and short wavelengths respectively. The first responds the most to light of the longer red
wavelengths, peaking at about 560
nm. The majority of the human cones are of the long type. The second most common type responds the most to light of yellow to green medium-wavelength, peaking at 530 nm. M cones make up about a third of cones in the human eye. The third type responds the most to blue short-wavelength light, peaking at 420 nm and make up only around 2% of the cones in the human retina. The three types have peak wavelengths in the range of 564–580 nm, 534–545 nm, and 420–440 nm, respectively, depending on the individual. Such a difference is caused by the different
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most ...
s they carry,
OPN1LW,
OPN1MW
Green-sensitive opsin is a protein that in humans is encoded by the ''OPN1MW'' gene.
OPN1MW2 is a similar opsin.
See also
* Opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, ...
,
OPN1SW, respectively, the forms of which affect the absorption of
retinal . The
CIE 1931 color space is an often-used model of spectral sensitivities of the three cells of an average human.
While it has been discovered that there exists a mixed type of
bipolar cells
A bipolar neuron, or bipolar cell, is a type of neuron that has two extensions (one axon and one dendrite). Many bipolar cells are specialized sensory neurons for the transmission of sense. As such, they are part of the sensory pathways for smell, ...
that bind to both rod and cone cells, bipolar cells still predominantly receive their input from cone cells.
Other animals might have different number of cone types, see
Color vision.
Shape and arrangement
Cone cells are somewhat shorter than rods, but wider and tapered, and are much less numerous than rods in most parts of the retina, but greatly outnumber rods in the
fovea
Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure.
Human anatomy
*Fovea centralis of the retina
* Fovea buccalis or Dimple
* Fovea of the femoral head
* Trochlear fovea of the fr ...
. Structurally, cone cells have a
cone-like shape at one end where a pigment filters incoming light, giving them their different response curves. They are typically 40–50
µm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
long, and their diameter varies from 0.5 to 4.0 µm, being smallest and most tightly packed at the center of the eye at the
fovea
Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure.
Human anatomy
*Fovea centralis of the retina
* Fovea buccalis or Dimple
* Fovea of the femoral head
* Trochlear fovea of the fr ...
. The S cone spacing is slightly larger than the others.
Photobleaching
In optics, photobleaching (sometimes termed fading) is the photochemical alteration of a dye or a fluorophore molecule such that it is permanently unable to fluoresce. This is caused by cleaving of covalent bonds or non-specific reactions between ...
can be used to determine cone arrangement. This is done by exposing dark-adapted retina to a certain wavelength of light that paralyzes the particular type of cone sensitive to that wavelength for up to thirty minutes from being able to dark-adapt making it appear white in contrast to the grey dark-adapted cones when a picture of the retina is taken. The results illustrate that S cones are randomly placed and appear much less frequently than the M and L cones. The ratio of M and L cones varies greatly among different people with regular vision (e.g. values of 75.8% L with 20.0% M versus 50.6% L with 44.2% M in two male subjects).
Like rods, each cone cell has a synaptic terminal, inner (nearer the brain) and outer segments, as well as an interior nucleus and various
mitochondria. The synaptic terminal forms a
synapse with a neuron
bipolar cell. The inner and outer segments are connected by a
cilium
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proj ...
.
The inner segment contains
organelles and the cell's nucleus, while the outer segment contains the light-absorbing materials.
The outer segments of cones have invaginations of their
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
s 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 worn off at the end of the outer segment, to be consumed and recycled by
phagocytic cells.
Function
The difference in the signals received from the three cone types allows the brain to perceive a continuous range of colors, through the
opponent process
The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are thr ...
of
color vision. (
Rod cells have a peak sensitivity at 498 nm, roughly halfway between the peak sensitivities of the S and M cones.)
All of the receptors contain the protein
photopsin
Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to ''opsin'', as they were the first opsins discovered and are still th ...
, with variations in its conformation causing differences in the optimum wavelengths absorbed.
The color yellow, for example, is perceived when the L cones are stimulated slightly more than the M cones, and the color red is perceived when the L cones are stimulated significantly more than the M cones. Similarly, blue and violet hues are perceived when the S receptor is stimulated more. S Cones are most sensitive to light at wavelengths around 420 nm. However, the
lens and
cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical ...
of the human eye are increasingly absorptive to shorter wavelengths, and this sets the short wavelength limit of human-visible light to approximately 380 nm, which is therefore called '
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
' light. People with
aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range. At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) of the three kinds of cones almost equally. At lower light levels, where only the
rod cells function, the sensitivity is greatest at a blueish-green wavelength.
Cones also tend to possess a significantly elevated visual acuity because each cone cell has a lone connection to the optic nerve, therefore, the cones have an easier time telling that two stimuli are isolated. Separate connectivity is established in the
inner plexiform layer so that each connection is parallel.
The response of cone cells to light is also directionally nonuniform, peaking at a direction that receives light from the center of the pupil; this effect is known as the
Stiles–Crawford effect.
It is possible that S cones may play a role in the regulation of the
circadian system and the secretion of
melatonin but this role is not clear yet. The exact contribution of S cone activation to circadian regulation is unclear but any potential role would be secondary to the better established role of
melanopsin.
Color afterimage
Sensitivity to a prolonged stimulation tends to decline over time, leading to
neural adaptation. An interesting effect occurs when staring at a particular color for a minute or so. Such action leads to an exhaustion of the cone cells that respond to that color – resulting in the
afterimage. This vivid color aftereffect can last for a minute or more.
[Schacter, Daniel L. ''Psychology: the second edition.'' Chapter 4.9.]
Associated Diseases
*
Achromatopsia (Rod Monochromacy) - a form of
monochromacy with no functional cones
*
Blue cone monochromacy
Blue-cone monochromacy (BCM) is an inherited eye disease that causes severe color blindness, poor visual acuity, nystagmus and photophobia due to the absence of functional red (L) and green (M) cone photoreceptor cells in the retina. BCM is a ...
- a rare form of
monochromacy with only functional S-cones
*
Congenital red-green color blindness - partial color blindness include protanopia, deuteranopia, etc.
*
Oligocone trichromacy - poor visual acuity and impairment of cone function according to ERG, but without significant color vision loss.
*
Bradyopsia -
photopic vision cannot respond quickly to stimuli.
*
Bornholm eye disease - X-linked recessive
myopia,
astigmatism, impaired
visual acuity and red-green
dichromacy
Dichromacy (from Greek ''di'', meaning "two" and ''chromo'', meaning "color") is the state of having two types of functioning photoreceptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats requir ...
.
*
Cone dystrophy - a degenerative loss of cone cells
*
Retinoblastoma - a type of cancer originating from cone precursor cells
See also
*
Disc shedding
Disc shedding is the process by which photoreceptor cells in the retina are renewed. The disc formations in the outer segment of photoreceptors, which contain the photosensitive opsins, are completely renewed every ten days.
Photoreceptors
The ret ...
*
Double cones
*
RG color space
*
Tetrachromacy
Tetrachromacy (from Greek ''tetra'', meaning "four" and ''chromo'', meaning "color") is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with te ...
*
Melanopsin
*
Color vision
References
External links
Cell Centered Database – Cone cellNIF Search – Cone Cellvia the
Neuroscience Information Framework
Model and image of cone cell
{{Authority control
Color vision
Photoreceptor cells
Human eye anatomy
Human cells
Neurons