Normalized responsivity spectra of human cone cells, S, M, and L types

Cone cells, or cones, are photoreceptor cells in the retina of the eye which function best in relatively bright light. Responsivity measures the input–output Gain of a detector system A photoreceptor, or photoreceptor cell, is a specialized type of Neuron (nerve cell found in the Eye 's Retina that is capable of The vertebrate retina is a light sensitive part inside the inner layer of the Eye. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The cone cells gradually become more sparse towards the periphery of the retina.

A commonly cited figure of six million in the human eye was found by Osterberg^[1] in 1935. Oyster's textbook (1999) cites work by Curcio et al. (1990) indicating an average closer to 4. 5 million cone cells and 90 million rod cells in the human retina. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type

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. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit 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. ^[2] Because humans usually have three kinds of cones, with different photopsins, which have different response curves, and thus respond to variation in color in different ways, they have trichromatic vision. Photopsins (also known as iodopsins) are the Photoreceptor proteins found in the Cone cells of the Retina that are the basis of Color Trichromacy is the condition of possessing three independent channels for conveying Color information derived from the three different cone types Being color blind can change this, and there have been reports of people with four or more types of cones, giving them tetrachromatic vision. Color blindness, a Color vision deficiency is the inability to perceive differences between some of the Colors that others can distinguish Tetrachromacy is the condition of possessing four independent channels for conveying Color information or possessing four different cones.

Types

Humans normally have three kinds of cones. The first responds most to light of long wavelengths, peaking in the yellow region; this type is designated L for long. The second type responds most to light of medium-wavelength, peaking at green, and is abbreviated M for medium. The third type responds most to short-wavelength light, of a violet color, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a ^[3][4] The difference in the signals received from the three cone types allows the brain to perceive all possible colors, through the opponent process of color vision. The color opponent process is a Color theory that states that the human Visual system interprets information about Color by processing signals from

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 than the other two.

The S cones are most sensitive to light at wavelengths around 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 lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays People with aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range. ^[5] At moderate to bright light levels where the cones functions, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common 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. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type

Structure

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. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. Structurally, cone cells have a cone-like shape at one end where a pigment filters incoming light, giving them their different response curves. A cone is a three-dimensional Geometric shape that tapers smoothly from a flat round base to a point called the apex or vertex They are typically 40-50 µm long, and their diameter varies from . A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre 50 to 4. 0 µm, being smallest and most tightly packed at the center of the eye at the fovea. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. The S cones are a little larger than the others.

Photobleaching can be used to determine cone arrangement. Photobleaching is the photochemical destruction of a Fluorophore. 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. ^[6]

Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment as well as an interior nucleus and various mitochondria. The synaptic terminal forms a synapse with a neuron such as a bipolar cell. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands A bipolar cell is a type of Neuron which has two extensions Bipolar cells are specialized sensory neurons for the transmission of special senses The inner and outer segments are connected by a cilium. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately ^[2] The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the back of the eye, contains the light-absorbing materials. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed ^[2]

Like rods, the outer segments of cones have invaginations of their cell membranes that create stacks of membranous disks. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Photopigments exist as transmembrane proteins within these disks, which provide more surface area for light to affect the pigments. A transmembrane protein is a Protein that spans the entire Biological membrane. 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. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole

Response to light

Activation of a photoreceptor cell is actually a hyperpolarization; when they are not being stimulated, rods and cones depolarize and release a neurotransmitter spontaneously, and activation of photopigments by light sends a signal by preventing this. In biology depolarization is a decrease in the Absolute value of a cell's Membrane potential. See Chemical synapse for an introduction to concepts and terminology used in this article Depolarization occurs due to the fact that in the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens ion channels (largely sodium channels, though Calcium can enter through these channels as well). Cyclic guanosine monophosphate ( cGMP) is a Cyclic nucleotide derived from Guanosine triphosphate (GTP Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living Sodium channels are Integral membrane proteins that form Ion channels, conducting sodium ions ( Na+) through a cell's Plasma membrane Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 The positive charges of the ions that enter the cell down its electrochemical gradient change the cell's membrane potential, cause depolarization, and lead to the release of the neurotransmitter glutamate. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its Glutamate can depolarize some neurons and hyperpolarize others, allowing photoreceptors to interact in an antagonistic manner.

When light hits photoreceptive pigments within the photoreceptor cell, the pigment changes shape. The pigment, called iodopsin (rhodopsin is found in rod cells) consists of a large protein called opsin (situated in the plasma membrane), attached to which is a covalently-bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes it to activate a regulatory protein called transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Transducin (also called Gt is a heterotrimeric G protein that is naturally expressed in Vertebrate Retina rods and cones (a different Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters (Kandel et al. , 2000). Though cone cells primarily use the transmitter substance acetyl choline, rod cells use a variety. The entire process by which light initiates a sensory response is called visual phototransduction. Visual phototransduction is a process by which Light is converted into Electrical signals in the Rod cells Cone cells and Photosensitive ganglion

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. The Stiles–Crawford effect (or Stiles–Crawford effect of the first kind) is a property of the cone photoreceptors of the human Eye.

Table

Comparison of rod and cone cells, from Kandel. ^[2]

See also

• Color blindness
• Color vision
• Cone dystrophy
• Tetrachromacy
• Rod cell

References

1. ^ G. Scotopic vision is the Monochromatic vision of the Eye in low light Photopic vision is the vision of the Eye under well-lit conditions Nyctalopia (Greek for " night blindness " is a condition making it difficult or impossible to see in relatively low light Blindness is the condition of lacking Visual perception due to Physiological or Neurological factors The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. Photosensitivity is the amount to which an object reacts upon receiving Photons especially Visible light. Photosensitivity is the amount to which an object reacts upon receiving Photons especially Visible light. Color blindness, a Color vision deficiency is the inability to perceive differences between some of the Colors that others can distinguish Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit A cone dystrophy is an Inherited ocular disorder characterized by the loss of Cone cells, the photoreceptors responsible for both central and Tetrachromacy is the condition of possessing four independent channels for conveying Color information or possessing four different cones. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type Osterberg (1935). “Topography of the layer of rods and cones in the human retina,” Acta Ophthalmol. , Suppl. 6, pp. 1–103.
2. ^ ^{a b c d} Kandel, E. R. ; Schwartz, J. H, and Jessell, T. M. (2000). Principles of Neural Science, 4th ed. , New York: McGraw-Hill, 507-513.  
3. ^ Wyszecki, Günther; Stiles, W. S. (1982). Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. , New York: Wiley Series in Pure and Applied Optics. ISBN 0-471-02106-7.  
4. ^ R. W. G. Hunt (2004). The Reproduction of Colour, 6th ed. , Chichester UK: Wiley–IS&T Series in Imaging Science and Technology, 11–12. ISBN 0-470-02425-9.  
5. ^ Let the light shine in: You don't have to come from another planet to see ultraviolet light EducationGuardian. co. uk, David Hambling (May 30, 2002)
6. ^ Roorda, A. and Williams, D. R. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397, 520-522.

External links

• Webvision's Photoreceptors