A photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina that is capable of phototransduction. More specifically, the photoreceptor sends signals to other neurons by a change in its membrane potential when it absorbs photons. Eventually, this information will be used by the visual system to form a complete representation of the visual world. Described here is a vertebrate photoreceptor. Invertebrate photoreceptor in organisms such as insects and mollusks are different in both their morphological organization and their underlying biochemical pathways. Drawing by Santiago Ramón y Cajal of neurons in the pigeon cerebellum. ... The human eye. ... Human eye cross-sectional view. ... Visual phototransduction is a process by which light is converted into electrical signals in the rod cells and cone cells of the retina of the eye. ... This article or section is in need of attention from an expert on the subject. ... In physics, the photon (from Greek φως, phōs, meaning light) is the quantum of the electromagnetic field; for instance, light. ... The visual system is the part of the nervous system which allows organisms to see. ... Classes and Clades See below Vertebrates are members of the subphylum Vertebrata (within the phylum Chordata), specifically, those chordates with backbones or spinal columns. ... Orders See taxonomy Insects are invertebrates that are taxonomically referred to as the class Insecta. ... Classes Caudofoveata Aplacophora Polyplacophora Monoplacophora Bivalvia Scaphopoda Gastropoda Cephalopoda † Rostroconchia The mollusks or molluscs are the large and diverse phylum Mollusca, which includes a variety of familiar creatures well-known for their decorative shells or as seafood. ...

They 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: rods and cones. Cones are adapted to detect colors, and function well in bright light; rods are more sensitive, but do not detect color well, being adapted for low light. The human retina contains about 125 million rod cells and 6 million cone cells. The number and ratio of rods to cones varies among animals, dependent on whether the animal is primarily diurnal or nocturnal. Certain owls have a tremendous number of rods in their retinas — the eyes of the tawny owl are approximately 100 times more sensitive at night than those of humans.^[1] A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... MRI scan of human eye showing optic nerve. ... Normalised absoption spectra of human rod (R) and cone (S,M,L) cells. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Cone cells, or cones, are cells in the retina of the eye which only function in relatively bright light. ... Diurnal may mean: in biology, a diurnal animal is an animal that is active in the daytime. ... A nocturnal animal is one that sleeps during the day and is active at night - the opposite of the human (diurnal) schedule. ... Binomial name Strix aluco Linnaeus, 1758 The Tawny Owl (Strix aluco) is a species of owl resident in much of Europe and southern Russia. ...

Histology

Photoreceptors have the same basic structure. Closest to the visual field (and farthest from the brain) is the axon terminal, which releases a neurotransmitter called glutamate to bipolar cells. Farther back is the Cell body, which contains the cell's organelles. Farther back still is the inner segment, a specialized part of the cell full of mitochondria. The chief function of the inner segment is to provide ATP (energy) for the sodium-potassium pump. Finally, closest to the brain (and farthest from the visual field) is the outer segment, the part of the photoreceptor that actually absorbs light. Outer segments are actually modified cilia that contain disks filled with opsin, the molecule that actually absorbs photons, as well as voltage-gated sodium channels. An axon or nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ... Chemical structure of D-Aspartic Acid, a common Amino Acid neurotransmitter. ... Glutamate is the anion of glutamic acid. ... As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. ... The soma is the bulbous end of a neuron, containing the nucleus. ... Schematic of typical animal cell, showing subcellular components. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell. Cells in culture, stained for keratin (red) and DNA (green). ... Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes In cell biology, a mitochondrion (plural mitochondria) (from Greek μιτος or mitos, thread + κουδριον or khondrion, granule) is a membrane-enclosed organelle, found in most eukaryotic cells. ... Adenosine 5-triphosphate (ATP), discovered in 1929 by Karl Lohmann,[1] is a multifunctional nucleotide primarily known in biochemistry as the molecular currency of intracellular energy transfer. ... The title given to this article is incorrect due to technical limitations. ... In animals, the brain, or encephalon (Greek for in the head), is the control center of the central nervous system. ... The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength[1]. The elementary particle that defines light is the photon. ... cross-section of two cilia, showing 9+2 structure A cilium (plural cilia) is a fine projection from a eukaryotic cell that constantly beats in one direction. ... A rhodopsin molecule in the cell membrane. ... Sodium channels are integral membrane proteins that exist in a cells plasma membrane and regulate the flow of sodium (Na+) ions into it. ...

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. A rhodopsin molecule in the cell membrane. ... In the vision system, retinal, technically called retinene1 or retinaldehyde, is a light-sensitive retinene molecule found in the photoreceptor cells of the retina. ... A rhodopsin molecule (yellow) with bound retinal (orange), embedded in a cell membrane (lipids shown as green, head groups as red/blue). ... This article or section does not cite its references or sources. ... In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. ...

Humans

In humans, the visual system uses millions of photoreceptors to view, perceive, and analyze the visual world. Moreover, the photoreceptor is the only neuron in humans capable of phototransduction (with an exception being the recently discovered photosensitive ganglion cell). All photoreceptors in humans are found in the outer nuclear layer in the retina at the back of each eye, while the bipolar and ganglion cells that transmit information from photoreceptors to the brain are in front of them. This arrangement requires two specializations: a fovea in each retina (for high visual acuity) and a blind spot in each eye, where axons from the ganglion cells can go back through the retina to the brain. A ganglion cell (or sometimes called a gangliocyte) is a type of neuron located in the retina that receives visual information from photoreceptors via various intermediate cells such as bipolar cells, amacrine cells, and horizontal cells. ... Human eye cross-sectional view. ... The fovea, a part of the eye, is a spot located in the center of the macula. ...

Normalized typical human cone responses (and the rod response) to monochromatic spectral stimuli

Humans have two types of photoreceptors: rods and cones. Both are neurons that transduce light into a change in membrane potential through the same signal transduction pathway (see below). However, they differ in the nature of the opsin they contain, and therefore in their function. Rods are used primarily to see at low levels of light, while cones are used to determine color, depth, and intensity. Furthermore, there are three types of cones, which differ in the spectrum of wavelengths of photons over which they absorb (see graph). A single cone or rod cannot tell color; color vision requires interactions of three types of cones (see below). Spectral absorption curves of the short (S), medium (M) and long (L) wavelength pigments in human cone and rod (R) cells. ... Spectral absorption curves of the short (S), medium (M) and long (L) wavelength pigments in human cone and rod (R) cells. ... Normalised absoption spectra of human rod (R) and cone (S,M,L) cells. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Cone cells, or cones, are cells in the retina of the eye which only function in relatively bright light. ... This article or section is in need of attention from an expert on the subject. ... In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. ... Color is an important part of the visual arts. ... Distance is a numerical description of how far apart things lie. ... In physics, intensity is a measure of the time-averaged energy flux. ... The wavelength is the distance between repeating units of a wave pattern. ... 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. ...

Phototransduction

Phototransduction is the complex process whereby the energy of a photon is used to change the inherent membrane potential of the photoreceptor -- and thereby signal to the nervous system that light is in the visual field. The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. ...

Dark current

Unstimulated (in the dark), the voltage-gated sodium channels in the outer segment are open because cyclic GMP (cGMP) is bound to them. This means that positively charged sodium ions are entering the photoreceptor, depolarizing it to about -40 mV (resting potential in other nerve cells is usually -65 mV). This depolarizing current is often known as dark current. Cyclic guanosine monophosphate (cGMP) is a second messenger derived from GTP. Cyclic guanosine monophosphate (cGMP) is a cyclic nucleotide derived from guanosine triphosphate (GTP). ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... An ion is an atom or group of atoms that normally are electrically neutral and achieve their status as an ion by loss or addition of one or more electrons. ... In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials, synaptic potentials, or other active changes in the membrane potential. ... Electric current is by definition the flow of electric charge. ... Dark current is the constant response exhibited by a receptor of radiation during periods when it is not actively being exposed to light. ...

Signal transduction pathway

The signal transduction pathway 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 steps in phototransduction that take place in the vertebrate photoreceptors eye, which constitute a signal transduction pathway, are then: In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. ... MRI scan of human eye showing optic nerve. ...

1. The opsin in the outer segment absorbs a photon, changing the configuration of a molecule inside the cell from the less-energetic cis-form to the more-energetic trans-form. (Light is absorbed by rhodopsin or by one of the various photopsins, causing the retinal to change shape.)
2. This results in a series of unstable intermediates, the last of which binds to the G protein in the membrane and activates transducin, a protein inside the cell. This is the first amplification step - each photoactivated rhodopsin triggers activation of about 100 transducins. (The shape change in the opsin activates a G protein called transducin.)
3. Each transducin then activates the enzyme cGMP-specific phosphodiesterase (PDE). (Transducin, in turn, activates the enzyme phosphodiesterase.)
4. PDE then catalyzes the hydrolysis of cGMP. This is the second amplification step because PDE hydrolyses about 1000 cGMP molecules. (The enzyme hydrolyzes the second messenger cGMP to GMP)
5. With the intracellular concentration of cGMP reduced, the net result is closing of ion channels in the photoreceptor membrane because cGMP was keeping the channels open. (Because cGMP acts to keep Na⁺ ion channels open, the conversion of cGMP to GMP closes the channels.)
6. As a result, sodium ions can no longer enter the cell, and the photoreceptor hyperpolarizes (its charge inside the membrane becomes more negative). (The closing of Na⁺ channels hyperpolarizes the cell.)
7. This hyperpolarization means that less glutamate is released to the bipolar cell than before (see below). (The hyperpolarization of the cell slows the release of the neurotransmitter glutamate, which can either excite or inhibit the postsynaptic bipolar cells.)
8. Reduction in the release of glutamate means one population of bipolar cells will be depolarized and a separate population of bipolar cells will be hyperpolarized, depending on the nature of receptors (ionotropic or metabotropic) in the postsynaptic terminal (see receptive field).

Thus, a photoreceptor actually releases less neurotransmitter when stimulated by light, because in the dark, the photoreceptor is at -40 mV, and photons, through a chemical process, hyperpolarize the cell. In chemistry, a molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds [1] [2] [3] [4] [5]. Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure... G-proteins, short for guanine nucleotide binding proteins, are a family of proteins involved in second messenger cascades. ... Drawing of a cell membrane A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... Transducin is the name given to the G-protein alpha-subunits that are naturally expressed in vertebrate retina rods and cones. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... G-proteins, short for guanine nucleotide binding proteins, are a family of proteins involved in second messenger cascades. ... Transducin is the name given to the G-protein alpha-subunits that are naturally expressed in vertebrate retina rods and cones. ... Transducin is the name given to the G-protein alpha-subunits that are naturally expressed in vertebrate retina rods and cones. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... A phosphodiesterase (PDE) is an enzyme that catalyzes the hydrolysis of phosphodiester bonds. ... A phosphodiesterase (PDE) is an enzyme that catalyzes the hydrolysis of phosphodiester bonds. ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay signals within a cell. ... Cyclic guanosine monophosphate (cGMP) is a second messenger derived from GTP. Cyclic guanosine monophosphate (cGMP) is a cyclic nucleotide derived from guanosine triphosphate (GTP). ... There are several meanings for GMP: Good Manufacturing Practice, a guideline for manufacturers of medications (including pharmaceuticals, biotechnology products, and active pharmaceutical ingredients) to assure that medications are of the required quality. ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ... Hyperpolarization has several meanings: In biology, hyperpolarization occurs when a cells membrane potential dips below its resting level. ... In biology, hyperpolarization is any change in a cells membrane potential that makes it more polarized. ... Glutamate is the anion of glutamic acid. ... As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. ... ... Metabotropic receptor is a transmembrane receptor, which starts some intracellular biochemical cascade after its activation by an agonistic ligand. ... Receptive fields are areas of the retina, producing a change in the firing of cells in the visual system. ...

ATP provided by the inner segment powers the sodium-potassium pump. This pump is necessary to reset the initial state of the outer segment by taking the sodium ions that are entering the cell and pumping them back out.

Although photoreceptors are neurons, they do not conduct action potentials. A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ...

Advantages

Phototransduction is unique in that the stimulus (in this case, light) actually reduces the cell's response or firing rate. (Nowhere does light increase the cell's response or firing rate, unlike in other sensory systems). However, this system provides several key advantages. In physiology, a stimulus is a detectable change in the internal or external environment. ...

First, the photoreceptor is depolarized in the dark, which means many sodium ions are flowing into the cell. Thus, the random opening or closing of sodium channels will not affect the membrane potential of the cell; only the closing of a large amount of channels, through absorption of a photon, will affect it and signal that light is in the visual field. Hence, the system is noiseless. Neuronal noise is the term that describes random activity of neurons that presumably is not associated with encoding of behaviorally relevant variables. ...

Second, there is a lot of amplification in two stages of phototransduction: one pigment will activate many molecules of transducin, and one PDE will cleave many cGMPs. This amplification means that even the absorption of one photon will affect membrane potential and signal to the brain that light is in the visual field. This the main feature which differentiates rod photoreceptors from cone photoreceptors. Rods are extremely sensitive and have the capacity of registering a single photon of light unlike cones. On the other hand, cones are known to have very fast kinetics in terms of rate of amplification of phototransduction unlike rods. For animal and plant pigments, see Pigment, biology. ... Transducin is the name given to the G-protein alpha-subunits that are naturally expressed in vertebrate retina rods and cones. ...

Function

Photoreceptors do not signal color; they only signal the presence of light in the visual field. Color is an important part of the visual arts. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength[1]. The elementary particle that defines light is the photon. ...

A given photoreceptor responds to both the wavelength and intensity of a light source. Hence, a red light source with a certain intensity may produce the exact same effect in a photoreceptor as a green light source with a different intensity. The wavelength is the distance between repeating units of a wave pattern. ... In physics, intensity is a measure of the time-averaged energy flux. ...

A single photoreceptor does not detect the color (wavelength) or intensity of a light source. The visual system computes color by comparing across a population of photoreceptors and intensity by determining how many photoreceptors are responding. This is the mechanism that allows trichromatic color vision in humans and some other animals. In physics, intensity is a measure of the time-averaged energy flux. ... Normalised absorption spectra of human cone (S,M,L) and rod (R) cells Trichromatic color vision is the ability of humans and some other animals to see different colors, mediated by interactions among three types of color-sensing cone cells. ...

Signaling

The photoreceptor signals its absorption of photons through a release of the neurotransmitter glutamate to bipolar cells at its axon terminal. Since the photoreceptor is depolarized in the dark, a high amount of glutamate is being released to bipolar cells in the dark. Absorption of a photon will hyperpolarize the photoreceptor and therefore result in the release of less glutamate at the postsynaptic terminal to the bipolar cell. A synapse is a connection between two neurons: presynaptic and postsynaptic. ...

Every photoreceptor releases the same neurotransmitter–glutamate. However, the effect of glutamate differs in the bipolar cells, depending upon the type of receptor imbedded in that cell's membrane. When glutamate binds to an ionotropic receptor, the bipolar cell will depolarize (and therefore will hyperpolarize with light as less glutamate is released). On the other hand, binding of glutamate to a metabotropic receptor results in a hyperpolarization, so this bipolar cell will depolarize to light as less glutamate is released. In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. ... Illustration of a cell membrane The cell membrane, also called the plasma membrane or plasmalemma, is a semipermeable lipid layer surrounding the cytoplasm of all living cells. ... Ligand-gated ion channel is a broad term that refers to any ion channel that is gated (i. ... Based on their structural and functional characteristics, neurotransmitter receptors can be classified into two broad categories: metabotropic and ionotropic receptors. ...

In essence, this property allows for one population of bipolar cells that gets excited by light and another population that gets inhibited by it, even though all photoreceptors show the same response to light. This complexity becomes both important and necessary for detecting color, contrast, edges, etc. This article or section is in need of attention from an expert on the subject. ... The goal of edge detection is to mark the points in a digital image at which the luminous intensity changes sharply. ...

Further complexity arises from the various interconnections among bipolar cells, horizontal cells, and amacrine cells in the retina. The final result is differing population of ganglion cells in the retina, each which convey different information to the brain, for the final synthesis of a visual world. As a part of the retina, the bipolar cell exists between photoreceptors (rod cells and cone cells) and ganglion cells. ... Plan of retinal neurons. ... Amacrine cell Retinal cell interneuron interacting at the Inner Plexiform Layer (IPL), the second synaptic retinal layer where bipolar cells and ganglion cells synapse. ...

See also

• Circadian rhythm
• G protein-coupled receptor
• Sensory system
• Photosensitive
• Photosensitive ganglion cell

A circadian rhythm is a roughly-24-hour cycle in the physiological processes of living beings, including plants, animals, fungi and cyanobacteria. ... Figure 1. ... This article or section may be confusing for some readers, and should be edited to be clearer or more simplified. ... Photosensitivity is the amount to which an object reacts upon receiving photons of light. ... This article needs to be cleaned up to conform to a higher standard of quality. ...

Bibliography

• Campbell, Neil A., and Reece, Jane B. (2002). Biology: 1064-1067.
• Freeman, Scott. (2002). Biological Science: 835-837.

References