Visual phototransduction is the
sensory transduction
In physiology, transduction is the translation of arriving stimulus into an action potential by a sensory receptor. It begins when stimulus changes the membrane potential of a receptor cell.
A receptor cell converts the energy in a stimulus into ...
process of the
visual system
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...
by which
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 tera ...
is detected to yield
nerve impulses
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...
in the
rod cell
Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in per ...
s and
cone cell
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...
s 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 then ...
of the
eye in humans and other vertebrates. It relies on the
visual cycle
The visual cycle is a process in the retina that replenishes the molecule retinal for its use in vision. Retinal is the chromophore of most visual opsins, meaning it captures the photons to begin the phototransduction cascade. When the proton i ...
, a sequence of biochemical reactions in which a molecule of
retinal
Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).
Some microorganisms use retin ...
bound to
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 pro ...
undergoes
photoisomerization
In chemistry, photoisomerization is a form of isomerization induced by photoexcitation. Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a re ...
, initiates a cascade that signals detection of the photon, and is indirectly restored to its photosensitive isomer for reuse. Phototransduction in some invertebrates such as
fruit flies
Fruit fly may refer to:
Organisms
* Drosophilidae, a family of small flies, including:
** ''Drosophila'', the genus of small fruit flies and vinegar flies
** ''Drosophila melanogaster'' or common fruit fly
** '' Drosophila suzukii'' or Asian frui ...
relies on similar processes.
Photoreceptors
The
photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
s involved in vertebrate vision are the
rods, the
cones
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines conn ...
, and the
photosensitive ganglion cell
Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence ...
s (ipRGCs). These cells contain a
chromophore (
11-''cis''-retinal, the
aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
of
vitamin A1
Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xerophtha ...
and light-absorbing portion) that is bound to a cell membrane protein,
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 pro ...
. Rods deal with low light level and do not mediate color vision. Cones, on the other hand, can code the color of an image through comparison of the outputs of the three different types of cones. Each cone type responds best to certain
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, tro ...
s, or colors, of light because each type has a slightly different opsin. The three types of cones are L-cones, M-cones and S-cones that respond optimally to long wavelengths (reddish color), medium wavelengths (greenish color), and short wavelengths (bluish color) respectively. Humans have
trichromatic
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 ...
photopic vision
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 108 cd/m2). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visua ...
consisting of three
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 ...
channels that enable
color vision
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths (i.e., different spectral power distributions) independently of light intensity. Color perception is a part of ...
.
Visual cycle
The visual cycle occurs via
G-protein coupled receptor
G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of protein family, evolution ...
s called
retinylidene protein
Retinylidene proteins, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include a ...
s which consists of a visual
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 pro ...
and a
chromophore 11-''cis''-retinal. The 11-''cis''-retinal is covalently linked to the
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 pro ...
receptor via
Schiff base
In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimine ...
. When it absorbs a
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
, 11-''cis''-retinal undergoes
photoisomerization
In chemistry, photoisomerization is a form of isomerization induced by photoexcitation. Both reversible and irreversible photoisomerizations are known for photoswitchable compounds. The term "photoisomerization" usually, however, refers to a re ...
to
all-''trans''-retinal, which changes the conformation of the opsin
GPCR
G protein-coupled receptors (GPCRs), also known as seven-(pass)-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptors, and G protein-linked receptors (GPLR), form a large group of evolutionarily-related p ...
leading to
signal transduction
Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellula ...
cascades which causes closure of cyclic GMP-gated cation channel, and hyperpolarization of the photoreceptor cell.
Following photoisomerization, all-''trans''-retinal is released from the opsin protein and reduced to all-''trans''-
retinol
Retinol, also called vitamin A1, is a fat-soluble vitamin in the vitamin A family found in food and used as a dietary supplement. As a supplement it is used to treat and prevent vitamin A deficiency, especially that which results in xeroph ...
, which travels to the
retinal pigment epithelium
The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual ce ...
to be "recharged". It is first
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...
ified by
lecithin retinol acyltransferase
Lecithin retinol acyltransferase is an enzyme that in humans is encoded by the ''LRAT'' gene.
Function
Lecithin retinol acyltransferase is a microsomal enzyme that catalyzes the esterification of all-trans-retinol into all-trans-retinyl ester ...
(LRAT) and then converted to 11-''cis''-retinol by the isomerohydrolase
RPE65
Retinal pigment epithelium-specific 65 kDa protein, also known as retinoid isomerohydrolase, is an enzyme of the vertebrate visual cycle that is encoded in humans by the ''RPE65'' gene. RPE65 is expressed in the retinal pigment epithelium (RPE, a ...
. The isomerase activity of RPE65 has been shown; it is uncertain whether it also acts as the hydrolase. Finally, it is oxidized to 11-''cis''-retinal before traveling back to the
photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
outer segment where it is again conjugated to an
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 pro ...
to form new, functional visual pigment (
retinylidene protein
Retinylidene proteins, are proteins that use retinal as a chromophore for light reception. They are the molecular basis for a variety of light-sensing systems from phototaxis in flagellates to eyesight in animals. Retinylidene proteins include a ...
), namely
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 ...
or
rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
.
Transduction process
![1415 Retinal Isomers](https://upload.wikimedia.org/wikipedia/commons/f/f8/1415_Retinal_Isomers.jpg)
To understand the photoreceptor's behavior to light intensities, it is necessary to understand the roles of different currents.
There is an ongoing outward
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
current through nongated K
+-selective channels. This outward current tends to hyperpolarize the photoreceptor at around −70 mV (the equilibrium potential for K
+).
There is also an inward sodium current carried by
cGMP-gated
sodium channel
Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the channel ...
s. This "
dark current" depolarizes the cell to around −40 mV. Note that this is significantly more depolarized than most other neurons.
A high density of
Na+-K+ pumps enables the photoreceptor to maintain a steady intracellular concentration of Na
+ and K
+.
In the dark
Photoreceptor cells
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
are unusual cells in that they depolarize in response to absence of stimuli or scotopic conditions (darkness). In photopic conditions (light), photoreceptors hyperpolarize to a potential of −60 mV.
In the dark, cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarized at about −40 mV, leading to
glutamate
Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...
release which inhibits excitation of neurons.
The depolarization of the cell membrane in
scotopic
In the study of human visual perception, scotopic vision (or scotopia) is the vision of the eye under low-light conditions. The term comes from Greek ''skotos'', meaning "darkness", and ''-opia'', meaning "a condition of sight". In the human eye, ...
conditions opens voltage-gated calcium channels. An increased intracellular concentration of
Ca2+ causes
vesicles
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane
* Synaptic vesicle
; In human embryology
* Vesicle (embryology), bulge-like features o ...
containing glutamate, a
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neuro ...
, to merge with the cell membrane, therefore releasing glutamate into the
synaptic cleft
Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous syste ...
, an area between the end of one cell and the beginning of another
neuron
A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...
. Glutamate, though usually excitatory, functions here as an inhibitory neurotransmitter.
In the cone pathway, glutamate:
* Hyperpolarizes on-center
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 ...
. Glutamate that is released from the photoreceptors in the dark binds to metabotropic glutamate receptors (
mGluR6
Glutamate receptor, metabotropic 6, also known as GRM6 or mGluR6, is a protein which in humans is encoded by the ''GRM6'' gene.
Function
L-glutamate is the major excitatory neurotransmitter in the central nervous system and activates both ion ...
), which, through a G-protein coupling mechanism, causes non-specific cation channels in the cells to close, thus hyperpolarizing the bipolar cell.
* Depolarizes off-center bipolar cells. Binding of glutamate to ionotropic glutamate receptors results in an inward cation current that depolarizes the bipolar cell.
In the light
In summary: ''Light'' closes cGMP-gated sodium channels, reducing the influx of both Na
+ and Ca
2+ ions. Stopping the influx of Na
+ ions effectively switches ''off'' the dark current. Reducing this dark current causes the photoreceptor to ''hyperpolarise'', which reduces glutamate release which thus reduces the ''inhibition'' of retinal nerves, leading to ''excitation'' of these nerves. This reduced Ca
2+ influx during phototransduction enables deactivation and recovery from phototransduction, as discussed below in .
# A light photon interacts with a
retinal
Retinal (also known as retinaldehyde) is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).
Some microorganisms use retin ...
molecule in an
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 pro ...
complex in a
photoreceptor cell
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiatio ...
. The retinal undergoes
isomerisation, changing from the 11-''cis''-retinal to the all-''trans''-retinal configuration.
# Opsin therefore undergoes a conformational change to metarhodopsin II.
# Metarhodopsin II activates a
G protein
G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior. Their act ...
known as
transducin
Transducin (Gt) is a protein naturally expressed in vertebrate retina rods and cones and it is very important in vertebrate phototransduction. It is a type of heterotrimeric G-protein with different α subunits in rod and cone photoreceptors.
L ...
. This causes transducin to dissociate from its bound
GDP
Gross domestic product (GDP) is a monetary measure of the market value of all the final goods and services produced and sold (not resold) in a specific time period by countries. Due to its complex and subjective nature this measure is ofte ...
, and bind
GTP; then the alpha subunit of transducin dissociates from the beta and gamma subunits, with the GTP still bound to the alpha subunit.
# The alpha subunit-GTP complex activates
phosphodiesterase
A phosphodiesterase (PDE) is an enzyme that breaks a phosphodiester bond. Usually, ''phosphodiesterase'' refers to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below. However, there are many oth ...
, also known as PDE6. It binds to one of two regulatory subunits of PDE (which itself is a tetramer) and stimulates its activity.
# PDE hydrolyzes
cGMP, forming
GMP
GMP may refer to:
Finance and economics
* Gross metropolitan product
* Guaranteed maximum price
* Guaranteed Minimum Pension
Science and technology
* GNU Multiple Precision Arithmetic Library, a software library
* Granulocyte-macrophage progenito ...
. This lowers the intracellular concentration of cGMP and therefore the sodium channels close.
# Closure of the sodium channels causes hyperpolarization of the cell due to the ongoing efflux of potassium ions.
# Hyperpolarization of the cell causes voltage-gated calcium channels to close.
# As the calcium level in the photoreceptor cell drops, the amount of the neurotransmitter glutamate that is released by the cell also drops. This is because calcium is required for the glutamate-containing vesicles to fuse with cell membrane and release their contents (see
SNARE proteins).
# A decrease in the amount of glutamate released by the photoreceptors causes depolarization of on-center bipolar cells (rod and cone On bipolar cells) and hyperpolarization of cone off-center bipolar cells.
Deactivation of the phototransduction cascade
In light, low cGMP levels close Na
+ and Ca
2+ channels, reducing intracellular Na
+ and Ca
2+.
During recovery (
dark adaptation
Darkness, the direct opposite of lightness, is defined as a lack of illumination, an absence of visible light, or a surface that absorbs light, such as black or brown.
Human vision is unable to distinguish colors in conditions of very low l ...
), the low Ca
2+ levels induce recovery (termination of the phototransduction cascade), as follows:
# Low intracellular Ca
2+ causes Ca
2+ to dissociate from
guanylate cyclase activating protein (GCAP). The liberated GCAP ultimately restores depleted cGMP levels, which re-opens the cGMP-gated cation channels (restoring dark current).
# Low intracellular Ca
2+ causes Ca
2+ to dissociate from
GTPase-activating protein GTPase-activating proteins or GTPase-accelerating proteins (GAPs) are a family of regulatory proteins whose members can bind to activated G proteins and stimulate their GTPase activity, with the result of terminating the signaling event. GAPs are ...
(GAP), also known as
regulator of G protein signaling
Regulators of G protein signaling (RGS) are protein structural domains or the proteins that contain these domains, that function to activate the GTPase activity of heterotrimeric G-protein α-subunits.
RGS proteins are multi-functional, GTPase-a ...
. The liberated GAP deactivates transducin, terminating the phototransduction cascade (restoring dark current).
# Low intracellular Ca
2+ makes intracellular Ca-recoverin-RK dissociate into Ca
2+ and
recoverin
Recoverin (abbreviated Recov) is a 23 kilodalton (kDa) neuronal calcium-binding protein that is primarily detected in the photoreceptor cells of the eye. It plays a key role in the inhibition of rhodopsin kinase, a molecule which regulates the p ...
and
rhodopsin kinase
Rhodopsin kinase (, ''rod opsin kinase'', ''G-protein-coupled receptor kinase 1'', ''GPCR kinase 1'', ''GRK1'', ''opsin kinase'', ''opsin kinase (phosphorylating)'', ''rhodopsin kinase (phosphorylating)'', ''RK'', ''STK14'') is a serine/threonine-s ...
(RK). The liberated RK then phosphorylates the Metarhodopsin II, reducing its binding affinity for transducin.
Arrestin
Arrestins (abbreviated Arr) are a small family of proteins important for regulating signal transduction at G protein-coupled receptors.
Arrestins were first discovered as a part of a conserved two-step mechanism for regulating the activity of ...
then completely deactivates the phosphorylated-metarhodopsin II, terminating the phototransduction cascade (restoring dark current).
# Low intracellular Ca
2+ make the Ca
2+/
calmodulin
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bind ...
complex within the cGMP-gated cation channels more sensitive to low cGMP levels (thereby, keeping the cGMP-gated cation channel open even at low cGMP levels, restoring dark current)
In more detail:
GTPase Accelerating Protein (GAP) of RGS (regulators of G protein signaling) interacts with the alpha subunit of transducin, and causes it to hydrolyse its bound GTP to GDP, and thus halts the action of phosphodiesterase, stopping the transformation of cGMP to GMP. This deactivation step of the phototransduction cascade (the deactivation of the G protein transducer) was found to be the rate limiting step in the deactivation of the phototransduction cascade.
[.]
In other words: Guanylate Cyclase Activating Protein (GCAP) is a calcium binding protein, and as the calcium levels in the cell have decreased, GCAP dissociates from its bound calcium ions, and interacts with Guanylate Cyclase, activating it. Guanylate Cyclase then proceeds to transform GTP to cGMP, replenishing the cell's cGMP levels and thus reopening the sodium channels that were closed during phototransduction.
Finally, Metarhodopsin II is deactivated. Recoverin, another calcium binding protein, is normally bound to Rhodopsin Kinase when calcium is present. When the calcium levels fall during phototransduction, the calcium dissociates from recoverin, and rhodopsin kinase is released and phosphorylates
metarhodopsin II, which decreases its affinity for transducin. Finally, arrestin, another protein, binds the phosphorylated metarhodopsin II, completely deactivating it. Thus, finally, phototransduction is deactivated, and the dark current and glutamate release is restored. It is this pathway, where Metarhodopsin II is phosphorylated and bound to arrestin and thus deactivated, which is thought to be responsible for the S2 component of dark adaptation. The S2 component represents a linear section of the dark adaptation function present at the beginning of dark adaptation for all bleaching intensities.
Phototransduction in invertebrates
Phototransduction process in
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...
s like the
fruit fly is different from the vertebrates.
PI(4,5)P2 cycle underlies the phototransduction process. Here, light induces the conformational change into
rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduction ...
and converts it into meta-rhodopsin. This helps in dissociation of G-protein complex. Alpha sub-unit of this complex activates the
PLC enzyme (PLC-beta) which hydrolyze the
PIP2
Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)''P''2, also known simply as PIP2 or PI(4,5)P2, is a minor phospholipid component of cell membranes. PtdIns(4,5)''P''2 is enriched at the plasma membrane where it is a substrate for a number of ...
into
DAG. This hydrolysis leads to opening of
TRP channels and influx of calcium.
References
* Moiseyev G, Chen Y, Takahashi Y, Wu BX, Ma JX. ''RPE65 is the isomerohydrolase in the retinoid visual cycle.'' Proc. Natl. Acad. Sci. 200
Article
* Jin M, Li S, Moghrabi WN, Sun H, Travis GH. ''Rpe65 is the retinoid isomerase in bovine retinal pigment epithelium.'' Cell. 200
Article
External links
Visual pigments and visual transduction at med.utah.eduTransduction of Light Prezi*
{{Signal transduction
Visual system
Nervous system
Sensory receptors
Metabolism