Editing Photoreceptor cell (section)

== Signaling ==
[[File:1415 Retinal Isomers.jpg|thumb|right|400px|The absorption of light leads to an isomeric change in the retinal molecule.]]
{{main|Visual phototransduction}}

The path of a visual signal is described by the [[phototransduction cascade]], 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 that apply to the phototransduction pathway from vertebrate rod/cone photoreceptors are:

#The [[Vertebrate visual opsin]] in the disc membrane of the outer segment absorbs a photon, changing the configuration of a [[retinal]] [[Schiff base]] [[Cofactor (biochemistry)|cofactor]] inside the protein from the cis-form to the trans-form, causing the retinal to change shape.
#This results in a series of unstable intermediates, the last of which binds stronger to a [[G protein]] in the [[plasma membrane|membrane]], called [[transducin]], and activates it. This is the first amplification step – each photoactivated opsin triggers activation of about 100 transducins. 
#Each transducin then activates the [[enzyme]] cGMP-specific [[phosphodiesterase]] (PDE).
#PDE then catalyzes the hydrolysis of cGMP to 5' GMP. This is the second amplification step, where a single PDE hydrolyses about 1000 cGMP molecules.
#The net concentration of intracellular cGMP is reduced (due to its conversion to 5' GMP via PDE), resulting in the closure of cyclic nucleotide-gated Na<sup>+</sup> ion channels located in the photoreceptor outer segment membrane.
#As a result, sodium ions can no longer enter the cell, and the photoreceptor outer segment membrane becomes [[Hyperpolarization (biology)|hyperpolarized]], due to the charge inside the membrane becoming more negative.
#This change in the cell's membrane potential causes voltage-gated calcium channels to close. This leads to a decrease in the influx of calcium ions into the cell and thus the intracellular calcium ion concentration falls.
#A decrease in the intracellular calcium concentration means that less glutamate is released via calcium-induced exocytosis to the bipolar cell (see below). (The decreased calcium level slows the release of the neurotransmitter [[glutamate]], which excites the postsynaptic [[bipolar cell]]s and [[Retina horizontal cell|horizontal cells]].)
#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.

===Hyperpolarization===
Unlike most sensory receptor cells, photoreceptors actually become [[hyperpolarization (biology)|hyperpolarized]] when stimulated; and conversely are [[depolarization|depolarized]] when not stimulated. This means that glutamate is released continuously when the cell is unstimulated, and stimulus causes release to stop. In the dark, cells have a relatively high concentration of [[cyclic guanosine 3'-5' monophosphate]] (cGMP), which opens [[Cyclic nucleotide–gated ion channel#Photoreceptors|cGMP-gated ion channels]]. These channels are nonspecific, allowing movement of both sodium and calcium ions when open. The movement of these positively charged ions into the cell (driven by their respective [[electrochemical gradient]]) depolarizes the membrane, and leads to the release of the neurotransmitter [[glutamate]].

Unstimulated (in the dark), cyclic-nucleotide gated channels in the outer segment are open because [[cyclic GMP]] (cGMP) is bound to them. Hence, positively charged ions (namely [[sodium]] [[ion]]s) enter the photoreceptor, depolarizing it to about −40 mV ([[resting potential]] in other nerve cells is usually −65 mV). This depolarization [[Electric current|current]] is often known as dark current.

===Bipolar cells===
The photoreceptors (''rods'' and ''cones'') transmit to the bipolar cells, which transmit then to the retinal ganglion cells. Retinal ganglion cell axons collectively form the [[optic nerve]], via which they project to the brain.<ref name =Schacter137/>

The rod and cone photoreceptors signal their absorption of photons via a decrease in the 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 [[presynaptic]] terminal to the bipolar cell.

Every rod or cone photoreceptor releases the same neurotransmitter, glutamate. However, the effect of glutamate differs in the bipolar cells, depending upon the type of [[Receptor (biochemistry)|receptor]] imbedded in that [[cell membrane|cell's membrane]]. When glutamate binds to an [[Ligand-gated ion channel|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 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 [[opponent process|detecting color]], [[contrast (vision)|contrast]], [[edge detection|edges]], etc.

=== Advantages ===
Phototransduction in rods and cones is somewhat unusual in that the [[stimulus (physiology)|stimulus]] (in this case, light) reduces the cell's response or firing rate, different from most other sensory systems in which a stimulus increases the cell's response or firing rate. This difference has important functional consequences:
# the classic (rod or cone) 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 number of channels, through absorption of a photon, will affect it and signal that light is in the visual field. This system may have less noise relative to sensory transduction schema that increase rate of neural firing in response to stimulus, like [[Somatosensory system|touch]] and [[olfaction]].
# there is a lot of amplification in two stages of classic 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 is the main feature that 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.