Photoreceptors

A photoreceptor is a specialized type of neuron 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.

Structure
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.

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.

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).

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.

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 is usually -65 mV). This depolarizing current is often known as dark current.

Signal Transduction Pathway
Suppose a light source emits photons in the visual field. The steps in phototransduction, which constitute a signal transduction pathway, are then:


 * 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.
 * 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.
 * 3) Each transducin then activates the enzyme phosphodiesterase (PDE).
 * 4) PDE then catalyzes the hydrolysis of cGMP.
 * 5) The intracellular concentration of cGMP is reduced, and because cGMP was keeping the channels open, the net result is closing of 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).
 * 7) This hyperpolarization means that less glutamate is released to the bipolar cell than before (see below).
 * 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.

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.

Advantages
Phototransduction is very 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.

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.

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. Hence, the system is very sensitive.

Function
Photoreceptors do not signal color; they only signal the presence of light in the visual field.

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.

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.

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.

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 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.

Further complexity arises from the various interconnections among bipolar cells in the retina, as well as populations of 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.

Photorécepteur Фоторецептор Fotorreceptor