Photoreceptor cell

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 absorbs photons from the visual field, and through a specific and complex biochemical pathway, signals this information through a change in its membrane potential. Ultimately, 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 molluscs are different in both their morphological organization and their underlying biochemical pathways.

In vertebrates, photoreceptors come in two types: rods and cones, with major functional differences between the two. 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 120 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.^[2]

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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 absorbs photons, as well as voltage-gated sodium channels.

The membranous photoreceptor protein opsin 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. Three different classes of photopsins react to different ranges of light frequency, a differentation which eventually allows the nervous system to distinguish color. The function of the photoreceptor cell is to convert the light energy of the photon into a form of energy communicable to the nervous system and more readily usable to the organism: this conversion is called signal transduction.

Humans

In humans, the visual system uses millions of photoreceptors to view, perceive, and analyze the visual world. With the exception of melanopsin-containing photosensitive ganglion cells, ocular photoreceptors are the only neurons in humans capable of phototransduction. 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 inverted arrangement significantly reduces acuity,^{[citation needed]} as light must travel through the axons and cell bodies of other neurons before reaching the photoreceptors. The retina contains two specializations to deal with this issue. First, a region at the center of the retina, called the fovea, containing only photoreceptors, is used for high visual acuity. Second, each retina contains a blind spot, an area 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 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 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). Because cones respond to both the wavelength and intensity of light, a single cone cannot tell color; instead, color vision requires interactions of more than one type of cone (see below), primarily by comparing responses across different cone types.

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. Hence, positively charged sodium ions enter 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.

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:

1. The rhodopsin or iodopsin in the outer segment absorbs a photon, changing the configuration of a retinal Schiff base cofactor inside the protein from the cis-form to the trans-form, causing the retinal to change shape.
2. This results in a series of unstable intermediates, the last of which binds stronger 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, where a single 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 cyclic nucleotide-gated 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.

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 unique in that the stimulus (in this case, light) actually reduces the cell's response or firing rate, which is unusual for a sensory system where the stimulus usually increases the cell's response or firing rate. However, this system offers 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. This is 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.

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. For example, red light at a certain intensity can produce the same exact response in a photoreceptor as green light of a different intensity. Therefore, the response of a single photoreceptor is ambiguous when it comes to color.

To determine color, the visual system compares responses across a population of photoreceptors (specifically, the three different cones with differing absorption spectra). To determine intensity, the visual system computes 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 presynaptic 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, 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.

See also

• Circadian rhythm
• G protein-coupled receptor
• Sensory system
• Photosensitive
• Photosensitive ganglion cell
• Horizontal cell
• Bipolar cell
• Amacrine cell
• Ganglion cell

Bibliography

• Campbell, Neil A., and Reece, Jane B. (2002). Biology. San Francisco: Benjamin Cummings, 1064-1067. ISBN 0-8053-6624-5. 

• Freeman, Scott (2002). Biological Science (2nd Edition). Englewood Cliffs, N.J: Prentice Hall, 835-837. ISBN 0-13-140941-7. 

References