|Function||Low light photoreceptor|
|Morphology||Long and narrow with rod shaped end portion.|
|Postsynaptic connections||Bipolar Cells and Horizontal cells|
A rod cell, or rod, is any of the generally cylindrically or rod-shaped photoreceptor cells in the retina of the eye that are sensitive to dim light and lack the visual acuity and color-distinguishing ability of the other type of photoreceptor, cone cells. Since they can function in less intense light than cone cells, rods are responsible for night vision in humans and predominate in nocturnal vertebrates. Named for the cylindrical shape of the part of the cell that responds directly to light, these photosensitive cells are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 120 million rod cells in the human retina.
The human visual system of rods and cones is a complementary one, allowing one to see in both low light conditions (rods) and to see a diversity of colors in brighter light (cones). While cones allow humans to experience the great beauty that color adds to perceptions of the environment, rods allow perception in dim light, opening possibilities for experiencing the darkened world of a moonlit night or a cave, or seeing distant stars on a moonless night.
The retina contains two forms of photosensitive cells—rods and cones. Though structurally and metabolically similar, their functions are quite different. Rod cells are highly sensitive to light, allowing them to respond in dim light and dark conditions. These are the cells that allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). However, rod cells do not distinguish between colors, and have low visual acuity (measure of detail). This is why the darker conditions become, the less color and definition objects seem to have. Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different colors (wavelengths of light), which allows an organism to see color.
Rods and cones are both photosensitive, but respond differently to different frequencies of light. They both contain different pigmented photoreceptor complexes. Rod cells contain the protein-chromophore complex, rhodopsin and cone cells contain different complexes for each color-range. The process through which these complexes work is quite similar—upon being subjected to electromagnetic radiation of a particular wavelength and intensity, the chromophore, called retinal, undergoes a structural change that destabilizes the complex and thereby causes the protein, an opsin, to pass through a series of changes that concludes with the complex separating into separate retinal and opsin units. Rhodopsin, of rods, breaks down into opsin and retinal; the three photopsins of cones break down into retinal and three different opsins. All of the different opsins trigger a change in the membrane protein transducin, which in turn activates the enzyme phosphodiesterase, which catalyzes a molecular change that causes sodium ion channels in the cell membrane to close. This leads to the generation of an action potential (an impulse that will eventually reach the visual cortex in the brain).
This is the reason why cones and rods enable organisms to see in dark and light conditions—each of the photoreceptor complexes is broken down into its component parts by light of different intensity levels. Further, signals from hundreds or thousands of rod cells are combined and transmitted to the visual cortex through a single bipolar cell connected to a single ganglion cell leading to the brain. On the other hand, a single cone cell is connected to a single bipolar cell. Thus, action potentials from rods share neurons, whereas those from cones are given their own. This results in the high visual acuity, or the high ability to distinguish between detail, of cone cells and not rods. If a ray of light were to reach just one rod cell this may not be enough to stimulate an action potential in the bipolar cell. Only after a bipolar cell accumulates a sufficient number of neurotransmitter molecules received from different rod cells "converging" onto the synapse of the one bipolar cell, will the bipolar cell attain the threshold level to generate its own action potential that sends a signal to the ganglion.
Oyster (1999) cites evidence for an average of about 90 million rod cells and 4.5 million cone cells in the human retina.
Like cone cells, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, usually a bipolar cell. The inner and outer segments are connected by a cilium (Kandel et al. 2000). The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the front of the eye, contains the light-absorbing materials (Kandel et al. 2000).
Like the photo-sensitive parts of cone cells, the outer segments of rod cells have invaginations of the cell membranes that create stacks of membranous disks. Within the disks, photopigments exist as transmembrane proteins covalently bonded to the photosensitive molecule retinal. The surfaces of the membranous disks provide more surface area in which the photopigments can be collected. In the cone portions of cone cells, these disks are attached to the outer membrane, whereas they are pinched off and exist separately in rods. Neither rod cells nor cone cells divide, but their membranous disks wear out and are worn off at the end of the outer segment, to be consumed and recycled by phagocytic cells.
A rod cell is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than a cone cell. Since rod cells require less light to function than cone cells, they are therefore the primary source of visual information at night (scotopic vision). Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, collecting and amplifying the signals. However, this convergence comes at a cost to visual acuity (or Image resolution) since the pooled information from multiple cells is less distinct than it would be if the visual system received information from each rod cell individually. The convergence of rod cells also tends to make peripheral vision very sensitive to movement, and is responsible for the phenomenon of an individual seeing something vague occur out of the corner of his or her eye.
Rod cells also respond more slowly to light than do cone cells, so stimuli received by rod cells are added over about 100 milliseconds. While this makes rods more sensitive to smaller amounts of light, it also means that their ability to sense temporal changes, such as quickly changing images, is less accurate than that of cones (Kandel et al. 2000). However, if multiple flashes of sub-threshold light occur during the 100 millisecond period, the energy of the flashes of light would aggregate to produce a light that will reach threshold and send a signal to the brain.
Experiments by George Wald and others showed that rods are more sensitive to the blue area of the spectrum, and are completely insensitive to wavelengths above about 640 nm (red). This fact is responsible for the Purkinje effect, in which blue colors appear more intense relative to reds in darker light, when rods take over as the cells responsible for vision.
In humans, the fovea, directly behind the lens, consists mostly of densely-packed cone cells. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other objects, using central vision because the light from these is not enough to stimulate cone cells. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light can stimulate cells, allowing the individual to observe distant stars. Because they have only one type of light sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in color vision.
Activation of a photoreceptor cell is actually a hyperpolarization (inhibition) of the cell. When they are not being stimulated, such as in the dark, rod cells and cone cells depolarize and release a neurotransmitter spontaneously. This neurotransmitter hyperpolarizes the bipolar cell. Bipolar cells exist between photoreceptors and ganglion cells and act to transmit signals from the photoreceptors to the ganglion cells. As a result of the bipolar cell being hyperpolarized, it does not release its transmitter at the bipolar-ganglion synapse and the synapse is not excited.
Activation of photopigments by light sends a signal by hyperpolarizing the rod cell, leading to the rod cell not sending its neurotransmitter, which leads to the bipolar cell then releasing its transmitter at the bipolar-ganglion synapse and exciting the synapse.
Depolarization of rod cells (causing release of their neurotransmitter) occurs because in the dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens ion channels (largely sodium channels, though calcium can enter through these channels as well). The positive charges of the ions that enter the cell down its electrochemical gradient change the cell's membrane potential, cause depolarization, and lead to the release of the neurotransmitter glutamate. Glutamate can depolarize some neurons and hyperpolarize others, allowing photoreceptors to interact in an antagonistic manner.
When light hits photoreceptive pigments within the photoreceptor cell, the pigment changes shape. The pigment, called rhodopsin (photopsin is found in cone cells) comprises a large protein called opsin (situated in the plasma membrane), attached to which is a covalently-bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes a series of changes in the opsin that ultimately lead it to activate a regulatory protein called transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of neurotransmitters (Kandel et al., 2000). Though cone cells primarily use the neurotransmitter substance acetylcholine, rod cells use a variety. The entire process by which light initiates a sensory response is called visual phototransduction.
Activation of a single unit of rhodopsin, the photosensitive pigment in rods, can lead to a large reaction in the cell because the signal is amplified. Once activated, rhodopsin can activate hundreds of transducin molecules, each of which in turn activates a phosphodiesterase molecule, which can break down over a thousand cGMP molecules per second (Kandel et al. 2000). Thus, rods can have a large response to a small amount of light.
As the retinal component of rhodopsin is derived from vitamin A, a deficiency of vitamin A causes a deficit in the pigment needed by rod cells. Consequently, fewer rod cells are able to sufficiently respond in darker conditions, and as the cone cells are poorly adapted for sight in the dark, blindness can result. This is night-blindness.
Rods make use of three inhibitory mechanisms (negative feedback mechanisms) to allow a rapid revert to the resting state after a flash of light.
Firstly, there exists a rhodopsin kinase (RK), which phosphorylates (attaches a phosphate group to) the cytosolic (extending into the cell cytosol) tail of the activated rhodopsin on its multiple serines, partially inhibiting the activation of transducin. Also, an inhibitory protein, arrestin, then binds to the phosphorylated rhodopsins to further inhibit the rhodopsin's activity.
Secondly, while arrestin shuts off rhodopsin, a regulatory protein drives the transducin (a G-protein, which is essentially a protein on-off switch) into an "off" state by increasing the rate of hydrolysis of the bound GTP (guanine triphosphate) to GDP (guanine diphosphate).
Thirdly, with the decrease in concentration of cGMP, cGMP sensitive channels are closed, reducing the normal influx of calcium ions through the open cGMP sensitive channels, which also allow the influx of sodiumions. The decrease in the concentration of calcium ions stimulates the calcium ion-sensitive proteins, which would then activate the guanylyl cyclase (a transmembrane protein and enzyme) to replenish the cGMP, rapidly restoring its original concentration. The restoration opens the cGMP sensitive channels and causes a depolarization of the plasma membrane (Alberts et al. 2008).
When the rods are exposed to a high concentration of photons for a prolonged period, they become desensitized (adapted) to the environment.
As rhodopsin is phosphorylated by rhodopsin kinase (a member of the GPCR kinases (GRKs)), it binds with high affinity to the arrestin. The bound arrestin can contribute to the densitization process in at least two ways. First, it prevents the interaction between the G-protein and the activated receptor. Second, it serves as an adapter protein to aid the receptor to the clathrin-dependent endocytosis machinery (to induce receptor-mediated endocytosis) (Alberts et al. 2008).
Comparison of rod and cone cells, from Kandel et al. (2000).
|Used for night vision||Used for day vision|
|Highly sensitive to light; sensitive to scattered light (they have more pigment than cones)||At least a tenth of the rods' light sensitivity; sensitive only to direct light|
|Loss causes night blindness||Loss constitutes legal blindness|
|Low spatial resolution with higher noise||High spatial resolution with lower noise|
|Not present in the fovea||Concentrated in the fovea|
|Slower response to light; rods need to be exposed to light over time||Quicker response to light; can perceive more rapid changes in stimuli|
|Stacks of membrane-enclosed disks are unattached to the cell membrane||Disks are attached to the outer membrane|
|22 times as numerous as cones in the retina|
|One type of photosensitive pigment (monochromatic stimulus)||Three types of photosensitive pigment in humans (trichromatic stimulus)|
|Confer achromatic vision, with more emphasis on detecting motion||Confer colour vision, with more emphasis on detecting fine details|
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