| Rod cell | |
|---|---|
 |
|
| Location | Retina |
| Function | Low light photoreceptor |
| Morphology | rod shaped |
| Presynaptic connections | None |
| Postsynaptic connections | Bipolar Cells and Horizontal cells |
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. A photoreceptor, or photoreceptor cell, is a specialized type of Neuron (nerve cell found in the Eye 's Retina that is capable of The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism As a part of the Retina, the bipolar cell exists between photoreceptors ( Rod cells and Cone cells and Ganglion cells They act to Horizontal cells are the laterally interconnecting neurons in the outer plexiform layer of the Retina of mammalian eyes A photoreceptor, or photoreceptor cell, is a specialized type of Neuron (nerve cell found in the Eye 's Retina that is capable of The vertebrate retina is a light sensitive part inside the inner layer of the Eye. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. Since they are more light-sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. Peripheral vision is a part of vision that occurs outside the very center of gaze There are about 120 million rod cells in the human retina.
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 cones. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Since rods require less light to function than cones, they are therefore the primary source of visual information at night (scotopic vision). Scotopic vision is the Monochromatic vision of the Eye in low light 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. An interneuron (also called association neuron, local circuit neuron or relay neuron) is a neuron which connects Afferent neurons and Efferent 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. Image resolution describes the detail an Image holds The term applies equally to Digital images film images and other types of images The visual system is the part of the Nervous system which allows organisms to see. 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.
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.
Rod cells also respond more slowly to light than cones do, so stimuli they receive 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[1] However, if multiple flashes of sub-threshold light occurs during the 100 millisecond period, the energy of the flashes of light would summate to produce a light which 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). George Wald ( November 18, 1906 &ndash April 12, 1997) was an American Scientist who is best known for his work with pigments 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. The Purkinje effect (sometimes called the Purkinje shift, or dark adaptation and named after the Czech Anatomist Jan Evangelista Purkyně
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, for example a bipolar cell. Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands A bipolar cell is a type of Neuron which has two extensions Bipolar cells are specialized sensory neurons for the transmission of special senses The inner and outer segments are connected by a cilium. A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately [1] 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. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed In Cell biology, the nucleus (pl nuclei; from Latin la ''nucleus'' or la ''nuculeus'' "little nut" or kernel is a membrane-enclosed [1]
Response to light
Activation of a photoreceptor cell is actually a hyperpolarization; when they are not being stimulated, rods and cones depolarize and release a neurotransmitter spontaneously, and activation of photopigments by light sends a signal by preventing this. Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. In biology depolarization is a decrease in the Absolute value of a cell's Membrane potential. See Chemical synapse for an introduction to concepts and terminology used in this article Depolarization 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). Cyclic guanosine monophosphate ( cGMP) is a Cyclic nucleotide derived from Guanosine triphosphate (GTP Ion channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living Sodium channels are Integral membrane proteins that form Ion channels, conducting sodium ions ( Na+) through a cell's Plasma membrane Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 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. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Cellular biology, an electrochemical gradient is a spatial variation of both Electrical potential and chemical Concentration across a membrane Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its 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 (iodopsin is found in cone cells) consists of 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). Rhodopsin, also known as visual purple, is a Pigment of the Retina that is responsible for both the formation of the Photoreceptor cells and the Vitamin A refers to a family of similarly shaped molecules the Retinoids. 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 it to activate a regulatory protein called transducin, which leads to the activation of cGMP phosphodiesterase, which breaks cGMP down into 5'-GMP. Transducin (also called Gt is a heterotrimeric G protein that is naturally expressed in Vertebrate Retina rods and cones (a different 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 transmitter substance acetyl choline, rod cells use a variety. The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central The entire process by which light initiates a sensory response is called visual phototransduction. Visual phototransduction is a process by which Light is converted into Electrical signals in the Rod cells Cone cells and Photosensitive ganglion
Activation of a single molecule 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 activate a phosphodiesterase molecule, which can break down over a thousand cGMP molecules per second. [1] 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.
Revert to the resting state
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 would phosphorylate the cytosolic tail of the activated rhodopsin on the 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.
While arrestin shuts off rhodopsin, an RGS protein (functioning as a GTPase-activiating proteins(GAPs)) drives the transducin (G-protein) into an "off" state by increasing the rate of hydrolysis of the bounded GTP to GDP.
Also as the cGMP sensitive channels allow not only the influx of sodium ions, but also calcium ions, with the decrease in concentration of cGMP, cGMP sensitive channels are then closed and reducing the normal influx of calcium ions. The decrease in the concentration of calcium ions stimulates the calcium ion-sensitive proteins, which would then activiate the guanylyl cyclase to replenish the cGMP, rapidly restoring its original concentration. The restoration opens the cGMP sensitive channels and causes a depolarization of the plasma membrane. [2]
Desensitization
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 desensitization process in at least two ways. First, it prevents the interaction between the G protein and the activated receptor. Second, it serves as an adaptor protein to aid the receptor to the clathrin-dependent endocytosis machinery (to induce receptor-mediated endocytosis). [2]
Table
Comparison of rod and cone cells, from Kandel Kandel E. Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. R. , Schwartz, J. H. , Jessell, T. M. (2000). Principles of Neural Science, 4th ed. , pp. 507-513. McGraw-Hill, New York.
| Rods | Cones |
|---|---|
| Used for night vision | Used for day vision |
| Highly sensitive to light; sensitive to scattered light (they have more pigment than cones) | At least 1/10th 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 |
References
- ^ a b c d Kandel E. Nyctalopia (Greek for " night blindness " is a condition making it difficult or impossible to see in relatively low light Blindness is the condition of lacking Visual perception due to Physiological or Neurological factors is a one volume manga created by Tsutomu Nihei as a prequel to his ten-volume work Blame!. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. R. , Schwartz, J. H. , Jessell, T. M. (2000). Principles of Neural Science, 4th ed. , pp. 507-513. McGraw-Hill, New York.
- ^ a b Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter (2008). Molecular Biology of The Cell, 5th ed. , pp. 919-921. Garland Science.
See also
In a Sensory system, a sensory receptor is a structure that recognizes a stimulus in the internal or external environment of an Organism.Dictionary
rod cell
-noun
- (anatomy) Any of the cylindrical photoreceptor cells in the retina that function in dim light levels.
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