1.
Visual system
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The visual system is the part of the central nervous system which gives organisms the ability to process visual detail, as well as enabling the formation of several non-image photo response functions. It detects and interprets information from visible light to build a representation of the surrounding environment, the psychological process of visual information is known as visual perception, a lack of which is called blindness. Non-image forming visual functions, independent of perception, include the pupillary light reflex. This article mostly describes the system of mammals, humans in particular. Together the cornea and lens refract light into a small image, the retina transcribes this image into electrical pulses using rods and cones. The optic nerve then carries these pulses through the optic canal, upon reaching the optic chiasm the nerve fibers decussate. The fibers then branch and terminate in three places, most end in the lateral geniculate nucleus. Before the LGN forwards the pulses to V1 of the visual cortex it gauges the range of objects, the LGN also sends some fibers to V2 and V3. V1 performs edge-detection to understand spatial organization, V2 both forwards pulses to V1 and receives them. Pulvinar is responsible for saccade and visual attention, V2 serves much the same function as V1, however, it also handles illusory contours, determining depth by comparing left and right pulses, and foreground distinguishment. V3 helps process ‘global motion’ of objects, V3 connects to V1, V2, and the inferior temporal cortex. V4 recognizes simple shapes, gets input from V1, V2, V3, LGN, v5’s outputs include V4 and its surrounding area, and eye-movement motor cortices. V5’s functionality is similar to that of the other V’s, however, V6 works in conjunction with V5 on motion analysis. V5 analyzes self-motion, whereas V6 analyzes motion of objects relative to the background, v6’s primary input is V1, with V5 additions. V6 houses the topographical map for vision, V6 outputs to the region directly around it. V6A has direct connections to arm-moving cortices, including the premotor cortex, the inferior temporal gyrus recognizes complex shapes, objects, and faces or, in conjunction with the hippocampus, creates new memories. The pretectal area is seven unique nuclei, anterior, posterior and medial pretectal nuclei inhibit pain, aid in REM, and aid the accommodation reflex, respectively. The Edinger-Westphal nucleus moderates pupil dilation and aids in convergence of the eyes, nuclei of the optic tract are involved in smooth pursuit eye movement and the accommodation reflex, as well as REM
2.
Light
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Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to light, which is visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m. This wavelength means a range of roughly 430–750 terahertz. The main source of light on Earth is the Sun, sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the used by living things. Historically, another important source of light for humans has been fire, with the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence, for example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey. Visible light, as all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term sometimes refers to electromagnetic radiation of any wavelength. In this sense, gamma rays, X-rays, microwaves and radio waves are also light, like all types of light, visible light is emitted and absorbed in tiny packets called photons and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality, the study of light, known as optics, is an important research area in modern physics. Generally, EM radiation, or EMR, is classified by wavelength into radio, microwave, infrared, the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths, when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. There exist animals that are sensitive to various types of infrared, infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it, above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the rods and cones located in the retina of the eye cannot detect the very short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm
3.
Photoreceptor cell
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A photoreceptor cell is a specialized type of cell found in the retina that is capable of phototransduction. The great biological importance of photoreceptors is that they convert light signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, there are currently three known types of photoreceptor cells in mammalian eyes, rods, cones, and photosensitive retinal ganglion cells. The two classic photoreceptor cells are rods and cones, each contributing information used by the system to form a representation of the visual world. The rods are narrower than the cones and distributed differently across the retina, a third class of mammalian photoreceptor cell was discovered during the 1990s, the photosensitive ganglion cells. These cells do not contribute to sight directly, but are thought to support circadian rhythms, there are major functional differences between the rods and cones. Rods are extremely sensitive, and can be triggered by a single photon, at very low light levels, visual experience is based solely on the rod signal. This explains why colors cannot be seen at low light levels, cones require significantly brighter light in order to produce a signal. In humans, there are three different types of cell, distinguished by their pattern of response to different wavelengths of light. Color experience is calculated from three distinct signals, perhaps via an opponent process. The three types of cone cell respond to light of short, medium, and long wavelengths, note that, due to the principle of univariance, the firing of the cell depends upon only the number of photons absorbed. The different responses of the three types of cells are determined by the likelihoods that their respective photoreceptor proteins will absorb photons of different wavelengths. So, for example, an L cone cell contains a protein that more readily absorbs long wavelengths of light. Light of a wavelength can also produce the same response. The human retina contains about 120 million rod cells, and 6 million cone cells, the number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal. Certain owls, such as the owl, have a tremendous number of rods in their retinae. In addition, there are about 2.4 million to 3 million ganglion cells in the visual system,1 to 2% of them photosensitive. The axons of cells form the two optic nerves
4.
Wavelength
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In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above
5.
Protein
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Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, a linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide, short polypeptides, containing less than 20–30 residues, are rarely considered to be proteins and are commonly called peptides, or sometimes oligopeptides. The individual amino acid residues are bonded together by peptide bonds, the sequence of amino acid residues in a protein is defined by the sequence of a gene, which is encoded in the genetic code. In general, the code specifies 20 standard amino acids, however. Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors, proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes. Once formed, proteins only exist for a period of time and are then degraded and recycled by the cells machinery through the process of protein turnover. A proteins lifespan is measured in terms of its half-life and covers a wide range and they can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal and or misfolded proteins are degraded more rapidly due to being targeted for destruction or due to being unstable. Like other biological macromolecules such as polysaccharides and nucleic acids, proteins are essential parts of organisms, many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism. Proteins also have structural or mechanical functions, such as actin and myosin in muscle and the proteins in the cytoskeleton, other proteins are important in cell signaling, immune responses, cell adhesion, and the cell cycle. In animals, proteins are needed in the diet to provide the essential amino acids that cannot be synthesized, digestion breaks the proteins down for use in the metabolism. Methods commonly used to study structure and function include immunohistochemistry, site-directed mutagenesis, X-ray crystallography, nuclear magnetic resonance. Most proteins consist of linear polymers built from series of up to 20 different L-α-amino acids, all proteinogenic amino acids possess common structural features, including an α-carbon to which an amino group, a carboxyl group, and a variable side chain are bonded. Only proline differs from this structure as it contains an unusual ring to the N-end amine group. The amino acids in a chain are linked by peptide bonds. Once linked in the chain, an individual amino acid is called a residue, and the linked series of carbon, nitrogen. The peptide bond has two forms that contribute some double-bond character and inhibit rotation around its axis, so that the alpha carbons are roughly coplanar
6.
Potassium
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Potassium is a chemical element with symbol K and atomic number 19. It was first isolated from potash, the ashes of plants, in the periodic table, potassium is one of the alkali metals. Potassium in nature only in ionic salts. It is found dissolved in sea water, and is part of many minerals, naturally occurring potassium is composed of three isotopes, of which 40K is radioactive. Traces of 40K are found in all potassium, and it is the most common radioisotope in the human body, Potassium is chemically very similar to sodium, the previous element in Group 1 of the periodic table. They have a similar energy, which allows for each atom to give up its sole outer electron. That they are different elements combine with the same anions to make similar salts was suspected in 1702. Most industrial applications of potassium exploit the high solubility in water of potassium compounds, heavy crop production rapidly depletes the soil of potassium, and this can be remedied with agricultural fertilizers containing potassium, accounting for 95% of global potassium chemical production. Potassium ions are necessary for the function of all living cells, fresh fruits and vegetables are good dietary sources of potassium. Potassium is the second least dense metal after lithium and it is a soft solid with a low melting point, and can be easily cut with a knife. Freshly cut potassium is silvery in appearance, but it begins to tarnish toward gray immediately on exposure to air, in a flame test, potassium and its compounds emit a lilac color with a peak emission wavelength of 766.5 nanometers. Neutral potassium atoms have 19 electrons, one more than the stable configuration of the noble gas argon. This process requires so little energy that potassium is readily oxidized by atmospheric oxygen, in contrast, the second ionization energy is very high, because removal of two electrons breaks the stable noble gas electronic configuration. Potassium therefore does not readily form compounds with the state of +2 or higher. Potassium is an active metal that reacts violently with oxygen in water. With oxygen it forms potassium peroxide, and with water potassium forms potassium hydroxide, the reaction of potassium with water is dangerous because of its violent exothermic character and the production of hydrogen gas. Hydrogen reacts again with atmospheric oxygen, producing water, which reacts with the remaining potassium and this reaction requires only traces of water, because of this, potassium and the liquid sodium-potassium — NaK — are potent desiccants that can be used to dry solvents prior to distillation. Because of the sensitivity of potassium to water and air, reactions with other elements are only in an inert atmosphere such as argon gas using air-free techniques
7.
Signal
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A signal as referred to in communication systems, signal processing, and electrical engineering is a function that conveys information about the behavior or attributes of some phenomenon. The IEEE Transactions on Signal Processing states that the signal includes audio, video, speech, image, communication, geophysical, sonar, radar. Typically, signals are provided by a sensor, and often the form of a signal is converted to another form of energy using a transducer. For example, a microphone converts a signal to a voltage waveform. The formal study of the content of signals is the field of information theory. The information in a signal is accompanied by noise. The term noise usually means an undesirable random disturbance, but is extended to include unwanted signals conflicting with the desired signal. The prevention of noise is covered in part under the heading of signal integrity, the separation of desired signals from a background is the field of signal recovery, one branch of which is estimation theory, a probabilistic approach to suppressing random disturbances. Engineering disciplines such as electrical engineering have led the way in the design, study, and implementation of systems involving transmission, storage, definitions specific to sub-fields are common. For example, in theory, a signal is a codified message, that is. In the context of signal processing, arbitrary binary data streams are not considered as signals, in a communication system, a transmitter encodes a message to a signal, which is carried to a receiver by the communications channel. For example, the words Mary had a little lamb might be the message spoken into a telephone, the telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the telephone by wires, at the receiver it is reconverted into sounds. In telephone networks, signalling, for example common-channel signaling, refers to number and other digital control information rather than the actual voice signal. Signals can be categorized in various ways, the most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are often referred to as series in other fields. Continuous-time signals are often referred to as continuous signals even when the functions are not continuous. A second important distinction is between discrete-valued and continuous-valued, particularly in digital signal processing a digital signal is sometimes defined as a sequence of discrete values, that may or may not be derived from an underlying continuous-valued physical process
8.
George Wald
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George David Wald was an American scientist who was best known for his work with pigments in the retina. He won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline, as a postdoctoral researcher, Wald discovered that vitamin A was a component of the retina. His further experiments showed that when the pigment rhodopsin was exposed to light, it yielded the protein opsin and this suggested that vitamin A was essential in retinal function. In the 1950s, Wald and his colleagues used chemical methods to extract pigments from the retina, then, using a spectrophotometer, they were able to measure the light absorbance of the pigments. Since the absorbance of light by retina pigments corresponds to the wavelengths that best activate photoreceptor cells, however, since rod cells make up most of the retina, what Wald and his colleagues were specifically measuring was the absorbance of rhodopsin, the main photopigment in rods. Later, with a technique called microspectrophotometry, he was able to measure the absorbance directly from cells and this allowed Wald to determine the absorbance of pigments in the cone cells. George Wald was born in New York City, the son of Ernestine and Isaac Wald and he was a member of the first graduating class of the Brooklyn Technical High School in New York in 1922. He received his Bachelor of Science degree from New York University in 1927, after graduating, he received a travel grant from the US National Research Council. Wald used this grant to work in Germany with Otto Heinrich Warburg where he identified vitamin A in the retina, Wald then went on to work in Zurich, Switzerland with the discoverer of vitamin A, Paul Karrer. In 1934, Wald went to Harvard University where he became an instructor and he was elected to the National Academy of Sciences in 1950 and in 1967 was awarded the Nobel Prize for Physiology or Medicine for his discoveries in vision. In 1966 he was awarded the Frederic Ives Medal by the OSA, Wald spoke out on many political and social issues and his fame as a Nobel laureate brought national and international attention to his views. He was an opponent of the Vietnam War and the nuclear arms race. Speaking at MIT in 1969 Wald bemoaned that Our government has become preoccupied with death, with the business of killing and being killed, in 1980, Wald served as part of Ramsey Clarks delegation to Iran during the Iran hostage crisis. With a small number of other Nobel laureates, he was invited in 1986 to fly to Moscow to advise Mikhail Gorbachev on a number of environmental questions. While there, he questioned Gorbachev about the arrest, detention and exile to Gorki of Yelena Bonner and her husband, Wald reported that Gorbachev said he knew nothing about it. Bonner and Sakharov were released thereafter, in December,1986. He was married twice, in 1931 to Frances Kingsley and in 1958 to the biochemist Ruth Hubbard and he had two sons with Kingsley—Michael and David, he and Hubbard had a son—the award-winning musicologist and musician Elijah Wald—and a daughter, Deborah, a prominent family law attorney. List of Jewish Nobel laureates Retinal Nobel Prize Biography John E. Dowling, George Wald, 1906–1997, A Biographical Memoir in Biographical Memoirs, Washington, the National Academy Press, Volume 78,298,317
9.
Signal transduction
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Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term sensor is used. The changes elicited by ligand binding in a receptor give rise to a cascade of events along a signalling pathway. When signalling pathways interact with one another they form networks, which allow cellular responses to be coordinated and these molecular events are the basic mechanisms controlling cell growth, proliferation, metabolism and many other processes. In multicellular organisms, signal transduction pathways have evolved to regulate cell communication in a variety of ways. Each component of a pathway is classified according to the role it plays with respect to the initial stimulus. Ligands are termed first messengers, while receptors are the signal transducers, such effectors are often linked to second messengers, which can activate secondary effectors, and so on. Depending on the efficiency of the nodes, a signal can be amplified, as with other signals, the transduction of biological signals is characterised by delay, noise and interference, which can range from negligible to pathological. With the advent of computational biology, the analysis of signalling pathways and networks has become a tool to understand cellular functions. The basis for signal transduction is the transformation of a certain stimulus into a biochemical signal, traditionally, signals that reach the central nervous system are classified as senses. These are transmitted from neuron to neuron in a process called synaptic transmission, many other intercellular signal relay mechanisms exist in multicellular organisms, such as those that govern embryonic development. The majority of signal transduction pathways involve the binding of signalling molecules, known as ligands, the combination of a signaling molecule with a receptor causes a change in the conformation of the receptor, known as receptor activation. Most ligands are soluble molecules from the medium which bind to cell surface receptors. These include growth factors, cytokines and neurotransmitters, components of the extracellular matrix such as fibronectin and hyaluronan can also bind to such receptors. In addition, some such as steroid hormones are lipid-soluble. In the case of hormone receptors, their stimulation leads to binding to the promoter region of steroid-responsive genes. Not all classifications of signalling molecules take into account the nature of each class member. For example, odorants belong to a range of molecular classes, as do neurotransmitters. Moreover, some molecules may fit more than one class, e. g. epinephrine is a neurotransmitter when secreted by the central nervous system
