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2423) Neuromuscular junction

Gist

A neuromuscular junction (NMJ) is the specialized synapse where a motor neuron transmits a signal to a skeletal muscle fiber, triggering muscle contraction. This happens when the nerve impulse reaches the neuron's end, causing it to release the neurotransmitter acetylcholine into the synaptic cleft. Acetylcholine then binds to receptors on the muscle fiber, initiating a series of events that lead to a muscle action potential and subsequent contraction. 

When a nerve impulse arrives, it causes the release of the neurotransmitter acetylcholine, which binds to receptors on the muscle fiber, generating an action potential and leading to muscle movement. Disorders affecting the NMJ can lead to muscle weakness and paralysis. 

Summary

A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber.

It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction.

Muscles require innervation to function—and even just to maintain muscle tone, avoiding atrophy. In the neuromuscular system, nerves from the central nervous system and the peripheral nervous system are linked and work together with muscles. Synaptic transmission at the neuromuscular junction begins when an action potential reaches the presynaptic terminal of a motor neuron, which activates voltage-gated calcium channels to allow calcium ions to enter the neuron. Calcium ions bind to sensor proteins (synaptotagmins) on synaptic vesicles, triggering vesicle fusion with the cell membrane and subsequent neurotransmitter release from the motor neuron into the synaptic cleft. In vertebrates, motor neurons release acetylcholine (ACh), a small molecule neurotransmitter, which diffuses across the synaptic cleft and binds to nicotinic acetylcholine receptors (nAChRs) on the cell membrane of the muscle fiber, also known as the sarcolemma. nAChRs are ionotropic receptors, meaning they serve as ligand-gated ion channels. The binding of ACh to the receptor can depolarize the muscle fiber, causing a cascade that eventually results in muscle contraction.

Neuromuscular junction diseases can be of genetic and autoimmune origin. Genetic disorders, such as Congenital myasthenic syndrome, can arise from mutated structural proteins that comprise the neuromuscular junction, whereas autoimmune diseases, such as myasthenia gravis, occur when antibodies are produced against nicotinic acetylcholine receptors on the sarcolemma.

Details

The body contains over 600 different skeletal muscles and each consists of thousands of muscle fibres ranging in length from a few millimetres to several centimetres. The motor nerve fibres innervating them, which arise in the spinal cord, can be more than 1 m in length. However, the area of apposition between the terminal tip of each nerve fibre and the ‘endplate’ of each muscle fibre is usually less than 50 μm (1/20 mm) in diameter. This particular type of synapse is called the neuromuscular junction (see figure). In order to generate the complex and finely controlled movements that we all take for granted, there has to be a very efficient, fail-safe, and one-way transmission between the nerve and the muscle that ensures that muscle contractions faithfully follow commands from the central nervous system. Such neuromuscular transmission depends on the release from the motor nerve terminal of the chemical acetylcholine (ACh), and its binding to a protein receiver on the surface of the muscle, called the acetylcholine receptor (AChR).

When a nerve impulse reaches the motor nerve terminal, specialized proteins forming ion-channels in its cell membrane open transiently, allowing a short-lived entry of calcium into the terminal. Stored inside the nerve terminal, and attached to special sites on the inside of the cell membrane, are small round vesicles filled with ACh. The sudden inrush of calcium causes some of the vesicle membranes to fuse with the nerve terminal membrane, and to release their contents into the synaptic cleft between the nerve and the surface of the muscle fibre.

ACh diffuses rapidly across the ultramicroscopic 50 nm gap and binds to the AChRs that are very densely packed on the tops of the synaptic folds on the muscle fibre. When two ACh molecules bind to each AChR, its central pore (channel) opens, allowing small positively charged ions, mainly sodium, to enter the muscle, resulting in a local reduction in the potential across the membrane (depolarization). The release of many ACh-containing vehicles by a nerve impulse leads to a large depolarization called the endplate potential, which in turn opens the voltage-sensitive sodium channels situated at the base of each synaptic fold. These are responsible for starting an ‘all or nothing’ action potential that is propagated along the muscle fibre in each direction and initiates muscle contraction.

After about a millisecond, the AChR pore closes and ACh unbinds and is broken down by an enzyme, ACh esterase (AChE), that sits in the synaptic cleft. Choline is then taken back into the nerve terminal by special transporters, and used to make more ACh; this is stored in newly-formed synaptic vesicles, themselves made up of recycled nerve terminal membrane. The whole sequence of events, from the inrush of calcium to the initiation of the action potential, takes place in less than two milliseconds.

Many of the earliest studies on chemical synaptic transmission began with the autonomic nervous system, but they were soon extended to skeletal muscles when Dale and his colleagues (1936) showed that stimulation of motor nerves released ACh, and that ACh can induce muscle contraction. The action of ACh could be increased by using a drug, eserine, that inhibits the ACh esterase, and the action of ACh on the muscle could be blocked by the arrow poison, curare. Katz and his co-workers subsequently used intracellular micro-electrodes to measure the endplate potentials and showed that these followed the release of many vesicles of ACh, and that a similar depolarization of the muscle occurred when ACh was applied directly onto the neuromuscular junction with a micropipette.

The neuromuscular junction, unlike most of the nervous system, is accessible to factors circulating in the blood. This can be both an advantage and a disadvantage. For many surgical operations, one of the important roles of the anesthetist is to relax the patient's muscles using an intravenous injection of the otherwise poisonous curare-like drugs — whilst taking over artificially the muscular function of breathing. Similarly, many species of venomous animals, such as snakes and scorpions, make toxins that paralyse their prey, and in some parts of the world this can also be a serious hazard for humans. Such toxins are rapidly absorbed and carried to the neuromuscular junction where they bind with extraordinary efficiency to the AChRs and other ion channel proteins, leading to muscle paralysis which can prevent breathing. Another important toxin is botulinum, which is produced by bacteria contaminating certain foods. Botulinum toxin blocks the release of ACh from the motor nerve terminals, and can cause fatal paralysis in babies; on the other hand it has recently found use as a treatment by local injection into muscles that are subject to uncontrollable severe spasm.

These neurotoxins have also provided marvellous tools for investigating function. For instance, a particular snake toxin, alpha-bungarotoxin, binds very strongly to AChRs and has been of immense use in the study of diseases that affect neuromuscular transmission. In myasthenia gravis (mys: muscle: aesthenia: weakness), the patient suffers from serious weakness and fatigue that can be life-threatening if it involves swallowing and breathing muscles. Myasthenia was first described in 1672 by the very distinguished London physician and anatomist, Thomas Willis. Over three hundred years later, Jim Patrick and Jon Lindstrom at the Salk Institute in California induced a myasthenia gravis-like disease in rabbits by injecting them with AChR protein purified from the electric organs of certain fish. The rabbits responded to the ‘foreign’ protein by making antibodies to it, but these antibodies gained access to the rabbits' neuromuscular junctions, recognized the muscle AChRs, and reduced their function, producing muscle weakness. Following these experimental observations, radioactively-labelled alpha-bungarotoxin was used to show that patients with myasthenia have reduced numbers of AChRs at their neuromuscular junctions, and subsequently that this is caused by serum antibodies that bind to AChRs — just as in the rabbits. Drugs that inhibit the ACh esterase enzyme cause clinical improvement because they prolong the action of ACh, as first demonstrated in 1934 by Mary Walker, a young doctor in London, but nowadays the most important treatment is to reduce the circulating antibodies that bind AChR.

Additional Information

Nneuromuscular junction is a site of chemical communication between a nerve fibre and a muscle cell. The neuromuscular junction is analogous to the synapse between two neurons. A nerve fibre divides into many terminal branches; each terminal ends on a region of muscle fibre called the end plate. Embedded in the end plate are thousands of receptors, which are long protein molecules that form channels through the membrane. Upon stimulation by a nerve impulse, the terminal releases the chemical neurotransmitter acetylcholine from synaptic vesicles. Acetylcholine then binds to the receptors, the channels open, and sodium ions flow into the end plate. This initiates the end-plate potential, the electrical event that leads to contraction of the muscle fibre.

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#5819. What does the noun feat mean?

#5820. What does the adjective febrile mean?

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#9772.

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2623.

2370) Murray Gell-Mann

Gist:

Work

During the 1950s and 1960s, new accelerators and apparatuses helped identify many new elementary particles. In theoretical works from the same period, Murray Gell-Mann classified particles and their interactions. He proposed that observed particles are in fact composite, that is, comprised of smaller building blocks called quarks. According to this theory, as-yet-undiscovered particles should exist. When these were later found in experiments, the theory was accepted.

Summary

Murray Gell-Mann (born September 15, 1929, New York, New York, U.S.—died May 24, 2019, Santa Fe, New Mexico) was an American physicist, winner of the Nobel Prize for Physics in 1969 for his work pertaining to the classification of subatomic particles and their interactions.

At age 15 Gell-Mann entered Yale University, and, after graduating from Yale with a B.S. in physics in 1948, he earned a Ph.D. (1951) at the Massachusetts Institute of Technology. His doctoral research on subatomic particles was influential in the later work of the Nobel laureate (1963) Eugene P. Wigner. In 1952 Gell-Mann joined the Institute for Nuclear Studies at the University of Chicago. The following year he introduced the concept of “strangeness,” a quantum property that accounted for previously puzzling decay patterns of certain mesons. As defined by Gell-Mann, strangeness is conserved when any subatomic particle interacts via the strong force—i.e., the force that binds the components of the atomic nucleus. Gell-Mann joined the faculty of the California Institute of Technology in Pasadena in 1955 and was appointed the Robert Andrews Millikan Professor of Theoretical Physics in 1967 (emeritus, 1993).

In 1961 Gell-Mann and Yuval Ne’eman, an Israeli theoretical physicist, independently proposed a scheme for classifying previously discovered strongly interacting particles into a simple orderly arrangement of families. Called the Eightfold Way (after Buddha’s Eightfold Path to Enlightenment and bliss), the scheme grouped mesons and baryons (e.g., protons and neutrons) into multiplets of 1, 8, 10, or 27 members on the basis of various properties. All particles in the same multiplet are to be thought of as variant states of the same basic particle. Gell-Mann speculated that it should be possible to explain certain properties of known particles in terms of even more fundamental particles, or building blocks. He later called these basic bits of matter “quarks,” adopting the fanciful term from James Joyce’s novel Finnegans Wake. One of the early successes of Gell-Mann’s quark hypothesis was the prediction and subsequent discovery of the omega-minus particle (1964). Over the years, research has yielded other findings that have led to the wide acceptance and elaboration of the quark concept.

Gell-Mann published a number of works on this phase of his career, notable among which were The Eightfold Way (1964), written in collaboration with Ne’eman, and Broken Scale Variance and the Light Cone (1971), coauthored with K. Wilson.

In 1984 Gell-Mann cofounded the Santa Fe Institute, a nonprofit centre located in Santa Fe, New Mexico, that supports research concerning complex adaptive systems and emergent phenomena associated with complexity. In “Let’s Call It Plectics,” a 1995 article in the institute’s journal, Complexity, he coined the word plectics to describe the type of research supported by the institute. In The Quark and the Jaguar (1994), Gell-Mann gave a fuller description of the ideas concerning the relationship between the basic laws of physics (the quark) and the emergent phenomena of life (the jaguar).

Gell-Mann was a director of the MacArthur Foundation (1979–2002) and served on the President’s Committee of Advisors on Science and Technology (1994–2001). He also was a member of the board of directors of Encyclopædia Britannica, Inc.

Details

Murray Gell-Mann (September 15, 1929 – May 24, 2019) was an American theoretical physicist who played a preeminent role in the development of the theory of elementary particles. Gell-Mann introduced the concept of quarks as the fundamental building blocks of the strongly interacting particles, and the renormalization group as a foundational element of quantum field theory and statistical mechanics. Murray Gell-Mann received the 1969 Nobel Prize in Physics for his contributions and discoveries concerning the classification of elementary particles and their interactions.

Gell-Mann played key roles in developing the concept of chirality in the theory of the weak interactions and spontaneous chiral symmetry breaking in the strong interactions, which controls the physics of the light mesons. In the 1970s he was a co-inventor of quantum chromodynamics (QCD) which explains the confinement of quarks in mesons and baryons and forms a large part of the Standard Model of elementary particles and forces.

Life and education

Gell-Mann was born in Lower Manhattan to a family of Jewish immigrants from the Austro-Hungarian Empire, specifically from Czernowitz in present-day Ukraine. His parents were Pauline (née Reichstein) and Arthur Isidore Gelman, who taught English as a second language. Gell-Mann married J. Margaret Dow in 1955; they had a daughter and a son. Margaret died in 1981, and in 1992 he married Marcia Southwick, whose son became his stepson.

Propelled by an intense boyhood curiosity and love for nature and mathematics, he graduated valedictorian from the Columbia Grammar & Preparatory School aged 14 and subsequently entered Yale College as a member of Jonathan Edwards College. At Yale, he participated in the William Lowell Putnam Mathematical Competition and was on the team representing Yale University (along with Murray Gerstenhaber and Henry O. Pollak) that won the second prize in 1947.

Gell-Mann graduated from Yale with a bachelor's degree in physics in 1948 and intended to pursue graduate studies in physics. He sought to remain in the Ivy League for his graduate education and applied to Princeton University as well as Harvard University. He was rejected by Princeton and accepted by Harvard, but the latter institution was unable to offer him needed financial assistance. He was then accepted by the Massachusetts Institute of Technology (MIT) and received a letter from Victor Weisskopf urging him to attend MIT and become Weisskopf's research assistant. This would provide Gell-Mann with the financial assistance he required. Unaware of MIT's eminent status in physics research, Gell-Mann was "miserable" and in characteristic dark irony, said he first considered suicide.

Gell-Mann received his Ph.D. in physics from MIT in 1951 after completing a doctoral dissertation, titled "Coupling strength and nuclear reactions", under the supervision of Weisskopf.
Subsequently, Gell-Mann was a postdoctoral fellow at the Institute for Advanced Study at Princeton in 1951, and a visiting research professor at the University of Illinois at Urbana–Champaign from 1952 to 1953. He was a visiting associate professor at Columbia University and an associate professor at the University of Chicago in 1954–1955, before moving to the California Institute of Technology, where he taught from 1955 until he retired in 1993.

Gell-Mann died on May 24, 2019, at his home in Santa Fe, New Mexico.

2422) Nebula

Gist

A nebula is a giant cloud of dust and gas in space, which can be a birthplace for new stars or the remnants of a dying star. The term is Latin for "mist" or "cloud" and was once used for any diffuse-looking object, but today specifically refers to these interstellar clouds. Nebulae are often regions of star formation, where gas and dust clump together to form new stars and planetary systems. 

A nebula is a giant cloud of dust and gas in space, located between stars. These clouds can be the birthplace of new stars, earning them the nickname "star nurseries," or they can be the remnants of dying stars, such as from a supernova explosion. Gravity causes the material within a nebula to clump together, eventually leading to the formation of stars and planets. 

Summary

A nebula (Latin for 'cloud, fog'; pl. nebulae or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as the Pillars of Creation in the Eagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form stars. The remaining material is then thought to form planets and other planetary system objects.

Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula that is visible to the human eye from Earth would appear larger, but no brighter, from close by. The Orion Nebula, the brightest nebula in the sky and occupying an area twice the angular diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth ({10}^{5} to {10}^{7} molecules per cubic centimeter) – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Earth's air has a density of approximately {10}^{19} molecules per cubic centimeter; by contrast, the densest nebulae can have densities of {10}^{4} molecules per cubic centimeter. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters. Some nebulae are variably illuminated by T Tauri variable stars.

Originally, the term "nebula" was used to describe any diffused astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble, and others. Edwin Hubble discovered that most nebulae are associated with stars and illuminated by starlight. He also helped categorize nebulae based on the type of light spectra they produced.

Details

A nebula is any of the various tenuous clouds of gas and dust that occur in interstellar space. The term was formerly applied to any object outside the solar system that had a diffuse appearance rather than a pointlike image, as in the case of a star. This definition, adopted at a time when very distant objects could not be resolved into great detail, unfortunately includes two unrelated classes of objects: the extragalactic nebulae, now called galaxies, which are enormous collections of stars and gas, and the galactic nebulae, which are composed of the interstellar medium (the gas between the stars, with its accompanying small solid particles) within a single galaxy. Today the term nebula generally refers exclusively to the interstellar medium.

In a spiral galaxy the interstellar medium makes up 3 to 5 percent of the galaxy’s mass, but within a spiral arm its mass fraction increases to about 20 percent. About 1 percent of the mass of the interstellar medium is in the form of “dust”—small solid particles that are efficient in absorbing and scattering radiation. Much of the rest of the mass within a galaxy is concentrated in visible stars, but there is also some form of dark matter that accounts for a substantial fraction of the mass in the outer regions.

The most conspicuous property of interstellar gas is its clumpy distribution on all size scales observed, from the size of the entire Milky Way Galaxy (about {10}^{20} metres, or hundreds of thousands of light-years) down to the distance from Earth to the Sun (about {10}^{11} metres, or a few light-minutes). The large-scale variations are seen by direct observation, and the small-scale variations are observed by fluctuations in the intensity of radio waves, similar to the “twinkling” of starlight caused by unsteadiness in Earth’s atmosphere. Various regions exhibit an enormous range of densities and temperatures. Within the Galaxy’s spiral arms about half the mass of the interstellar medium is concentrated in molecular clouds, in which hydrogen occurs in molecular form (H2) and temperatures are as low as 10 kelvins (K). These clouds are inconspicuous optically and are detected principally by their carbon monoxide (CO) emissions in the millimetre wavelength range. Their densities in the regions studied by CO emissions are typically 1,000 H2 molecules per cubic cm. At the other extreme is the gas between the clouds, with a temperature of 10 million K and a density of only 0.001 H+ ion per cubic cm. Such gas is produced by supernovae, the violent explosions of unstable stars.

Classes of nebulae

All nebulae observed in the Milky Way Galaxy are forms of interstellar matter—namely, the gas between the stars that is almost always accompanied by solid grains of cosmic dust. Their appearance differs widely, depending not only on the temperature and density of the material observed but also on how the material is spatially situated with respect to the observer. Their chemical composition, however, is fairly uniform; it corresponds to the composition of the universe in general in that approximately 90 percent of the constituent atoms are hydrogen and nearly all the rest are helium, with oxygen, carbon, neon, nitrogen, and the other elements together making up about two atoms per thousand. On the basis of appearance, nebulae can be divided into two broad classes: dark nebulae and bright nebulae. Dark nebulae appear as irregularly shaped black patches in the sky and blot out the light of the stars that lie beyond them. Bright nebulae appear as faintly luminous glowing surfaces; they either emit their own light or reflect the light of nearby stars.

Dark nebulae are very dense and cold molecular clouds; they contain about half of all interstellar material. Typical densities range from hundreds to millions (or more) of hydrogen molecules per cubic centimetre. These clouds are the sites where new stars are formed through the gravitational collapse of some of their parts. Most of the remaining gas is in the diffuse interstellar medium, relatively inconspicuous because of its very low density (about 0.1 hydrogen atom per cubic cm) but detectable by its radio emission of the 21-cm line of neutral hydrogen.

Bright nebulae are comparatively dense clouds of gas within the diffuse interstellar medium. They have several subclasses: (1) reflection nebulae, (2) H II regions, (3) diffuse ionized gas, (4) planetary nebulae, and (5) supernova remnants.

Reflection nebulae reflect the light of a nearby star from their constituent dust grains. The gas of reflection nebulae is cold, and such objects would be seen as dark nebulae if it were not for the nearby light source.

H II regions are clouds of hydrogen ionized (separated into positive H+ ions and free electrons) by a neighbouring hot star. The star must be of stellar type O or B, the most massive and hottest of normal stars in the Galaxy, in order to produce enough of the radiation required to ionize the hydrogen.

Diffuse ionized gas, so pervasive among the nebular clouds, is a major component of the Galaxy. It is observed by faint emissions of positive hydrogen, nitrogen, and sulfur ions (H+, N+, and S+) detectable in all directions. These emissions collectively require far more power than the much more spectacular H II regions, planetary nebulae, or supernova remnants that occupy a tiny fraction of the volume.

Planetary nebulae are ejected from stars that are dying but are not massive enough to become supernovae—namely, red giant stars. That is to say, a red giant has shed its outer envelope in a less-violent event than a supernova explosion and has become an intensely hot star surrounded by a shell of material that is expanding at a speed of tens of kilometres per second. Planetary nebulae typically appear as rather round objects of relatively high surface brightness. Their name is derived from their superficial resemblance to planets—i.e., their regular appearance when viewed telescopically as compared with the chaotic forms of other types of nebula.

Supernova remnants are the clouds of gas expanding at speeds of hundreds or even thousands of kilometres per second from comparatively recent explosions of massive stars. If a supernova remnant is younger than a few thousand years, it may be assumed that the gas in the nebula was mostly ejected by the exploded star. Otherwise, the nebula would consist chiefly of interstellar gas that has been swept up by the expanding remnant of older objects.

Additional Information

A nebula is a giant cloud of dust and gas in space. Some nebulae (more than one nebula) come from the gas and dust thrown out by the explosion of a dying star, such as a supernova. Other nebulae are regions where new stars are beginning to form.

A nebula is a giant cloud of dust and gas in space. Some nebulae (more than one nebula) come from the gas and dust thrown out by the explosion of a dying star, such as a supernova. Other nebulae are regions where new stars are beginning to form. For this reason, some nebulae are called "star nurseries."

How do stars form in a nebula?

Nebulae are made of dust and gases—mostly hydrogen and helium. The dust and gases in a nebula are very spread out, but gravity can slowly begin to pull together clumps of dust and gas. As these clumps get bigger and bigger, their gravity gets stronger and stronger.

Eventually, the clump of dust and gas gets so big that it collapses from its own gravity. The collapse causes the material at the center of the cloud to heat up-and this hot core is the beginning of a star.

Where are nebulae?

Nebulae exist in the space between the stars—also known as interstellar space. The closest known nebula to Earth is called the Helix Nebula. It is the remnant of a dying star—possibly one like the Sun. It is approximately 700 light-years away from Earth. That means even if you could travel at the speed of light, it would still take you 700 years to get there!

How do we know what nebulae look like?

Astronomers use very powerful telescopes to take pictures of faraway nebulae. Space telescopes such as NASA's Spitzer Space Telescope and Hubble Space Telescope have captured many images of faraway nebulae.

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#5817. What does the verb (used without object) deign mean?

#5818. What does the noun deism mean?

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#9771.

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2622.