nucleus
American Heritage Dictionary:
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nu·cle·us
(nū'klē-əs, nyū'-) 
n., pl., -cle·i (-klē-ī'), or -cle·us·es.
n., pl., -cle·i (-klē-ī'), or -cle·us·es.
- A central or essential part around which other parts are gathered or grouped; a core: the nucleus of a city.
- Something regarded as a basis for future development and growth; a kernel: a few paintings that formed the nucleus of a great art collection.
- Biology. A large, membrane-bound, usually spherical protoplasmic structure within a living cell, containing the cell's hereditary material and controlling its metabolism, growth, and reproduction.
- Botany.
- The central kernel of a nut or seed.
- The center of a starch granule.
- Anatomy. A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.
- Physics. The positively charged central region of an atom, composed of protons and neutrons and containing almost all of the mass of the atom.
- Chemistry. A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.
- Astronomy.
- The central portion of the head of a comet.
- The central or brightest part of a nebula or galaxy.
- Meteorology. A particle on which water vapor molecules accumulate in free air to form a droplet or ice crystal.
- Linguistics. The part of a syllable having the greatest sonority. In the word middlemost (mĭd'l-mōst') the nuclei of the three syllables are (ĭ), (l), and (ō); in the Czech word krk ("neck"), the nucleus is (r).
[Latin nuculeus, nucleus, kernel, from nucula, little nut, diminutive of nux, nuc-, nut.]
Fowler's Modern English Usage:
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nucleus
Oxford Dictionary of Chemistry:
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nucleus
The central core of an atom that contains most of its mass. It is positively charged and consists of one or more nucleons (protons or neutrons). The positive charge of the nucleus is determined by the number of protons it contains (see atomic number) and in the neutral atom this is balanced by an equal number of electrons, which move around the nucleus. The simplest nucleus is the hydrogen nucleus, consisting of one proton only. All other nuclei also contain one or more neutrons. The neutrons contribute to the atomic mass (see nucleon number) but not to the nuclear charge. The most massive nucleus that occurs in nature is uranium–238, containing 92 protons and 146 neutrons. The symbol used for this nuclide is 23892U, the upper figure being the nucleon number and the lower figure the atomic number. In all nuclei the nucleon number (A) is equal to the sum of the atomic number (Z) and the neutron number (N), i.e. A=Z+N.
Wiley Book of Astronomy:
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atomic nucleus
Britannica Concise Encyclopedia:
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nucleus
Specialized structure occurring in most cells (except bacteria) and separated from the rest of the cell by the nuclear membrane. This membrane seems to be continuous with the cell's endoplasmic reticulum and has pores that permits the passage of large molecules. The nucleus controls and regulates the cell's activities (e.g., growth and metabolism) and carries the genes. Nucleoli are small bodies often seen within the nucleus that play an important part in the synthesis of RNA and protein. A cell normally contains only one nucleus.
For more information on nucleus, visit Britannica.com.
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nucleus
Antonyms by Answers.com:
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nucleus
n
Definition: core; basis for something's beginning
Antonyms: exterior, exteriority, outside
In ancient construction, the internal part of the flooring, consisting of a strong cement, over which the pavement was laid, bound with mortar.
Columbia Encyclopedia:
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nucleus
nucleus, in physics, the extremely dense central core of an atom.
The Nature of the Nucleus
Composition
Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons. A proton is simply the nucleus of an ordinary hydrogen atom, the lightest atom, and has a unit positive charge. A neutron is an uncharged particle of about the same mass as the proton. The number of protons in a given nucleus is the atomic number of that nucleus and determines which chemical element the nucleus will constitute when surrounded by electrons.
The total number of protons and neutrons together in a nucleus is the atomic mass number of the nucleus. Two nuclei may have the same atomic number but different mass numbers, thus constituting different forms, or isotopes, of the same element. The mass number of a given isotope is the nearest whole number to the atomic weight of that isotope and is approximately equal to the atomic weight (in the case of carbon-12, exactly equal).
Size and Density
The nucleus occupies only a tiny fraction of the volume of an atom (the radius of the nucleus being some 10,000 to 100,000 times smaller than the radius of the atom as a whole), but it contains almost all the mass. An idea of the extreme density of the nucleus is revealed by a simple calculation. The radius of the nucleus of hydrogen is on the order of 10−13 cm so that its volume is on the order of 10−39 cm3 (cubic centimeter); its mass is about 10−24 g (gram). Combining these to estimate the density, we have 10−24 g/10−39 cm3 ≈ 1015 g/cm3, or about a thousand trillion times the density of matter at ordinary scales (the density of water is 1 g/cm3).
Mass Defect, Binding Energy, and Nuclear Reactions
When nuclear masses are measured, the mass is always found to be less than the sum of the masses of the individual nucleons bound in the nucleus. The difference between the nuclear mass and the sum of the individual masses is known as the mass defect and is due to the fact that some of the mass must be converted to energy in order to make the nucleus stable. This nuclear binding energy is related to the mass defect by the famous formula from relativity, E = mc2, where E is energy, m is mass, and c is the speed of light. The binding energy of a nucleus increases with increasing mass number.
A more interesting property of a nucleus is the binding energy per nucleon, found by dividing the binding energy by the mass number. The average binding energy per nucleon is observed to increase rapidly with increasing mass number up to a mass number of about 60, then to decrease rather slowly with higher mass numbers. Thus, nuclei with mass numbers around 60 are the most stable, and those of very small or very large mass numbers are the least stable.
Two important phenomena result from this property of nuclei. Nuclear fission is the spontaneous splitting of a nucleus of large mass number into two nuclei of smaller mass numbers. Nuclear fusion, on the other hand, is the combining of two light nuclei to form a heavier single nucleus, again with an increase in the average binding energy per nucleon. In both cases, the change to a stable final state is accompanied by the release of a large amount of energy per unit mass of the reacting materials as compared to the energy released in chemical reactions (see nuclear energy).
Models of the Nucleus
Several models of the nucleus have evolved that fit certain aspects of nuclear behavior, but no single model has successfully described all aspects. One model is based on the fact that certain properties of a nucleus are similar to those of a drop of incompressible liquid. The liquid-drop model has been particularly successful in explaining details of the fission process and in evolving a formula for the mass of a particular nucleus as a function of its atomic number and mass number, the so-called semiempirical mass formula.
Another model is the Fermi gas model, which treats the nucleons as if they were particles of a gas restricted by the Pauli exclusion principle, which allows only two particles of opposite spin to occupy a particular energy level described by the quantum theory. These particle pairs will fill the lowest energy levels first, then successively higher ones, so that the "gas" is one of minimum energy. There are actually two independent Fermi gases, one of protons and one of neutrons. The tendency of nucleons to occupy the lowest possible energy level explains why there is a tendency for the numbers of protons and neutrons to be nearly equal in lighter nuclei. In heavier nuclei the effect of electrostatic repulsion among the larger number of charges from the protons raises the energy of the protons, with the result that there are more neutrons than protons (for uranium-235, for example, there are 143 neutrons and only 92 protons). The pairing of nucleons in energy levels also helps to explain the tendency of nuclei to have even numbers of both protons and neutrons.
Neither the liquid-drop model nor the Fermi gas model, however, can explain the exceptional stability of nuclei having certain values for either the number of protons or the number of neutrons, or both. These so-called magic numbers are 2, 8, 20, 28, 50, 82, and 126. Because of the similarity between this phenomenon and the stability of the noble gases, which have certain numbers of electrons that are bound in closed "shells," a shell model was suggested for the nucleus. There are major differences, however, between the electrons in an atom and the nucleons in a nucleus. First, the nucleus provides a force center for the electrons of an atom, while the nucleus itself has no single force center. Second, there are two different types of nucleons. Third, the assumption of independent particle motion made in the case of electrons is not as easily made for nucleons. The liquid-drop model is in fact based on the assumption of strong forces between the nucleons that considerably constrain their motion. However, these difficulties were solved and a good explanation of the magic numbers achieved on the basis of the shell model, which included the assumption of strong coupling between the spin angular momentum of a nucleon and its orbital angular momentum. Various attempts have been made, with partial success, to construct a model incorporating the best features of both the liquid-drop model and the shell model.
Scientific Notation for the Nucleus and Nuclear Reactions
A nucleus may be represented conveniently by the chemical symbol for the element together with a subscript and superscript for the atomic number and mass number. (The subscript is often omitted, since the element symbol fixes the atomic number.) The nucleus of ordinary hydrogen, i.e., the proton, is represented by 1H1, an alpha particle (a helium nucleus) is 2He4, the most common isotope of chlorine is 17Cl35, and the uranium isotope used in the atomic bomb is 92U235.
Nuclear reactions involving changes in atomic number or mass number can be expressed easily using this notation. For example, when Ernest Rutherford produced the first artificial nuclear reaction (1919), it involved bombarding a nitrogen nucleus with alpha particles and resulted in an isotope of oxygen with the release of a proton: 2He4+7N14→8O17+1H1. Note that the total of the atomic numbers on the left is equal to the total on the right (i.e., 2+7=8+1), and similarly for the mass numbers (4+14=17+1).
Scientific Investigations of the Nucleus
Following the discovery of radioactivity by A. H. Becquerel in 1896, Ernest Rutherford identified two types of radiation given off by natural radioactive substances and named them alpha and beta; a third, gamma, was later identified. In 1911 he bombarded a thin target of gold foil with alpha rays (subsequently identified as helium nuclei) and found that, although most of the alpha particles passed directly through the foil, a few were deflected by large amounts. By a quantitative analysis of his experimental results, he was able to propose the existence of the nucleus and estimate its size and charge.
After the discovery of the neutron in 1932, physicists turned their attention to the understanding of the strong interactions, or strong nuclear force, that bind protons and neutrons together in nuclei. This force must be great enough to overcome the considerable repulsive force existing between several protons because of their electrical charge. It must exist between nucleons without regard to their charge, since it acts equally on protons and neutrons, and it must not extend very far away from the nucleons (i.e., it must be a short-range force), since it has negligible effect on protons or neutrons outside the nucleus.
In 1935 Hideki Yukawa proposed a theory that this nuclear "glue" was produced by the exchange of a particle between nucleons, just as the electromagnetic force is produced by the exchange of a photon between charged particles. The range of a force is dependent on the mass of the particle carrying the force; the greater the mass of the particle, the shorter the range of the force. The range of the electromagnetic force is infinite because the mass of the photon is zero. From the known range of the nuclear force, Yukawa estimated the mass of the hypothetical carrier of the nuclear force to be about 200 times that of the electron. Given the name meson because its mass is between that of the electron and those of the nucleons, this particle was finally observed in 1947 and is now called the pi meson, or pion, to distinguish it from other mesons that have been discovered (see elementary particles).
Both the proton and the neutron are surrounded by a cloud of pions given off and reabsorbed again within an incredibly short interval of time. Certain other mesons are assumed to be created and destroyed in this way as well, all such particles being termed "virtual" because they exist in violation of the law of conservation of energy (see conservation laws) for a very short span of time allowed by the uncertainty principle. It is now known, however, that at a more fundamental level the actual carrier of the strong force is a particle called the gluon.
Bibliography
See G. Gamow, The Atom and Its Nucleus (1961); R. K. Adair, The Great Design: Particles, Fields, and Creation (1987).
Electronics Dictionary:
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nucleus
Core of an atom. The nucleus contains both positive (protons) and neutral (neutrons) subatomic particles.
Cosmic Lexicon:
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Nucleus
(see comet) Kilometer-sized "dirty snowball" composed of dust (refractory material) and primarily water-ice which gives rise to all of the features observers associate with comets. As the nucleus approaches the Sun, the temperatures rise sufficiently to cause the sublimation of the ices. As the gas leaves the nucleus, it is able to drag some of the dust with it in the low gravity. Sunlight reflected off the dust is what we see as the yellowish coma and tail of the comet, and interaction of the solar radiation with the gases gives us the characteristic blue appearance of the plasma tail.
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plur. nuclei
In biology, the central region of the cell, in which DNA is stored. The nucleus usually appears as a dark spot in the interior of the cell. Primitive cells (such as bacteria and blue-green algae) have no nuclei.
nucleus
plur. nuclei (nooh-klee-eye)
The small, dense center of the atom. The nucleus is composed of protons and neutrons and has a positive electrical charge.
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(pl. nuclei)
- (in biology) the most conspicuous organelle of a eukaryotic cell; it contains the chromosomes and (except for mitochondria and chloroplasts) is the sole site of DNA replication and of RNA synthesis in the cell. Usually a spheroidal body, it is separated from the cytoplasm by the nuclear envelope.
- (in physics) atomic nucleus the positively charged central part of an atom, with which is associated almost the whole of the mass of the atom but only a very small part of its volume. It is composed of nucleons, i.e. protons and neutrons.
- (in organic chemistry) a characteristic arrangement of atoms, especially a ring structure, that occurs in a series of related organic compounds; e.g. the benzene nucleus.
- a particle on which a crystal, droplet, or bubble forms in a fluid.
| nucleotidyltransferase, nucleotidyl, nucleotide-binding fold | |
| nuclide, nude, null |
Saunders Veterinary Dictionary:
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nucleus
Pl. nuclei [L.]
1. cell nucleus; a spheroid body within a cell, contained in a double membrane, the nuclear envelope, and containing the chromosomes and one or more nucleoli. The contents are collectively referred to as nucleoplasm. The chromosomes contain deoxyribonucleic acid (DNA), which is the genetic material that codes for the structure of all the proteins of the cell.
2. a mass of gray matter in the central nervous system, especially such a mass marking the central termination of a cranial nerve.
3. in organic chemistry, the combination of atoms forming the central element or basic framework of the molecule of a specific compound or class of compounds.
4. the dense core of an atom; called also atomic nucleus. It is made of protons and neutrons held together by the strong nuclear force. Traveling in orbit around the nucleus is a cloud of negatively charged particles called electrons. The number of protons in the atomic nucleus gives a substance its identity as a particular element (2).
- n. abducens — located in the floor of the fourth ventricle; its axons constitute the abducent nerve.
- n. ambiguus — the nucleus of origin of motor fibers of the glossopharyngeal, vagus and accessory nerves that supply the striated muscle of the pharynx and larynx. Found in the medulla oblongata.
- arcuate n., n. arcuati — small irregular areas of gray substance on the ventromedial aspect of the pyramid of the medulla oblongata.
- atomic n. — nucleus (3).
- basal n. — large brain nuclei, the caudate and lentiform nuclei, which combine with the white matter to form the corpus striatum. Important in the regulation of motor function.
- caudate n., n. caudatus — an elongated, arched gray mass closely related to the lateral ventricle throughout its entire extent, which, together with the putamen, forms the neostriatum.
- central nervous system n. — aggregations of neurons within the brain.
- cerebellar n. — there are a number of them; they are surrounded by the medulla oblongata caudal to the cerebellum.
- cochlear n. (dorsal and ventral) — the nuclei of termination of sensory fibers of the cochlear part of the vestibulocochlear (eighth cranial) nerve, which partly encircle the inferior cerebellar peduncle at the junction of the medulla oblongata and pons.
- cranial nerve n. — aggregations of cell bodies associated with the cranial nerves, which in general are organized as continuations of the four gray matter components of the spinal cord plus three others which appear in the medulla oblongata developed for innervation of the organs in the head.
- cuneate n. — medial and lateral nuclei are situated in the medulla oblongata.
- dentate n., n. dentatus — the largest of the deep cerebellar nuclei lying in the white matter of the cerebellum.
- Edinger–Westphal n. — located in the midbrain and a center for coordination of oculomotor activity.
- facial n. — in the medulla oblongata and the origin of the facial nerve.
- gracile n. — located in the medulla oblongata.
- habenular n. — the gray matter of the habenula.
- hypoglossal n. — located in the medulla oblongata, the origin of the hypoglossal nerve.
- lateral geniculate n. — concerned in the transmission of visual stimuli.
- lenticular n., lentiform n. — the part of the corpus striatum just lateral to the internal capsule, comprising the putamen and globus pallidus.
- medial geniculate n. — a nucleus within the thalamus; involved in transmission of auditory stimuli.
- motor n. — any collection of cells in the central nervous system giving origin to a motor nerve.
- oculomotor n. — the cells of the midbrain which make up the origin of the oculomotor nerve.
- olivary n., n. olivaris — 1. a folded band of gray matter enclosing a white core and producing the elevation (olive) on the medulla oblongata. — 2. olive (2).
- n. of origin — any collection of nerve cells giving origin to the fibers, or a part of the fibers, of a peripheral nerve.
- paraventricular n., n. paraventricularis — a band of cells in the wall of the third ventricle in the supraoptic part of the hypothalamus; many of its cells are neurosecretory in function and project to the neurohypophysis, where they secrete oxytocin (and, to a lesser extent, antidiuretic hormone).
- pontine n., n. pontis — groups of nerve cell bodies in the part of the pyramidal tract within the ventral part of the pons, upon which the fibers of the corticopontine tract synapse, and whose axons in turn cross to the opposite side and form the middle cerebellar peduncle.
- pulpy n., n. pulposus — a semifluid mass of fine white and elastic fibers forming the center of an intervertebral disk. It serves to distribute pressure over the vertebral body. It shows early age changes, may calcify and herniate through the fibrous rings that enclose it to cause disk disease.
- red n. — an oval mass of gray matter (pink in fresh specimens) in the anterior part of the tegmentum and extending into the posterior part of the hypothalamus; one of the important relay stations in the extrapyramidal motor pathway of the CNS; origin of the rubrospinal tract in the cord; called also nucleus ruber.
- n. ruber — see red nucleus (above).
- salivatory n. — groups of preganglionic parasympathetic neurons concerned with salivary secretion organized into a rostral nucleus, of the facial nerve, and the caudal nucleus, of the glossopharyngeal nerve.
- sensory n. — the nucleus of termination of the afferent (sensory) fibers of a peripheral nerve.
- supraoptic n., n. supraopticus — one just above the lateral part of the optic chiasm; many of its cells are neurosecretory in function and project to the neurohypophysis, where they secrete antidiuretic hormone (ADH) and, to a lesser extent, oxytocin; other cells are osmoreceptors that stimulate ADH release in response to increased osmotic pressure.
- tegmental n. — several nuclear masses of the reticular formations of the pons and midbrain, especially of the latter, where they are in close approximation to the superior cerebellar peduncles.
- thoracic n., n. thoracicus — a column of cells in the dorsal gray column of the spinal cord, extending from the seventh or eighth cervical segments to the second or third lumbar level.
- trapezoid body n. — a relay station in the auditory pathways.
- trigeminal n. — there are three sensory nuclei and one motor nucleus of the trigeminal nerve found in the brainstem; the sensory nuclei comprise the mesencephalic nucleus, the nucleus of the descending tract, and the principal sensory nucleus, but none is exclusive to the trigeminal nerve, all of them receiving sensory inputs from other cranial nerves.
- trochlear n. — source of the trochlear nerve; located in the tegmentum of the midbrain.
- vagus n. — source of the vagus nerve.
- vestibular n., n. vestibularis — the four cellular masses (superior, lateral, medial and inferior) in the floor of the fourth ventricle, in which the branches of the eighth cranial (vestibulocochlear) nerve terminate.
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categories related to 'nucleus'
- Cell Structure and Function - nucleus: control center for all cellular activities
- Nuclear and Particle Physics - nucleus: positively charged, dense core of an atom, containing protons and neutrons
- Substances, Particles, and Atomic Architecture - nucleus: small, dense, positively charged central core of an atom, constituting a tiny fraction of the atom’s volume but almost all of its mass, composed primarily of protons and neutrons
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Nucleus (neuroanatomy)
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This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009) |
In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up as a region of gray matter, often bordered by white matter. The vertebrate brain contains hundreds of distinguishable nuclei, varying widely in shape and size. A nucleus may itself have a complex internal structure, with multiple types of neurons arranged in clumps (subnuclei) or layers.
The term "nucleus" is in some cases used rather loosely, to mean simply an identifiably distinct group of neurons, even if they are spread over an extended area. The reticular nucleus of the thalamus, for example, is a thin layer of inhibitory neurons that surrounds the thalamus.
Some of the major anatomical components of the brain are organized as clusters of interconnected nuclei. Notable among these are the thalamus and hypothalamus, each of which contains several dozen distinguishable substructures. The medulla and pons also contain numerous small nuclei with a wide variety of sensory, motor, and regulatory functions.
In the peripheral nervous system, a cluster of neurons is referred to instead as a ganglion.
Examples
- Brainstem: red nucleus, vestibular nucleus, inferior olive
- Cerebellum: dentate nucleus, emboliform nucleus, globose nucleus, fastigial nucleus
- Basal ganglia: striatum (caudate and putamen), pallidum (globus pallidus, medial and lateral), substantia nigra, subthalamic nucleus
- Cranial nerve nuclei
See also
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This neuroanatomy article is a stub. You can help Wikipedia by expanding it. |
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Translations:
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Nucleus
Dansk (Danish)
n. - kerne, cellekerne, atomkerne, grundstamme
Nederlands (Dutch)
kern, celkern, kern van komeet
Deutsch (German)
n. - Kern, Grundstock
Português (Portuguese)
n. - núcleo (m), ponto (m) central
Español (Spanish)
n. - núcleo, parte central o medular
Svenska (Swedish)
n. - kärna, centrum, grundstomme, grundplåt
中文（简体）(Chinese (Simplified))
核心, 原子核, 核子
中文（繁體）(Chinese (Traditional))
n. - 核心, 原子核, 核子
日本語 (Japanese)
n. - 中心, 核, 原子核, 細胞核
العربيه (Arabic)
(الاسم) نواة
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