Thursday 26 January 2012
HERBARIUM
In botany, a herbarium (plural: herbaria ) – sometimes known by the Anglicized term herbar – is a collection of preserved plant specimens. These specimens may be whole plants or plant parts: these will usually be in a dried form, mounted on a sheet, but depending upon the material may also be kept in alcohol or other preservative. The same term is often used in mycology to describe an equivalent collection of preserved fungi, otherwise known as a fungarium . The term can also refer to the building where the specimens are stored, or the scientific institute that not only stores but researches these specimens. The specimens in a herbarium are often used as reference material in describing plant taxa ; some specimens may be types . A xylarium is a herbarium specialising in specimens of wood. A hortorium (as in the Liberty Hyde Bailey Hortorium) is one specialising in preserved specimens of cultivated plant. Specimen preservation Preparing a plant for mounting To preserve their form and color, plants collected in the field are spread flat on sheets of newsprint and dried, usually in a plant press, between blotters or absorbent paper. The specimens, which are then mounted on sheets of stiff white paper, are labeled with all essential data, such as date and place found, description of the plant, altitude, and special habitat conditions. The sheet is then placed in a protective case. As a precaution against insect attack, the pressed plant is frozen or poisoned and the case disinfected. Certain groups of plants are soft, bulky, or otherwise not amenable to drying and mounting on sheets. For these plants, other methods of preparation and storage may be used. For example, conifer cones and palm fronds may be stored in labeled boxes. Representative flowers or fruits may be pickled in formaldehyde to preserve their three-dimensional structure. Small specimens, such as mosses and lichens, are often air-dried and packaged in small paper envelopes. No matter the method of preservation, detailed information on where and when the plant was collected, habitat, color (since it may fade over time), and the name of collector is usually included. Collections management A large herbarium may have hundreds of cases filled with specimens. Most herbaria utilize a standard system of organizing their specimens into herbarium cases. Specimen sheets are stacked in groups by the species to which they belong and placed into a large lightweight folder that is labelled on the bottom edge. Groups of species folders are then placed together into larger, heavier folders by genus. The genus folders are then sorted by taxonomic family according to the standard system selected for use by the herbarium and placed into pigeonholes in herbarium cabinets. Locating a specimen filed in the herbarium requires knowing the nomenclature and classification used by the herbarium. It also requires familiarity with possible name changes that have occurred since the specimen was collected, since the specimen may be filed under an older name. Modern herbaria often maintain electronic databases of their collections. Many herbaria have initiatives to digitize specimens to produce a virtual herbarium . These records and images are made publicly accessible via the Internet when possible. Uses Herbaria are essential for the study of plant taxonomy , the study of geographic distributions, and the stabilizing of nomenclature. Thus it is desirable to include in a specimen as much of the plant as possible (e.g., flowers, stems, leaves, seed, and fruit). Linnaeus' herbarium now belongs to the Linnean Society in England. Specimens housed in herbaria may be used to catalogue or identify the flora of an area. A large collection from a single area is used in writing a field guide or manual to aid in the identification of plants that grow there. With more specimens available, the author of the guide will better understand the variability of form in the plants and the natural distribution over which the plants grow. Herbaria also preserve a historical record of change in vegetation over time. In some cases, plants become extinct in one area, or may become extinct altogether. In such cases, specimens preserved in an herbarium can represent the only record of the plant's original distribution. Environmental scientists make use of such data to track changes in climate and human impact. Many kinds of scientists use herbaria to preserve voucher specimens; representative samples of plants used in a particular study to demonstrate precisely the source of their data. They may also be a repository of viable seeds for rare species.[1] Largest herbaria The Swedish Museum of Natural History (S) . Main article: List of herbaria Many universities, museums, and botanical gardens maintain herbaria. Herbaria have also proven very useful as sources of plant DNA for use in taxonomy and molecular systematics . The largest herbaria in the world, in approximate order of decreasing size, are: Muséum National d'Histoire Naturelle (P) (Paris, France) New York Botanical Garden (NY) (Bronx, New York, USA) Komarov Botanical Institute (LE) (St. Petersburg, Russia) Royal Botanic Gardens (K) (Kew, England, UK) Conservatoire et Jardin botaniques de la Ville de Genève (G) (Geneva, Switzerland) Missouri Botanical Garden (MO) (St. Louis, Missouri, USA) British Museum of Natural History (BM) (London, England, UK) Harvard University (HUH) (Cambridge, Massachusetts, USA) Swedish Museum of Natural History (S) (Stockholm, Sweden) United States National Herbarium (Smithsonian Institution ) (US) (Washington, DC, USA) Nationaal Herbarium Nederland (L) (Leiden, the Netherlands) Université Montpellier (MPU) (Montpellier, France) Université Claude Bernard (LY) (Villeurbane Cedex, France) Herbarium Universitatis Florentinae (FI) (Florence, Italy) National Botanic Garden of Belgium (BR) (Meise, Belgium) University of Helsinki (H) (Helsinki, Finland) Botanischer Garten und Botanisches Museum Berlin- Dahlem, Zentraleinrichtung der Freien Universität Berlin (B) (Berlin, Germany) The Field Museum (F) (Chicago, Illinois, USA) University of Copenhagen (C) (Copenhagen, Denmark) Chinese National Herbarium, (Chinese Academy of Sciences) (PE) (Beijing, People's Republic of China) University and Jepson Herbaria (UC/JEPS) (Berkeley, California, USA) Herbarium Bogoriense (BO) (Bogor, West Java, Indonesia) Royal Botanic Garden, Edinburgh (E) (Edinburgh, Scotland, UK) See also Herbal List of herbaria Plant collecting Plant taxonomy Systematics Virtual herbarium External links Wikimedia Commons has media related to: Herbarium For links to a specific herbarium or institution, see the List of herbaria Index Herbariorum Linnean Herbarium Lamarck's Herbarium (online database with 20.000 sheets in HD) French Herbaria Network
NOMENCULATURE
Nomenclature is a term that applies to either a list of names or terms, or to the system of principles, procedures and terms related to naming - which is the assigning of a word or phrase to a particular object or property. [1][clarification needed] The principles of naming vary from the relatively informal conventions of everyday speech to the internationally- agreed principles, rules and recommendations that govern the formation and use of the specialist terms used in scientific and other disciplines. Naming "things" is a part of our general communication using words and language: it is an aspect of everyday taxonomy as we distinguish the objects of our experience, together with their similarities and differences, which we identify , name and classify. The use of names, as the many different kinds of nouns embedded in different languages, connects nomenclature to theoretical linguistics, while the way we mentally structure the world in relation to word meanings and experience relates to the philosophy of language . Onomastics, the study of proper names and their origins, includes: anthroponymy , concerned with human names, including personal names, surnames and nicknames; toponymy the study of place names; and etymology , the derivation, history and use of names as revealed through comparative and descriptive linguistics . The scientific need for simple, stable and internationally- accepted systems for naming objects of the natural world has generated many formal nomenclatural systems. Probably the best known of these nomenclatural systems are the five codes of biological nomenclature that govern the Latinized scientific names of organisms. Definition & criteria Nomenclature is a system of words used in particular discipline. It is used in respect of giving names systematically following the rules to all known living.It is applied to many chemical components, mainly used in carbon and hydrogen components. Etymology The word nomenclature is derived from the Latin nomen - name, calare - to call; the Ancient Greek ονοματοκλήτωρ from όνομα or onoma meaning name and equivalent to the Old English nama and Old High German namo which is derived from Sanskrit nama. The Latin term nomenclatura refers to a list of names as does the word nomenclator which can also indicate a provider or announcer of names. Onomastics and nomenclature Main article: Onomastics The study of proper names is known as onomastics,[2] which has a wide-ranging scope encompassing all names, all languages, all geographical and cultural regions. The distinction between onomastics and nomenclature is not readily clear: onomastics is an unfamiliar discipline to most people and the use of nomenclature in an academic sense is also not commonly known. Although the two fields integrate, nomenclature concerns itself more with the rules and conventions that are used for the formation of names.[citation needed] Naming as a cultural activity Main article: Philosophy of language Names provide us with a way of structuring and mapping the world in our minds so, in some way, they mirror or represent the objects of our experience. Names, words, language and meaning Main articles: Proper name (philosophy) and Semantics Elucidating the connections between language (especially names and nouns), meaning and the way we perceive the world has provided a rich field of study for philosophers and linguists. Relevant areas of study include: the distinction between proper names and proper nouns;[3] and the relationship between names, [4] their referents, [5] meanings (semantics), and the structure of language. Folk taxonomy Main articles: Folk taxonomy and Binomial nomenclature Modern scientific taxonomy has been described as "basically a Renaissance codification of folk taxonomic principles."[6] Formal scientific nomenclatural and classification systems are exemplified by biological classification. All classification systems are established for a purpose. The scientific classification system anchors each organism within the nested hierarchy of internationally-accepted classification categories. Maintenance of this system involves formal rules of nomenclature and periodic international meetings of review. This modern system evolved from the folk taxonomy of pre-history. [7] Folk taxonomy can be illustrated through the Western tradition of horticulture and gardening. Unlike scientific taxonomy, folk taxonomies serve many purposes. Examples in horticulture would be the grouping of plants, and naming of these groups, according to their properties and uses: annuals, biennials and perennials (nature of life cycle); vegetables , fruits, culinary herbs and spices (culinary use); herbs, trees and shrubs (growth habit); wild and cultivated plants (whether they are managed or not), and weeds (whether they are considered to be a nuisance or not) and so on. Folk taxonomy is generally associated with the way rural or indigenous peoples use language to make sense of and organise the objects around them. Ethnobiology frames this interpretation through either "utilitarianists" like Bronislaw Malinowski who maintain that names and classifications reflect mainly material concerns, and "intellectualists" like Claude Lévi-Strauss who hold that they spring from innate mental processes.[8] The literature of ethnobiological classifications was reviewed in 2006.[9] Folk classification is defined by the way in which members of a language community name and categorize plants and animals whereas ethnotaxonomy refers to the hierarchical structure, organic content, and cultural function of biological classification that ethnobiologists find in every society around the world. [10] Ethnographic studies of the naming and classification of animals and plants in non- Western societies have revealed some general principles that indicate pre- scientific man’s conceptual and linguistic method of organising the biological world in a hierarchical way. [11][12][13][14] Such studies indicate that the urge to classify is a basic human instinct.[15][16] in all languages natural groups of organisms are distinguished (present- day taxa) these groups are arranged into more inclusive groups or ethnobiological categories in all languages there are about five or six ethnobiological categories of graded inclusiveness these groups (ethnobiological categories) are arranged hierarchically, generally into mutually exclusive ranks the ranks at which particular organisms are named and classified is often similar in different cultures The levels are — moving from the most to least inclusive: level 1 - "unique beginner" --e.g. plant or animal. A single all- inclusive name rarely used in folk taxonomies but loosely equivalent to an original living thing, a "common ancestor" level 2 - “life form” --------------e.g. tree, bird, grass and fish These are usually primary lexemes (basic linguistic units) loosely equivalent to a phylum or major biological division. level 3 - "generic name" ------e.g. oak, pine, robin, catfish This is the most numerous and basic building block of all folk taxonomies, the most frequently referred to, the most important psychologically, and among the first learned by children. These names can usually be associated directly with a second level group. Like life-form names these are primary lexemes. level 4 - "specific name" ------e.g. white fir, post oak More or less equivalent to species. A secondary lexeme and generally less frequent than generic names. level 5 - "varietal name"--------e.g. baby lima bean, butter lima bean. In almost all cultures objects are named using one or two words equivalent to "kind" (genus) and "particular kind" (species). [6] When made up of two words (a binomial) the name usually consists of a noun (like salt, dog or star) and an adjectival second word that helps describe the first, and therefore makes the name, as a whole, more "specific", for example, lap dog, sea salt, or film star. The meaning of the noun used for a common name may have been lost or forgotten (whelk, elm, lion, shark, pig) but when the common name is extended to two or more words much more is conveyed about the organism's use, appearance or other special properties (sting ray, poison apple, giant stinking hogweed, hammerhead shark). These noun-adjective binomials are just like our own names with a family or surname like Simpson and another adjectival Christian- or forename name that specifies which Simpson, say Homer Simpson. It seems reasonable to assume that the form of scientific names we call binomial nomenclature is derived from this simple and practical way of constructing common names - but with the use of Latin as a universal language. In keeping with the "utilitarianist" view other authors maintain that ethnotaxonomies resemble more a "complex web of resemblances" than a neat hierarchy.
METABOLISM
Metabolism (from Greek: μεταβολή "metabolē",
"change" or Greek: μεταβολισμός metabolismos,
"outthrow") is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and
reproduce, maintain their
structures, and respond to
their environments.
Metabolism is usually divided
into two categories. Catabolism breaks down organic matter, for example
to harvest energy in cellular respiration. Anabolism uses energy to construct
components of cells such as proteins and nucleic acids. The chemical reactions of
metabolism are organized into metabolic pathways , in which one chemical is transformed
through a series of steps into
another chemical, by a
sequence of enzymes . Enzymes are crucial to
metabolism because they
allow organisms to drive
desirable reactions that
require energy and will not occur by themselves, by coupling them to spontaneous reactions that release energy. As enzymes act as catalysts they allow these reactions to
proceed quickly and
efficiently. Enzymes also
allow the regulation of metabolic pathways in
response to changes in the cell's environment or signals from other cells. The metabolism of an
organism determines which
substances it will find nutritious and which it will find poisonous. For example, some prokaryotes use hydrogen sulfide as a nutrient, yet this gas is poisonous to animals.[1] The speed of metabolism, the metabolic rate, influences how much food an organism will require,
and also affects how it is able
to obtain that food. A striking feature of
metabolism is the similarity of
the basic metabolic pathways
and components between
even vastly different species. [2] For example, the set of carboxylic acids that are best known as the intermediates in
the citric acid cycle are present in all known organisms, being
found in species as diverse as
the unicellular bacteria Escherichia coli and huge multicellular organisms like elephants.[3] These striking similarities in metabolic
pathways are likely due to
their early appearance in evolutionary history , and being retained because of their efficacy. [4][5] Key biochemicals Further information: Biomolecule, cell (biology) and biochemistry Structure of a triacylglycerol lipid Most of the structures that
make up animals, plants and
microbes are made from three
basic classes of molecule: amino acids, carbohydrates and lipids (often called fats). As these molecules are vital
for life, metabolic reactions
either focus on making these
molecules during the
construction of cells and
tissues, or breaking them down and using them as a
source of energy, in the
digestion and use of food.
Many important biochemicals
can be joined together to
make polymers such as DNA and proteins. These macromolecules are essential. Type of molecule Name of monomer forms Name of polymer forms Examples of polymer forms Amino acids Amino acids Proteins (also called polypeptides) Fibrous proteins and globular
proteins Carbohydrates Monosaccharides Polysaccharides Starch, glycogen and cellulose Nucleic acids Nucleotides Polynucleotides DNA and RNA Amino acids and proteins Proteins are made of amino acids arranged in a linear chain and joined together by peptide bonds. Many proteins are the enzymes that catalyze the chemical reactions in
metabolism. Other proteins
have structural or mechanical
functions, such as the proteins
that form the cytoskeleton , a system of scaffolding that maintains the cell shape.[6] Proteins are also important in cell signaling, immune responses, cell adhesion, active transport across membranes, and the cell cycle .[7] Lipids Lipids are the most diverse group of biochemicals. Their
main structural uses are as
part of biological membranes such as the cell membrane, or as a source of energy. [7] Lipids are usually defined as hydrophobic or amphipathic biological molecules that will
dissolve in organic solvents such as benzene or chloroform.[8] The fats are a large group of compounds
that contain fatty acids and glycerol ; a glycerol molecule attached to three fatty acid esters is a triacylglyceride .[9] Several variations on this basic
structure exist, including
alternate backbones such as sphingosine in the sphingolipids, and hydrophilic groups such as phosphate in phospholipids. Steroids such as cholesterol are another major class of lipids that are made in cells.[10] Carbohydrates Glucose can exist in both a straight-chain and ring form. Carbohydrates are aldehydes or ketones with many hydroxyl groups that can exist as straight chains or
rings. Carbohydrates are the
most abundant biological
molecules, and fill numerous
roles, such as the storage and
transport of energy (starch, glycogen ) and structural components (cellulose in plants, chitin in animals).[7] The basic carbohydrate units
are called monosaccharides and include galactose, fructose, and most importantly glucose. Monosaccharides can be linked
together to form polysaccharides in almost limitless ways.
VERNALISATION
Vernalization (from Latin: vernus, of the spring) is the acquisition of a plant's ability to flower or germinate in the spring by exposure to the prolonged cold of winter. After vernalization, plants have acquired the ability to flower, but they may require additional seasonal cues or weeks of growth before they will actually flower. Many plants grown in temperate climates require vernalization and must experience a period of low winter temperature to initiate or accelerate the flowering process. This ensures that reproductive development and seed production occurs in spring and summer, rather than in autumn.[1] The needed cold is often expressed in chill hours. Typical vernalization temperatures are between 5 and 10 degrees Celsius (40 and 50 degrees Fahrenheit). For many perennial plants, such as fruit tree species, a period of cold is needed to break dormancy, prior to flowering. Many monocarpic annuals and biennials, including some ecotypes of Arabidopsis thaliana [2] and winter cereals such as wheat, must go through a prolonged period of cold before flowering occurs. History of vernalization research In the history of agriculture , farmers observed a traditional distinction between "winter cereals," whose seeds require chilling, and "spring cereals," whose seeds can be sown in spring and flower soon thereafter. [3] The word "vernalization" is a translation of "Jarovization," a word coined by Trofim Lysenko to describe a chilling process he used to make the seeds of winter cereals behave like spring cereals ("Jarovoe" in Russian).[3] Scientists had also discussed how some plants needed cold temperatures to flower, as early as the 18th century, with the German plant physiologist Gustav Gassner often mentioned for his 1918 paper.[3][4] Lysenko's 1928 paper on vernalization and plant physiology drew wide attention due to its practical consequences for Russian agriculture. Severe cold and lack of winter snow had destroyed many early winter wheat seedlings. By treating wheat seeds with moisture as well as cold, Lysenko induced them to bear a crop when planted in spring.[4] Later however, Lysenko inaccurately asserted that the vernalized state could be inherited - i.e., that the offspring of a vernalized plant would behave as if they themselves had also been vernalized and would not require vernalization in order to flower quickly. [5] Early research on vernalization focused on plant physiology; the increasing availability of molecular biology has made it possible to unravel its underlying mechanisms.[5] For example, longer days as well as cold temperatures are required for winter wheat plants to go from the vegetative to the reproductive state; the three interacting genes are called VRN1, VRN2, and FT (VRN3). [6] Due to plant flowering requiring the successful co- operation of several metabolic pathways , computer models that incorporate vernalization have also been made. [7] Vernalization in Arabidopsis thaliana Arabidopsis thaliana rosette before vernalization, with no floral spike Arabidopsis thaliana, also known as "thale cress," is a much-studied model species. In 2000, the entire genome of its five chromosomes was completely sequenced. [8] Some variants, called "winter annuals", require vernalization to flower; others ("summer annuals") do not.[9] The genes that underlie this difference in plant physiology have been intensively studied. [5] The reproductive phase change of A. thalliana occurs by a sequence of two related events: first, the bolting transition (flower stalk elongates), then the floral transition (first flower appears).[10] Bolting is a robust predictor of flower formation, and hence a good indicator for vernalization research.[10] In arabidopsis winter annuals, the apical meristem is the part of the plant that needs to be chilled. Vernalization of the meristem appears to confer competence to respond to floral inductive signals on the meristem. A vernalized meristem retains competence for as long as 300 days in the absence of an inductive signal. [9] Before vernalization, flowering is repressed by the action of a gene called Flowering Locus Controller (FLC).[1] Vernalization activates a gene called Frigida (FRI), which progressively turns off FLC expression over a period of six weeks. Since vernalization also occurs in flc mutants (lacking FLC), vernalization must also activate a non-FLC pathway. [11] A day-length mechanism is also important.
Monday 23 January 2012
PHOTOPERIODISM
Photoperiodism is the physiological reaction of
organisms to the length of
day or night. It occurs in plants and animals. Photoperiodism can also be
defined as the developmental
responses of plants to the
relative lengths of the light
and dark periods. Here it
should be emphasized that photoperiodic effects relate
directly to the timing of both
the light and dark periods. In plants Many flowering plants use a photoreceptor protein , such as phytochrome or cryptochrome , to sense seasonal changes in night
length, or photoperiod, which
they take as signals to flower.
In a further subdivision,
obligate photoperiodic plants
absolutely require a long or short enough night before
flowering, whereas
facultative photoperiodic
plants are more likely to
flower under the appropriate
light conditions, but will eventually flower regardless
of night length. Photoperiodic flowering
plants are classified as long-
day plants or short-day
plants, though the regulatory
mechanism is actually
governed by hours of darkness, not the length of
the day. Modern biologists believe [citation needed] that it is the coincidence of the active
forms of phytochrome or
cryptochrome, created by
light during the daytime, with
the rhythms of the circadian clock that allows plants to measure the length of the
night. Other than flowering,
photoperiodism in plants
includes the growth of stems
or roots during certain
seasons, or the loss of leaves. Long-day plants A long-day plant requires
fewer than a certain number
of hours of darkness in each
24-hour period to induce
flowering. These plants
typically flower in the northern hemisphere during late spring or early summer as
days are getting longer. In the
northern hemisphere, the
longest day of the year is on
or about 21 June (solstice). After that date, days grow
shorter (i.e. nights grow
longer) until 21 December
(solstice). This situation is
reversed in the southern
hemisphere (i.e. longest day is 21 December and shortest day
is 21 June). In some parts of
the world, however, "winter"
or "summer" might refer to
rainy versus dry seasons,
respectively, rather than the coolest or warmest time of
year. Some long-day obligate plants
are:Carnation (Dianthus) Henbane (Hyoscyamus) Oat (Avena) Ryegrass (Lolium) Clover (Trifolium) Bellflower (Campanula carpatica) Some long-day facultative
plants are: Pea (Pisum sativum) Barley (Hordeum vulgare) Lettuce (Lactuca sativa) Wheat (Triticum aestivum, spring wheat cultivars) Turnip (Brassica rapa) Arabidopsis thaliana (model organism) Short-day plants Short-day plants flower when
the night is longer than a
critical length. They cannot
flower under long days or if a
pulse of artificial light is shone
on the plant for several minutes during the middle of
the night; they require a
consolidated period of
darkness before floral
development can begin.
Natural nighttime light, such as moonlight or lightning, is
not of sufficient brightness or
duration to interrupt
flowering. In general, short-day (i.e.
long-night) plants flower as
days grow shorter (and
nights grow longer) after 21
June in the northern
hemisphere, which is during summer or fall. The length of
the dark period required to
induce flowering differs
among species and varieties of
a species. Photoperiod affects flowering
when the shoot is induced to
produce floral buds instead of
leaves and lateral buds. Note
that some species must pass
through a "juvenile" period during which they cannot be
induced to flower—common
cocklebur is an example of a
plant species with a
remarkably short period of
juvenility and plants can be induced to flower when quite
small. Some short-day obligate
plants are: Chrysanthemum Coffee Poinsettia Strawberry Tobacco, var. Maryland Mammouth Common duckweed , (Lemna minor) Cocklebur (Xanthium) Maize – tropical cultivars only[citation needed] Some short-day facultative
plants are: Hemp (Cannabis) Cotton (Gossypium) Rice Sugar cane Day-neutral plants Day-neutral plants, such as cucumbers, roses and tomatoes, do not initiate
flowering based on
photoperiodism at all; they
flower regardless of the night
length. They may initiate
flowering after attaining a certain overall developmental
stage or age, or in response to
alternative environmental
stimuli, such as vernalisation (a period of low
temperature), rather than in
response to photoperiod.
ETHYLENE
Ethylene (IUPAC name: ethene ) is a gaseous organic compound with the formula C2H4. It is the simplest alkene (older name: olefin from its
oil-forming property). Because
it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon . Ethylene is widely used in industry and is also a plant hormone.[3] Ethylene is the most produced organic compound in the world; global production of
ethylene exceeded 107 million tonnes in 2005.[4] To meet the ever increasing demand for
ethylene, sharp increases in
production facilities are added
globally, particularly in the Persian Gulf countries and in China.[5] Structure and
properties Orbital description of bonding between ethylene and a transition metal. This hydrocarbon has four hydrogen atoms bound to a pair of carbon atoms that are connected by a double bond. All six atoms that comprise
ethylene are coplanar. The H-C- H angle is 119°, close to the 120° for ideal sp² hybridized carbon. The molecule is also
relatively rigid: rotation about
the C-C bond is a high energy
process that requires breaking
the π-bond. The π-bond in the ethylene
molecule is responsible for its
useful reactivity. The double
bond is a region of high electron density , thus it is susceptible to attack by
electrophiles. Many reactions
of ethylene are catalyzed by
transition metals, which bind
transiently to the ethylene
using both the π and π* orbitals. Being a simple molecule,
ethylene is spectroscopically
simple. Its UV-vis spectrum is still used as a test of theoretical methods. [6] Uses Major industrial reactions of
ethylene include in order of
scale: 1) polymerization , 2) oxidation , 3) halogenation and hydrohalogenation , 4) alkylation , 5) hydration , 6) oligomerization, and 7) hydroformylation . In the United States and Europe, approximately 90% of
ethylene is used to produce
three chemical compounds— ethylene oxide , ethylene dichloride, and ethylbenzene —and a variety of kinds of polyethylene .[7] Main industrial uses of ethylene. Clockwise from the upper right: its conversions to ethylene oxide , precursor to ethylene glycol, to ethylbenzene , precursor to styrene , to various kinds of polyethylene , to ethylene dichloride, precursor to vinyl chloride. Polymerization See also: Ziegler-Natta catalyst and Polyethylene Polyethylenes of various
types consume more than half
of world ethylene supply.
Polyethylene, also called
polythene, is the world's most
widely-used plastic, being primarily used to make films
used in packaging, carrier bags and trash liners. Linear alpha- olefins, produced by oligomerization (formation of short polymers) are used as precursors, detergents, plasticisers, synthetic lubricants, additives, and also as co-monomers in the
production of polyethylenes. [7] Oxidation Ethylene is oxidized to produce ethylene oxide , a key raw material in the
production of surfactants and detergents by ethoxylation . Ethylene oxide also
hydrolyzed to produce ethylene glycol , widely used as an automotive antifreeze as
well as higher molecular
weight glycols and glycol ethers. Main article: Wacker process Ethylene undergoes oxidation
by palladium to give acetaldehyde . This conversion remains a major industrial process (10M kg/y). [8] The process proceeds via the initial
complexation of ethylene to a
Pd(II) center. Halogenation and
hydrohalogenation Major intermediates from the halogenation and hydrohalogenation of ethylene include ethylene dichloride, ethyl chloride and ethylene dibromide . The addition of chlorine entails
"oxychlorination," i.e. chlorine
itself is not used. Some
products derived from this
group are polyvinyl chloride , trichloroethylene , perchloroethylene , methyl chloroform, polyvinylidiene chloride and copolymers , and ethyl bromide .[9] Alkylation Major chemical intermediates
from the alkylation with ethylene is ethylbenzene , precursor to styrene . Styrene is used principally in polystyrene for packaging and insulation, as well as in styrene-butadiene rubber for tires and footwear. On a
smaller scale, ethyl toluene, ethylanilines, 1,4-hexadiene,
and aluminium alkyls. Products of these
intermediates include polystyrene , unsaturated polyesters and ethylene- propylene terpolymers .[9] Oxo reaction The hydroformylation (oxo reaction) of ethylene results in propionaldehyde , a precursor to propionic acid and n-propyl alcohol.[9] Hydration Ethylene can be hydrated to
give ethanol, but this method is rarely used industrially. Niche uses An example of a niche use is
as an anesthetic agent (in an 85% ethylene/15% oxygen ratio).[10] It can also be used to hasten fruit ripening, as well as a welding gas. [7][11] Production In 2006, global ethylene
production was 109 million tonnes.[12] By 2010 ethylene was produced by at least 117 companies in 55 countries.[5] Ethylene is produced in the petrochemical industry by steam cracking . In this process, gaseous or light liquid
hydrocarbons are heated to
750–950 °C, inducing numerous free radical reactions followed by immediate quench to stop these reactions. This process
converts large hydrocarbons
into smaller ones and
introduces unsaturation.
Ethylene is separated from the
resulting complex mixture by repeated compression and distillation. In a related process used in oil refineries,
high molecular weight
hydrocarbons are cracked
over zeolite catalysts. Heavier feedstocks, such as naphtha and gas oils require at least
two "quench towers"
downstream of the cracking
furnaces to recirculate
pyrolysis-derived gasoline and
process water. When cracking a mixture of ethane and
propane, only one water quench tower is required. [9] The areas of an ethylene plant
are: 1. steam cracking furnaces: 2. primary and secondary
heat recovery with quench; 3. a dilution steam recycle
system between the
furnaces and the quench
system; 4. primary compression of the
cracked gas (3 stages of
compression); 5. hydrogen sulfide and
carbon dioxide removal
(acid gas removal); 6. secondary compression (1
or 2 stages); 7. drying of the cracked gas; 8. cryogenic treatment; 9. all of the cold cracked gas
stream goes to the
demethanizer tower. The
overhead stream from the
demethanizer tower
consists of all the hydrogen and methane that was in
the cracked gas stream.
Cryogenically (−250 °F
(−157 °C)) treating this
overhead stream separates
hydrogen from methane. Methane recovery is critical
to the economical operation
of an ethylene plant. 0. the bottom stream from
the demethanizer tower
goes to the deethanizer
tower. The overhead
stream from the
deethanizer tower consists of all the C2,'s that were in the cracked gas stream. The
C2 stream contains acetylene, which is
explosive above 200 kPa (29 psi).[13] If the partial pressure of acetylene is
expected to exceed these
values, the C 2 stream is partially hydrogenated. The
C2's then proceed to a C2 splitter. The product
ethylene is taken from the
overhead of the tower and
the ethane coming from
the bottom of the splitter is
recycled to the furnaces to be cracked again; 1. the bottom stream from
the de-ethanizer tower
goes to the depropanizer
tower. The overhead
stream from the
depropanizer tower consists of all the C 3's that were in the cracked gas
stream. Before feeding the
C3's to the C3 splitter, the stream is hydrogenated to
convert the methylacetylene and propadiene (allene) mix. This stream is then sent to
the C3 splitter. The overhead stream from the
C3 splitter is product propylene and the bottom
stream is propane which is
sent back to the furnaces
for cracking or used as fuel. 2. The bottom stream from
the depropanizer tower is
fed to the debutanizer
tower. The overhead
stream from the
debutanizer is all of the C 4's that were in the cracked
gas stream. The bottom
stream from the
debutanizer (light pyrolysis
gasoline) consists of
everything in the cracked gas stream that is C 5 or heavier. [9] Since ethylene production is
energy intensive, much effort
has been dedicated to
recovering heat from the gas
leaving the furnaces. Most of
the energy recovered from the cracked gas is used to
make high pressure (1200
psig) steam. This steam is in
turn used to drive the
turbines for compressing
cracked gas, the propylene refrigeration compressor, and
the ethylene refrigeration
compressor. An ethylene
plant, once running, does not
need to import steam to drive
its steam turbines. A typical world scale ethylene plant
(about 1.5 billion pounds of
ethylene per year) uses a
45,000 horsepower (34,000
kW) cracked gas compressor,
a 30,000 hp (22,000 kW) propylene compressor, and a
15,000 hp (11,000 kW)
ethylene compressor.
CYTOKININ
Cytokinins (CK) are a class of plant growth substances (phytohormones ) that promote cell division , or cytokinesis , in plant roots and shoots. They are involved
primarily in cell growth and differentiation , but also affect apical dominance, axillary bud growth, and leaf senescence. Folke Skoog discovered their effects using coconut milk in the 1940s at the University of Wisconsin–Madison. [1] There are two types of
cytokinins: adenine-type
cytokinins represented by kinetin , zeatin, and 6- benzylaminopurine , and phenylurea-type cytokinins
like diphenylurea and
thidiazuron (TDZ). Most
adenine-type cytokinins are
synthesized in roots. [2]Cambium and other actively dividing tissues also synthesize cytokinins. [3] No phenylurea cytokinins have been found in plants.[4] Cytokinins participate in local
and long-distance signalling,
with the same transport
mechanism as purines and nucleosides.[5] Typically, cytokinins are transported in the xylem .[2] Cytokinins act in concert with auxin , another plant growth hormone.[2] Mode of Action The ratio of auxin to
cytokinin plays an important
role in the effect of cytokinin
on plant growth. Cytokinin
alone has no effect on parenchyma cells. When cultured with auxin but no
cytokinin, they grow large
but do not divide. When
cytokinin is added, the cells
expand and differentiate.
When cytokinin and auxin are present in equal levels, the
parenchyma cells form an
undifferentiated callus. More cytokinin induces growth of shoot buds, while more auxin induces root formation.[2] Cytokinins are involved in
many plant processes,
including cell division and
shoot and root
morphogenesis. They are
known to regulate axillary bud growth and apical
dominance. The "direct
inhibition hypothesis" posits
that these effects result from
the cytokinin to auxin ratio.
This theory states that auxin from apical buds travels down
shoots to inhibit axillary bud
growth. This promotes shoot
growth, and restricts lateral
branching. Cytokinin moves
from the roots into the shoots, eventually signaling
lateral bud growth. Simple
experiments support this
theory. When the apical bud is
removed, the axillary buds
are uninhibited, lateral growth increases, and plants
become bushier. Applying
auxin to the cut stem again inhibits lateral dominance.[2] While cytokinin action in vascular plants is described as pleiotropic, this class of plant hormones specifically induces
the transition from apical
growth to growth via a
three-faced apical cell in moss protonema. This bud induction can be pinpointed to differentiation of a specific single cell, and thus is a very specific effect of cytokinin. [6] Cytokinins have been shown
to slow aging of plant organs
by preventing protein breakdown, activating
protein synthesis, and
assembling nutrients from nearby tissues. [2] A study that regulated leaf senescence
in tobacco leaves found that
wild-type leaves yellowed
while transgenic leaves remained mostly green. It
was hypothesized that
cytokinin may affect
enzymes that regulate protein synthesis and degradation. [7] Biosynthesis Adenosine phosphate-
isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins. It may use ATP, ADP, or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl
diphosphate (HMBDP) as prenyl donors .[8] This reaction is the rate-limiting step in cytokinin biosynthesis.
DMAPP and HMBDP used in
cytokinin biosynthesis are
produced by the
methylerythritol phosphate pathway (MEP). [8] Cytokinins can also be
produced by recycled tRNAs in plants and bacteria.[8][9] tRNAs with anticodons that start with a uridine and carrying an already-
prenylated adenosine adjacent
to the anticodon release on
degradation the adenosine as a cytokinin. [8] The prenylation of these adenines is carried out
by tRNA- isopentenyltransferase .[9] Auxin is known to regulate
the biosynthesis of cytokinin. [10] Uses Because cytokinin promotes
plant cell division and growth,
produce farmers use it to
increase crops. One study
found that applying cytokinin
to cotton seedlings led to a 5– 10% yield increase under drought conditions. [11] Cytokinins have recently been
found to play a role in plant
pathogenesis.
MESISTEM
A meristem is the tissue in most plants consisting of undifferentiated cells
(meristematic cells ), found in zones of the plant where
growth can take place. The meristematic cells give rise
to various organs of the plant,
and keep the plant growing.
The Shoot Apical Meristem
(SAM) gives rise to organs like
the leaves and flowers. The cells of the apical meristems -
SAM and RAM (Root Apical
Meristem) - divide rapidly and
are considered to be
indeterminate, in that they do
not possess any defined end fate. In that sense, the
meristematic cells are
frequently compared to the stem cells in animals, that have an analogous behavior
and function. The term meristem was first
used in 1858 by Karl Wilhelm von Nägeli (1817–1891) in his book Beiträge zur Wissenschaftlichen Botanik. [1] It is derived from the Greek
word merizein (μερίζειν),
meaning to divide, in
recognition of its inherent
function. In general, differentiated
plant cells cannot divide or
produce cells of a different
type. Therefore, cell division in the meristem is required to
provide new cells for
expansion and differentiation
of tissues and initiation of
new organs, providing the
basic structure of the plant body. Meristematic cells are
incompletely or not at all differentiated , and are capable of continued cellular division
(youthful). Furthermore, the
cells are small and protoplasm fills the cell completely. The vacuoles are extremely small. The cytoplasm does not contain differentiated plastids (chloroplasts or chromoplasts), although they are present in rudimentary
form (proplastids). Meristematic cells are packed
closely together without
intercellular cavities. The cell
wall is a very thin primary cell
wall. Maintenance of the cells
requires a balance between
two antagonistic processes:
organ initiation and stem cell
population renewal. Meristematic zones Apical meristems are the
completely undifferentiated
(indeterminate) meristems in
a plant. These differentiate
into three kinds of primary
meristems. The primary meristems in turn produce the
two secondary meristem
types. These secondary
meristems are also known as
lateral meristems because
they are involved in lateral growth. At the meristem summit,
there is a small group of
slowly dividing cells, which is
commonly called the central
zone. Cells of this zone have a
stem cell function and are essential for meristem
maintenance. The proliferation
and growth rates at the
meristem summit usually
differ considerably from those
at the periphery. Meristems also are induced in
the roots of legumes such as soybean , Lotus japonicus, pea, and Medicago truncatula after infection with soil bacteria
commonly called Rhizobium. Cells of the inner or outer
cortex in the so-called
"window of nodulation" just
behind the developing root tip
are induced to divide. The
critical signal substance is the lipo-oligosaccharide Nod- factor, decorated with side
groups to allow specificity of
interaction. The Nod factor
receptor proteins NFR1 and
NFR5 were cloned from
several legumes including Lotus japonicus, Medicago
truncatula and soybean
(Glycine max). Regulation of
nodule meristems utilizes long
distance regulation commonly
called "Autoregulation of Nodulation" (AON). This
process involves a leaf-
vascular tissue located LRR receptor kinases (LjHAR1, GmNARK and MtSUNN), CLE peptide signalling , and KAPP interaction, similar to that
seen in the CLV1,2,3 system.
LjKLAVIER also exhibits a
nodule regulation phenotype though it is not yet known
how this relates to the other
AON receptor kinases Apical meristems Organisation of an apical meristem (growing tip) 1 - Central zone 2 - Peripheral zone 3 - Medullary (i.e. central) meristem 4 - Medullary tissue The apical meristem , or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of roots in plants. Its main function is to begin growth of new cells in
young seedlings at the tips of
roots and shoots (forming
buds, among other things).
Specifically, an active apical
meristem lays down a growing root or shoot behind itself, pushing itself forward.
Apical meristems are very
small, compared to the
cylinder-shaped lateral
meristems (see 'Secondary
Meristems' below). Apical meristems are
composed of several layers.
The number of layers varies
according to plant type. In
general the outermost layer is
called the tunica while the innermost layers are the corpus . In monocots, the tunica determine the physical
characteristics of the leaf edge
and margin. In dicots, layer two of the corpus determine
the characteristics of the edge
of the leaf. The corpus and
tunica play a critical part of
the plant physical appearance
as all plant cells are formed from the meristems. Apical
meristems are found in two
locations: the root and the
stem. Some Arctic plants have
an apical meristem in the
lower/middle parts of the plant. It is thought that this
kind of meristem evolved
because it is advantageous in
Arctic conditions[citation needed]. Shoot apical meristems The source of all above-
ground organs. Cells at the
shoot apical meristem summit
serve as stem cells to the
surrounding peripheral region,
where they proliferate rapidly and are incorporated
into differentiating leaf or
flower primordia. The shoot apical meristem is
the site of most of the
embryogenesis in flowering
plants. Primordia of leaves, sepals, petals, stamens and
ovaries are initiated here at
the rate of one every time
interval, called a plastochron. It is where the first
indications that flower
development has been
evoked are manifested. One of
these indications might be the
loss of apical dominance and the release of otherwise
dormant cells to develop as
axillary shoot meristems, in
some species in axils of
primordia as close as two or
three away from the apical dome. The shoot apical
meristem consists of 4 distinct
cell groups: -. Stem cells The immediate daughter
cells of the stem cells A subjacent organising
centre Founder cells for organ
initiation in surrounding
regions The four distinct zones
mentioned above are
maintained by a complex
signalling pathway. In Arabidopsis thaliana , 3 interacting CLAVATA genes
are required to regulate the
size of the stem cell reservoir in the shoot apical meristem
by controlling the rate of cell division .[2] CLV1 and CLV2 are predicted to form a receptor
complex (of the LRR receptor
like kinase family) to which CLV3 is a ligand.[3][4][5] CLV3 shares some homology with the ESR proteins of maize, with a short 14 amino acid region being conserved between the proteins. [6][7] Proteins that contain these
conserved regions have been
grouped into the CLE family of proteins.[6][7] CLV1 has been shown to
interact with several cytoplasmic proteins that are most likely involved in downstream signalling . For example, the CLV complex has
been found to be associated
with Rho/Rac small GTPase- related proteins.[2] These proteins may act as an
intermediate between the CLV
complex and a mitogen- activated protein kinase (MAPK), which is often
involved in signalling cascades. [8] KAPP is a kinase-associated protein phosphatase that has
been shown to interact with CLV1.[9] KAPP is thought to act as a negative regulator of
CLV1 by dephosphorylating it. [9] Another important gene in
plant meristem maintenance is
WUSCHEL (shortened to WUS),
which is a target of CLV signalling.[10]WUS is expressed in the cells below
the stem cells of the meristem
and its presence prevents the differentiation of the stem cells.[10] CLV1 acts to promote cellular differentiation by
repressing WUS activity
outside of the central zone
containing the stem cells. [10]STM also acts to prevent the differentiation of stem
cells by repressing the
expression of Myb genes that
are involved in cellular differentiation.
Sunday 22 January 2012
STROKE
A stroke , also known as a cerebrovascular accident (CVA ), is the rapid loss of brain function(s) due to disturbance in the blood supply to the brain. This can be due to ischemia (lack of blood flow) caused by
blockage (thrombosis, arterial embolism), or a hemorrhage (leakage of blood). [1] As a result, the affected area of the
brain cannot function, which
might result in an inability to move one or more limbs on
one side of the body , inability to understand or formulate speech, or an inability to see one side of the visual field .[2] A stroke is a medical emergency and can cause permanent neurological damage, complications, and
death. It is the leading cause
of adult disability in the United States and Europe and the second leading cause of death worldwide. [3]Risk factors for stroke include old age, hypertension (high blood pressure), previous stroke or transient ischemic attack (TIA), diabetes, high cholesterol, cigarette smoking and atrial fibrillation .[2] High blood pressure is the most
important modifiable risk factor of stroke. [2] A silent stroke is a stroke that does not have any outward
symptoms, and the patients
are typically unaware they
have suffered a stroke.
Despite not causing
identifiable symptoms, a silent stroke still causes damage to
the brain, and places the
patient at increased risk for
both transient ischemic attack and major stroke in the
future. Conversely, those who
have suffered a major stroke
are at risk of having silent strokes. [4] In a broad study in 1998, more than 11 million
people were estimated to
have experienced a stroke in
the United States.
Approximately 770,000 of
these strokes were symptomatic and 11 million
were first-ever silent MRI
infarcts or hemorrhages. Silent strokes typically cause lesions which are detected via the use
of neuroimaging such as MRI. Silent strokes are estimated to
occur at five times the rate of symptomatic strokes. [5][6] The risk of silent stroke
increases with age, but may
also affect younger adults and
children, especially those with acute anemia.[5][7] An ischemic stroke is
occasionally treated in a
hospital with thrombolysis (also known as a "clot
buster"), and some
hemorrhagic strokes benefit
from neurosurgery. Treatment to recover any lost
function is termed stroke rehabilitation, ideally in a stroke unit and involving health professions such as speech and language therapy, physical therapy and occupational therapy . Prevention of recurrence may
involve the administration of antiplatelet drugs such as aspirin and dipyridamole , control and reduction of hypertension , and the use of statins. Selected patients may benefit from carotid endarterectomy and the use of anticoagulants.[2] Classification A slice of brain from the autopsy of a person who suffered an acute middle cerebral artery (MCA) stroke Strokes can be classified into
two major categories: ischemic and hemorrhagic.[8] Ischemic strokes are those
that are caused by
interruption of the blood
supply, while hemorrhagic
strokes are the ones which
result from rupture of a blood vessel or an abnormal vascular structure. About 87% of
strokes are caused by
ischemia, and the remainder
by hemorrhage. Some
hemorrhages develop inside
areas of ischemia ("hemorrhagic
transformation"). It is
unknown how many
hemorrhages actually start as ischemic stroke. [2] Ischemic Main articles: Cerebral infarction and Brain ischemia In an ischemic stroke, blood
supply to part of the brain is
decreased, leading to
dysfunction of the brain tissue
in that area. There are four
reasons why this might happen: 1. Thrombosis (obstruction of
a blood vessel by a blood
clot forming locally) 2. Embolism (obstruction due
to an embolus from
elsewhere in the body, see below), [2] 3. Systemic hypoperfusion
(general decrease in blood supply, e.g., in shock)[9] 4. Venous thrombosis.[10] Stroke without an obvious
explanation is termed
"cryptogenic" (of unknown
origin); this constitutes 30-40% of all ischemic strokes. [2][11] There are various classification
systems for acute ischemic
stroke. The Oxford
Community Stroke Project
classification (OCSP, also
known as the Bamford or Oxford classification) relies
primarily on the initial
symptoms; based on the
extent of the symptoms, the
stroke episode is classified as total anterior circulation
infarct (TACI), partial anterior circulation infarct (PACI), lacunar infarct (LACI) or posterior circulation infarct (POCI). These four entities
predict the extent of the
stroke, the area of the brain
affected, the underlying cause, and the prognosis.[12][13] The TOAST (Trial of Org 10172 in Acute Stroke Treatment)
classification is based on
clinical symptoms as well as
results of further
investigations; on this basis, a
stroke is classified as being due to (1) thrombosis or
embolism due to atherosclerosis of a large artery, (2) embolism of cardiac origin, (3) occlusion of a small
blood vessel, (4) other
determined cause, (5)
undetermined cause (two
possible causes, no cause
identified, or incomplete investigation). [2][14] Hemorrhagic Main articles: Intracranial hemorrhage and intracerebral hemorrhage An intraparenchymal bleed (bottom arrow) with surrounding edema (top arrow) Intracranial hemorrhage is the
accumulation of blood
anywhere within the skull
vault. A distinction is made
between intra-axial hemorrhage (blood inside the brain) and extra-axial hemorrhage (blood inside the skull but outside the brain).
Intra-axial hemorrhage is due
to intraparenchymal hemorrhage or intraventricular hemorrhage (blood in the ventricular
system). The main types of
extra-axial hemorrhage are epidural hematoma (bleeding between the dura mater and the skull, subdural hematoma (in the subdural space) and subarachnoid hemorrhage (between the arachnoid mater and pia mater). Most of the hemorrhagic stroke
syndromes have specific
symptoms (e.g., headache, previous head injury). Signs and symptoms Stroke symptoms typically
start suddenly, over seconds
to minutes, and in most cases
do not progress further. The
symptoms depend on the area
of the brain affected. The more extensive the area of
brain affected, the more
functions that are likely to be
lost. Some forms of stroke can
cause additional symptoms.
For example, in intracranial hemorrhage, the affected area
may compress other
structures. Most forms of
stroke are not associated with headache, apart from subarachnoid hemorrhage and
cerebral venous thrombosis
and occasionally intracerebral
hemorrhage. Early recognition Various systems have been
proposed to increase
recognition of stroke by
patients, relatives and
emergency first responders. A systematic review , updating a previous systematic review
from 1994, looked at a
number of trials to evaluate
how well different physical examination findings are able to predict the presence or
absence of stroke. It was
found that sudden-onset face
weakness, arm drift (i.e., if a
person, when asked to raise
both arms, involuntarily lets one arm drift downward) and
abnormal speech are the
findings most likely to lead to
the correct identification of a
case of stroke (+ likelihood ratio of 5.5 when at least one of these is present). Similarly,
when all three of these are
absent, the likelihood of
stroke is significantly
decreased (– likelihood ratio of 0.39).[15] While these findings are not perfect for
diagnosing stroke, the fact
that they can be evaluated
relatively rapidly and easily
make them very valuable in
the acute setting. Proposed systems include FAST (stroke) (face, arm, speech, and time),[16] as advocated by the Department of Health (United Kingdom) and The Stroke Association , the American Stroke
Association
(www.strokeassociation.org) ,
National Stroke Association
(US www.stroke.org), the Los
Angeles Prehospital Stroke Screen (LAPSS) [17] and the Cincinnati Prehospital Stroke Scale (CPSS).[18] Use of these scales is recommended by professional guidelines.[19] For people referred to the emergency room , early recognition of stroke is
deemed important as this can
expedite diagnostic tests and
treatments. A scoring system
called ROSIER (recognition of
stroke in the emergency room) is recommended for
this purpose; it is based on
features from the medical
history and physical examination. [19][20] Subtypes If the area of the brain
affected contains one of the
three prominent central nervous system pathways — the spinothalamic tract, corticospinal tract, and dorsal column (medial lemniscus), symptoms may include: hemiplegia and muscle weakness of the face numbness reduction in sensory or
vibratory sensation initial flaccidity
(hypotonicity), replaced by
spasticity (hypertonicity),
hyperreflexia, and obligatory synergies. [21] In most cases, the symptoms
affect only one side of the
body ( unilateral). Depending on the part of the brain
affected, the defect in the
brain is usually on the
opposite side of the body.
However, since these
pathways also travel in the spinal cord and any lesion there can also produce these
symptoms, the presence of
any one of these symptoms
does not necessarily indicate a
stroke. In addition to the above CNS
pathways, the brainstem give rise to most of the twelve cranial nerves . A stroke affecting the brain stem and
brain therefore can produce
symptoms relating to deficits
in these cranial nerves: altered smell, taste, hearing,
or vision (total or partial) drooping of eyelid ( ptosis) and weakness of ocular muscles decreased reflexes: gag,
swallow, pupil reactivity
to light decreased sensation and
muscle weakness of the
face balance problems and nystagmus altered breathing and heart
rate weakness in sternocleidomastoid muscle with inability to turn head
to one side weakness in tongue
(inability to protrude and/
or move from side to side) If the cerebral cortex is involved, the CNS pathways
can again be affected, but also
can produce the following
symptoms: aphasia (difficulty with verbal expression, auditory
comprehension, reading
and/or writing Broca's or Wernicke's area typically involved) dysarthria (motor speech disorder resulting from
neurological injury) apraxia (altered voluntary movements) visual field defect memory deficits
(involvement of temporal lobe) hemineglect (involvement of parietal lobe) disorganized thinking,
confusion, hypersexual gestures (with
involvement of frontal
lobe) anosognosia (persistent
denial of the existence of a,
usually stroke-related,
deficit) If the cerebellum is involved, the patient may have the
following: trouble walking altered movement
coordination vertigo and or disequilibrium Associated symptoms Loss of consciousness, headache, and vomiting
usually occurs more often in
hemorrhagic stroke than in
thrombosis because of the
increased intracranial pressure
from the leaking blood compressing the brain. If symptoms are maximal at
onset, the cause is more likely
to be a subarachnoid
hemorrhage or an embolic
stroke.
CERIBRAL HEMMORAGE
A cerebral hemorrhage or haemorrhage (or intracerebral hemorrhage , ICH ) is a subtype of intracranial hemorrhage that occurs within the brain tissue itself. Intracerebral
hemorrhage can be caused by brain trauma, or it can occur spontaneously in hemorrhagic stroke . Non-traumatic intracerebral hemorrhage is a
spontaneous bleeding into the brain tissue.[1] A cerebral hemorrhage is an intra-axial hemorrhage ; that is, it occurs within the
brain tissue rather than
outside of it. The other
category of intracranial
hemorrhage is extra-axial
hemorrhage, such as epidural, subdural, and subarachnoid hematomas, which all occur within the skull but outside of
the brain tissue. There are two
main kinds of intra-axial
hemorrhages: intraparenchymal hemorrhage and intraventricular hemorrhages. As with other types of hemorrhages within
the skull, intraparenchymal
bleeds are a serious medical emergency because they can increase intracranial pressure, which if left untreated can
lead to coma and death. The mortality rate for intraparenchymal bleeds is over 40%. [2] Signs and symptoms Patients with
intraparenchymal bleeds have
symptoms that correspond to
the functions controlled by
the area of the brain that is damaged by the bleed. [3] Other symptoms include those
that indicate a rise in intracranial pressure due to a large mass putting pressure on the brain.[3] Intracerebral hemorrhages are often
misdiagnosed as subarachnoid hemorrhages due to the similarity in symptoms and
signs. A severe headache
followed by vomiting is one
of the more common
symptoms of intracerebral
hemorrhage. Some patients may also go into a coma
before the bleed is noticed. Causes CT scan showing hemorrhage in the posterior fossa[1] Intracerebral bleeds are the
second most common cause of stroke , accounting for 30–60% of hospital admissions for stroke. [1]High blood pressure raises the risks of spontaneous
intracerebral hemorrhage by two to six times. [1] More common in adults than in
children, intraparenchymal
bleeds due to trauma are
usually due to penetrating head trauma, but can also be due to depressed skull fractures. Acceleration- deceleration trauma,[4][5][6] rupture of an aneurysm or arteriovenous malformation (AVM), and bleeding within a tumor are additional causes. Amyloid angiopathy is a not
uncommon cause of
intracerebral hemorrhage in
patients over the age of 55. A
very small proportion is due
to cerebral venous sinus thrombosis. Infection with the k serotype of Streptococcus mutans may also be a risk factor, due to its
prevalence in stroke patients
and production of collagen- binding protein.[7] Risk factors for ICH include: [8] Hypertension Diabetes Menopause Current cigarette smoking Alcoholic drinks (≥2/day) Tramautic intracerebral
Hematomas are divided into
acute and delayed. Acute
intracerebral Hematomas
occur at the time of the injury
while delayed intracerebral Hematomas have been
reported from as early as 6
hours post injury to as long as
several weeks. It is important
to keep in mind that
intracerebral Hematomas can be delayed because if
symptoms begin to appear
several weeks after the
injury, concussion is no longer
considered and the symptoms
may not be connected to the injury. Diagnosis Spontaneous ICH with hydrocephalus on CT scan[1] Intraparenchymal
hemorrhage can be recognized
on CT scans because blood appears brighter than other
tissue and is separated from
the inner table of the skull by
brain tissue. The tissue
surrounding a bleed is often
less dense than the rest of the brain due to edema, and therefore shows up darker on
the CT scan. Treatment Treatment depends
substantially of the type of
ICH. Rapid CT scan and other diagnostic measures are used
to determine proper
treatment, which may include
both medication and surgery. Medication Antihypertensive therapy
in acute phases. The AHA/
ASA and EUSI guidelines
(American Heart
Association/American
Stroke Association guidelines and the European
Stroke Initiative guidelines)
have recommended
antihypertensive therapy
to stabilize the mean arterial pressure at 110 mmHg. One paper showed
the efficacy of this
antihypertensive therapy
without worsening
outcome in patients of
hypertensive intracerebral hemorrhage within 3 hours onset.[9] Giving Factor VIIa within 4 hours limits the bleeding
and formation of a hematoma. However, it also increases the risk of thromboembolism.[10] Mannitol is effective in acutely reducing raised
intracranial pressure. Acetaminophen may be needed to avoid hyperthermia , and to relieve headache. [10] Frozen plasma, vitamin K , protamine, or platelet transfusions are given in case of a coagulopathy.[10] Fosphenytoin or other anticonvulsant is given in case of seizures or lobar hemorrhage.[10] H2 antagonists or proton
pump inhibitors are
commonly given for stress
ulcer prophylaxis, a
condition somehow linked with ICH. [10] Corticosteroids, in concert with antihypertensives, reduces swelling. [11] Surgery Surgery is required if the hematoma is greater than 3 cm (1 in), if there is a
structural vascular lesion or lobar hemorrhage in a young patient.[10] A catheter may be passed into the brain vasculature to close off or dilate blood vessels, avoiding invasive surgical procedures.[12] Aspiration by stereotactic surgery or endoscopic drainage may be used in basal ganglia hemorrhages, although successful reports are limited.[10] Other treatment Tracheal intubation is indicated in patients with
decreased level of
consciousness or other risk of airway obstruction. [10] IV fluids are given to maintain fluid balance, using normotonic rather than hypotonic fluids. [10] Prognosis The risk of death from an
intraparenchymal bleed in
traumatic brain injury is
especially high when the
injury occurs in the brain stem.[2] Intraparenchymal bleeds within the medulla oblongata are almost always fatal, because they cause
damage to cranial nerve X, the vagus nerve , which plays an important role in blood circulation and breathing.[4] This kind of hemorrhage can
also occur in the cortex or subcortical areas, usually in
the frontal or temporal lobes when due to head injury, and
sometimes in the cerebellum. [4][13] For spontaneous ICH seen on
CT scan, the death rate
(mortality ) is 34–50% by 30 days after the insult, [1] and half of the deaths occur in the first 2 days. [14] The inflammatory response
triggered by stroke has been
viewed as harmful, focusing
on the influx and migration of
blood-borne leukocytes,
neutrophils, and macrophages. New area of interest are the Mast Cells.[15] Epidemiology It accounts for 20% of all cases
of cerebrovascular disease in the US, behind cerebral thrombosis (40%) and cerebral embolism (30%).[16] It is two or more times more
prevalent in black American patients than it is in white.
Subscribe to:
Posts (Atom)