hpprc/enwiki-paragraphs · Datasets at Hugging Face

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Apiaceae	957	Taxonomy	Apioideae is by far the largest subfamily with about 90% of the genera. Most subsequent studies have supported this division, although leaving some genera unplaced. A 2021 study suggested the relationships shown in the following cladogram.
Apiaceae	957	Taxonomy	The Platysace clade and the genera Klotzschia and Hermas fell outside the four subfamilies. It was suggested that they could be accommodated in subfamilies of their own. Phlyctidocarpa was formerly placed in the subfamily Apioideae, but if kept there makes Apioideae paraphyletic. It could be placed in an enlarged Saniculoideae, or restored to Apioideae if the latter were expanded to include Saniculoideae.
Apiaceae	957	Taxonomy	The subfamilies can be further divided into tribes and clades, with many clades falling outside formally recognized tribes.
Apiaceae	957	Taxonomy	The number of genera accepted by sources varies. As of December 2022, Plants of the World Online (PoWO) accepted 444 genera, while GRIN Taxonomy accepted 462. The PoWO genera are not a subset of those in GRIN; for example, Haloselinum is accepted by PoWO but not by GRIN, while Halosciastrum is accepted by GRIN but not by PoWO, which treats it as a synonym of Angelica. The Angiosperm Phylogeny Website had an "approximate list" of 446 genera.
Apiaceae	957	Ecology	The black swallowtail butterfly, Papilio polyxenes, uses the family Apiaceae for food and host plants for oviposition. The 22-spot ladybird is also commonly found eating mildew on these plants.
Apiaceae	957	Uses	Many members of this family are cultivated for various purposes. Parsnip (Pastinaca sativa), carrot (Daucus carota) and Hamburg parsley (Petroselinum crispum) produce tap roots that are large enough to be useful as food. Many species produce essential oils in their leaves or fruits and as a result are flavourful aromatic herbs. Examples are parsley (Petroselinum crispum), coriander (Coriandrum sativum), culantro, and dill (Anethum graveolens). The seeds may be used in cuisine, as with coriander (Coriandrum sativum), fennel (Foeniculum vulgare), cumin (Cuminum cyminum), and caraway (Carum carvi).
Apiaceae	957	Uses	Other notable cultivated Apiaceae include chervil (Anthriscus cerefolium), angelica (Angelica spp.), celery (Apium graveolens), arracacha (Arracacia xanthorrhiza), sea holly (Eryngium spp.), asafoetida (Ferula asafoetida), galbanum (Ferula gummosa), cicely (Myrrhis odorata), anise (Pimpinella anisum), lovage (Levisticum officinale), and hacquetia (Sanicula epipactis).
Apiaceae	957	Uses	Generally, all members of this family are best cultivated in the cool-season garden; they may not grow at all if the soils are too warm. Almost every widely cultivated plant of this group is a considered useful as a companion plant. One reason is that the tiny flowers, clustered into umbels, are well suited for ladybugs, parasitic wasps, and predatory flies, which drink nectar when not reproducing. They then prey upon insect pests on nearby plants. Some of the members of this family considered "herbs" produce scents that are believed to mask the odours of nearby plants, thus making them harder for insect pests to find.
Apiaceae	957	Uses	The poisonous members of the Apiaceae have been used for a variety of purposes globally. The poisonous Oenanthe crocata has been used as an aid in suicides, and arrow poisons have been made from various other family species.
Apiaceae	957	Uses	Daucus carota has been used as coloring for butter.
Apiaceae	957	Uses	Dorema ammoniacum, Ferula galbaniflua, and Ferula moschata (sumbul) are sources of incense.
Apiaceae	957	Uses	The woody Azorella compacta Phil. has been used in South America for fuel.
Apiaceae	957	Toxicity	Many species in the family Apiaceae produce phototoxic substances (called furanocoumarins) that sensitize human skin to sunlight. Contact with plant parts that contain furanocoumarins, followed by exposure to sunlight, may cause phytophotodermatitis, a serious skin inflammation. Phototoxic species include Ammi majus, Notobubon galbanum, the parsnip (Pastinaca sativa) and numerous species of the genus Heracleum, especially the giant hogweed (Heracleum mantegazzianum). Of all the plant species that have been reported to induce phytophotodermatitis, approximately half belong to the family Apiaceae.
Apiaceae	957	Toxicity	The family Apiaceae also includes a smaller number of poisonous species, including poison hemlock, water hemlock, spotted cowbane, fool's parsley, and various species of water dropwort.
Apiaceae	957	Toxicity	Some members of the family Apiaceae, including carrot, celery, fennel, parsley and parsnip, contain polyynes, an unusual class of organic compounds that exhibit cytotoxic effects.
Axon	958		An axon (from Greek ἄξων áxōn, axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons (pseudounipolar neurons), such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction can be the cause of many inherited and acquired neurological disorders that affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, and Type IV.
Axon	958		An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron; the other type is a dendrite. Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites receive signals whereas axons transmit them). Some types of neurons have no axon and transmit signals from their dendrites. In some species, axons can emanate from dendrites known as axon-carrying dendrites. No neuron ever has more than one axon; however in invertebrates such as insects or leeches the axon sometimes consists of several regions that function more or less independently of each other.
Axon	958		Axons are covered by a membrane known as an axolemma; the cytoplasm of an axon is called axoplasm. Most axons branch, in some cases very profusely. The end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendrite or cell body of another neuron forming a synaptic connection. Axons make contact with other cells – usually other neurons but sometimes muscle or gland cells – at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon closely adjoins the membrane of the target cell, and special molecular structures serve to transmit electrical or electrochemical signals across the gap. Some synaptic junctions appear along the length of an axon as it extends; these are called en passant ("in passing") synapses and can be in the hundreds or even the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches.
Axon	958		A single axon, with all its branches taken together, can target multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, and a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 200 million axons in the human brain.
Axon	958	Anatomy	Axons are the primary transmission lines of the nervous system, and as bundles they form nerves. Some axons can extend up to one meter or more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot. The diameter of axons is also variable. Most individual axons are microscopic in diameter (typically about one micrometer (µm) across). The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, which is specialized to conduct signals very rapidly, is close to 1 millimeter in diameter, the size of a small pencil lead. The numbers of axonal telodendria (the branching structures at the end of the axon) can also differ from one nerve fiber to the next. Axons in the central nervous system (CNS) typically show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain.
Axon	958	Anatomy	There are two types of axons in the nervous system: myelinated and unmyelinated axons. Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells: Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. Oligodendrocytes form the insulating myelin in the CNS. Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction.
Axon	958	Anatomy	The myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures. The largest of these is the corpus callosum that connects the two cerebral hemispheres, and this has around 20 million axons.
Axon	958	Anatomy	The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, and the axonal region as the other.
Axon	958	Anatomy	The axonal region or compartment, includes the axon hillock, the initial segment, the rest of the axon, and the axon telodendria, and axon terminals. It also includes the myelin sheath. The Nissl bodies that produce the neuronal proteins are absent in the axonal region. Proteins needed for the growth of the axon, and the removal of waste materials, need a framework for transport. This axonal transport is provided for in the axoplasm by arrangements of microtubules and intermediate filaments known as neurofilaments.
Axon	958	Anatomy	The axon hillock is the area formed from the cell body of the neuron as it extends to become the axon. It precedes the initial segment. The received action potentials that are summed in the neuron are transmitted to the axon hillock for the generation of an action potential from the initial segment.
Axon	958	Anatomy	The axonal initial segment (AIS) is a structurally and functionally separate microdomain of the axon. One function of the initial segment is to separate the main part of an axon from the rest of the neuron; another function is to help initiate action potentials. Both of these functions support neuron cell polarity, in which dendrites (and, in some cases the soma) of a neuron receive input signals at the basal region, and at the apical region the neuron's axon provides output signals.
Axon	958	Anatomy	The axon initial segment is unmyelinated and contains a specialized complex of proteins. It is between approximately 20 and 60 µm in length and functions as the site of action potential initiation. Both the position on the axon and the length of the AIS can change showing a degree of plasticity that can fine-tune the neuronal output. A longer AIS is associated with a greater excitability. Plasticity is also seen in the ability of the AIS to change its distribution and to maintain the activity of neural circuitry at a constant level.
Axon	958	Anatomy	The AIS is highly specialized for the fast conduction of nerve impulses. This is achieved by a high concentration of voltage-gated sodium channels in the initial segment where the action potential is initiated. The ion channels are accompanied by a high number of cell adhesion molecules and scaffold proteins that anchor them to the cytoskeleton. Interactions with ankyrin-G are important as it is the major organizer in the AIS.
Axon	958	Anatomy	The axoplasm is the equivalent of cytoplasm in the cell. Microtubules form in the axoplasm at the axon hillock. They are arranged along the length of the axon, in overlapping sections, and all point in the same direction – towards the axon terminals. This is noted by the positive endings of the microtubules. This overlapping arrangement provides the routes for the transport of different materials from the cell body. Studies on the axoplasm has shown the movement of numerous vesicles of all sizes to be seen along cytoskeletal filaments – the microtubules, and neurofilaments, in both directions between the axon and its terminals and the cell body.
Axon	958	Anatomy	Outgoing anterograde transport from the cell body along the axon, carries mitochondria and membrane proteins needed for growth to the axon terminal. Ingoing retrograde transport carries cell waste materials from the axon terminal to the cell body. Outgoing and ingoing tracks use different sets of motor proteins. Outgoing transport is provided by kinesin, and ingoing return traffic is provided by dynein. Dynein is minus-end directed. There are many forms of kinesin and dynein motor proteins, and each is thought to carry a different cargo. The studies on transport in the axon led to the naming of kinesin.
Axon	958	Anatomy	In the nervous system, axons may be myelinated, or unmyelinated. This is the provision of an insulating layer, called a myelin sheath. The myelin membrane is unique in its relatively high lipid to protein ratio.
Axon	958	Anatomy	In the peripheral nervous system axons are myelinated by glial cells known as Schwann cells. In the central nervous system the myelin sheath is provided by another type of glial cell, the oligodendrocyte. Schwann cells myelinate a single axon. An oligodendrocyte can myelinate up to 50 axons.
Axon	958	Anatomy	The composition of myelin is different in the two types. In the CNS the major myelin protein is proteolipid protein, and in the PNS it is myelin basic protein.
Axon	958	Anatomy	Nodes of Ranvier (also known as myelin sheath gaps) are short unmyelinated segments of a myelinated axon, which are found periodically interspersed between segments of the myelin sheath. Therefore, at the point of the node of Ranvier, the axon is reduced in diameter. These nodes are areas where action potentials can be generated. In saltatory conduction, electrical currents produced at each node of Ranvier are conducted with little attenuation to the next node in line, where they remain strong enough to generate another action potential. Thus in a myelinated axon, action potentials effectively "jump" from node to node, bypassing the myelinated stretches in between, resulting in a propagation speed much faster than even the fastest unmyelinated axon can sustain.
Axon	958	Anatomy	An axon can divide into many branches called telodendria (Greek for 'end of tree'). At the end of each telodendron is an axon terminal (also called a synaptic bouton, or terminal bouton). Axon terminals contain synaptic vesicles that store the neurotransmitter for release at the synapse. This makes multiple synaptic connections with other neurons possible. Sometimes the axon of a neuron may synapse onto dendrites of the same neuron, when it is known as an autapse.
Axon	958	Anatomy	In the normally developed brain, along the shaft of some axons are located pre-synaptic boutons also known as axonal varicosities and these have been found in regions of the hippocampus that function in the release of neurotransmitters. However, axonal varicosities are also present in neurodegenerative diseases where they interfere with the conduction of an action potential. Axonal varicosities are also the hallmark of traumatic brain injuries. Axonal damage is usually to the axon cytoskeleton disrupting transport. As a consequence protein accumulations such as amyloid-beta precursor protein can build up in a swelling resulting in a number of varicosities along the axon.
Axon	958	Action potentials	Most axons carry signals in the form of action potentials, which are discrete electrochemical impulses that travel rapidly along an axon, starting at the cell body and terminating at points where the axon makes synaptic contact with target cells. The defining characteristic of an action potential is that it is "all-or-nothing" – every action potential that an axon generates has essentially the same size and shape. This all-or-nothing characteristic allows action potentials to be transmitted from one end of a long axon to the other without any reduction in size. There are, however, some types of neurons with short axons that carry graded electrochemical signals, of variable amplitude.
Axon	958	Action potentials	When an action potential reaches a presynaptic terminal, it activates the synaptic transmission process. The first step is rapid opening of calcium ion channels in the membrane of the axon, allowing calcium ions to flow inward across the membrane. The resulting increase in intracellular calcium concentration causes synaptic vesicles (tiny containers enclosed by a lipid membrane) filled with a neurotransmitter chemical to fuse with the axon's membrane and empty their contents into the extracellular space. The neurotransmitter is released from the presynaptic nerve through exocytosis. The neurotransmitter chemical then diffuses across to receptors located on the membrane of the target cell. The neurotransmitter binds to these receptors and activates them. Depending on the type of receptors that are activated, the effect on the target cell can be to excite the target cell, inhibit it, or alter its metabolism in some way. This entire sequence of events often takes place in less than a thousandth of a second. Afterward, inside the presynaptic terminal, a new set of vesicles is moved into position next to the membrane, ready to be released when the next action potential arrives. The action potential is the final electrical step in the integration of synaptic messages at the scale of the neuron.
Axon	958	Action potentials	Extracellular recordings of action potential propagation in axons has been demonstrated in freely moving animals. While extracellular somatic action potentials have been used to study cellular activity in freely moving animals such as place cells, axonal activity in both white and gray matter can also be recorded. Extracellular recordings of axon action potential propagation is distinct from somatic action potentials in three ways: 1. The signal has a shorter peak-trough duration (~150μs) than of pyramidal cells (~500μs) or interneurons (~250μs). 2. The voltage change is triphasic. 3. Activity recorded on a tetrode is seen on only one of the four recording wires. In recordings from freely moving rats, axonal signals have been isolated in white matter tracts including the alveus and the corpus callosum as well hippocampal gray matter.
Axon	958	Action potentials	In fact, the generation of action potentials in vivo is sequential in nature, and these sequential spikes constitute the digital codes in the neurons. Although previous studies indicate an axonal origin of a single spike evoked by short-term pulses, physiological signals in vivo trigger the initiation of sequential spikes at the cell bodies of the neurons.
Axon	958	Action potentials	In addition to propagating action potentials to axonal terminals, the axon is able to amplify the action potentials, which makes sure a secure propagation of sequential action potentials toward the axonal terminal. In terms of molecular mechanisms, voltage-gated sodium channels in the axons possess lower threshold and shorter refractory period in response to short-term pulses.
Axon	958	Development and growth	The development of the axon to its target, is one of the six major stages in the overall development of the nervous system. Studies done on cultured hippocampal neurons suggest that neurons initially produce multiple neurites that are equivalent, yet only one of these neurites is destined to become the axon. It is unclear whether axon specification precedes axon elongation or vice versa, although recent evidence points to the latter. If an axon that is not fully developed is cut, the polarity can change and other neurites can potentially become the axon. This alteration of polarity only occurs when the axon is cut at least 10 μm shorter than the other neurites. After the incision is made, the longest neurite will become the future axon and all the other neurites, including the original axon, will turn into dendrites. Imposing an external force on a neurite, causing it to elongate, will make it become an axon. Nonetheless, axonal development is achieved through a complex interplay between extracellular signaling, intracellular signaling and cytoskeletal dynamics.
Axon	958	Development and growth	The extracellular signals that propagate through the extracellular matrix surrounding neurons play a prominent role in axonal development. These signaling molecules include proteins, neurotrophic factors, and extracellular matrix and adhesion molecules. Netrin (also known as UNC-6) a secreted protein, functions in axon formation. When the UNC-5 netrin receptor is mutated, several neurites are irregularly projected out of neurons and finally a single axon is extended anteriorly. The neurotrophic factors – nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NTF3) are also involved in axon development and bind to Trk receptors.
Axon	958	Development and growth	The ganglioside-converting enzyme plasma membrane ganglioside sialidase (PMGS), which is involved in the activation of TrkA at the tip of neutrites, is required for the elongation of axons. PMGS asymmetrically distributes to the tip of the neurite that is destined to become the future axon.
Axon	958	Development and growth	During axonal development, the activity of PI3K is increased at the tip of destined axon. Disrupting the activity of PI3K inhibits axonal development. Activation of PI3K results in the production of phosphatidylinositol (3,4,5)-trisphosphate (PtdIns) which can cause significant elongation of a neurite, converting it into an axon. As such, the overexpression of phosphatases that dephosphorylate PtdIns leads into the failure of polarization.
Axon	958	Development and growth	The neurite with the lowest actin filament content will become the axon. PGMS concentration and f-actin content are inversely correlated; when PGMS becomes enriched at the tip of a neurite, its f-actin content is substantially decreased. In addition, exposure to actin-depolimerizing drugs and toxin B (which inactivates Rho-signaling) causes the formation of multiple axons. Consequently, the interruption of the actin network in a growth cone will promote its neurite to become the axon.
Axon	958	Development and growth	Growing axons move through their environment via the growth cone, which is at the tip of the axon. The growth cone has a broad sheet-like extension called a lamellipodium which contain protrusions called filopodia. The filopodia are the mechanism by which the entire process adheres to surfaces and explores the surrounding environment. Actin plays a major role in the mobility of this system. Environments with high levels of cell adhesion molecules (CAMs) create an ideal environment for axonal growth. This seems to provide a "sticky" surface for axons to grow along. Examples of CAMs specific to neural systems include N-CAM, TAG-1 – an axonal glycoprotein – and MAG, all of which are part of the immunoglobulin superfamily. Another set of molecules called extracellular matrix-adhesion molecules also provide a sticky substrate for axons to grow along. Examples of these molecules include laminin, fibronectin, tenascin, and perlecan. Some of these are surface bound to cells and thus act as short range attractants or repellents. Others are difusible ligands and thus can have long range effects.
Axon	958	Development and growth	Cells called guidepost cells assist in the guidance of neuronal axon growth. These cells that help axon guidance, are typically other neurons that are sometimes immature. When the axon has completed its growth at its connection to the target, the diameter of the axon can increase by up to five times, depending on the speed of conduction required.
Axon	958	Development and growth	It has also been discovered through research that if the axons of a neuron were damaged, as long as the soma (the cell body of a neuron) is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells. This is also referred to as neuroregeneration.
Axon	958	Development and growth	Nogo-A is a type of neurite outgrowth inhibitory component that is present in the central nervous system myelin membranes (found in an axon). It has a crucial role in restricting axonal regeneration in adult mammalian central nervous system. In recent studies, if Nogo-A is blocked and neutralized, it is possible to induce long-distance axonal regeneration which leads to enhancement of functional recovery in rats and mouse spinal cord. This has yet to be done on humans. A recent study has also found that macrophages activated through a specific inflammatory pathway activated by the Dectin-1 receptor are capable of promoting axon recovery, also however causing neurotoxicity in the neuron.
Axon	958	Development and growth	Axons vary largely in length from a few micrometers up to meters in some animals. This emphasizes that there must be a cellular length regulation mechanism allowing the neurons both to sense the length of their axons and to control their growth accordingly. It was discovered that motor proteins play an important role in regulating the length of axons. Based on this observation, researchers developed an explicit model for axonal growth describing how motor proteins could affect the axon length on the molecular level. These studies suggest that motor proteins carry signaling molecules from the soma to the growth cone and vice versa whose concentration oscillates in time with a length-dependent frequency.
Axon	958	Classification	The axons of neurons in the human peripheral nervous system can be classified based on their physical features and signal conduction properties. Axons were known to have different thicknesses (from 0.1 to 20 µm) and these differences were thought to relate to the speed at which an action potential could travel along the axon – its conductance velocity. Erlanger and Gasser proved this hypothesis, and identified several types of nerve fiber, establishing a relationship between the diameter of an axon and its nerve conduction velocity. They published their findings in 1941 giving the first classification of axons.
Axon	958	Classification	Axons are classified in two systems. The first one introduced by Erlanger and Gasser, grouped the fibers into three main groups using the letters A, B, and C. These groups, group A, group B, and group C include both the sensory fibers (afferents) and the motor fibers (efferents). The first group A, was subdivided into alpha, beta, gamma, and delta fibers – Aα, Aβ, Aγ, and Aδ. The motor neurons of the different motor fibers, were the lower motor neurons – alpha motor neuron, beta motor neuron, and gamma motor neuron having the Aα, Aβ, and Aγ nerve fibers, respectively.
Axon	958	Classification	Later findings by other researchers identified two groups of Aa fibers that were sensory fibers. These were then introduced into a system (Lloyd classification) that only included sensory fibers (though some of these were mixed nerves and were also motor fibers). This system refers to the sensory groups as Types and uses Roman numerals: Type Ia, Type Ib, Type II, Type III, and Type IV.
Axon	958	Classification	Lower motor neurons have two kind of fibers:
Axon	958	Classification	Different sensory receptors innervate different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers and nociceptors and thermoreceptors by type III and IV sensory fibers.
Axon	958	Classification	The autonomic nervous system has two kinds of peripheral fibers:
Axon	958	Clinical significance	In order of degree of severity, injury to a nerve in the peripheral nervous system can be described as neurapraxia, axonotmesis, or neurotmesis. Concussion is considered a mild form of diffuse axonal injury. Axonal injury can also cause central chromatolysis. The dysfunction of axons in the nervous system is one of the major causes of many inherited and acquired neurological disorders that affect both peripheral and central neurons.
Axon	958	Clinical significance	When an axon is crushed, an active process of axonal degeneration takes place at the part of the axon furthest from the cell body. This degeneration takes place quickly following the injury, with the part of the axon being sealed off at the membranes and broken down by macrophages. This is known as Wallerian degeneration. Dying back of an axon can also take place in many neurodegenerative diseases, particularly when axonal transport is impaired, this is known as Wallerian-like degeneration. Studies suggest that the degeneration happens as a result of the axonal protein NMNAT2, being prevented from reaching all of the axon.
Axon	958	Clinical significance	Demyelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis.
Axon	958	Clinical significance	Dysmyelination is the abnormal formation of the myelin sheath. This is implicated in several leukodystrophies, and also in schizophrenia.
Axon	958	Clinical significance	A severe traumatic brain injury can result in widespread lesions to nerve tracts damaging the axons in a condition known as diffuse axonal injury. This can lead to a persistent vegetative state. It has been shown in studies on the rat that axonal damage from a single mild traumatic brain injury, can leave a susceptibility to further damage, after repeated mild traumatic brain injuries.
Axon	958	Clinical significance	A nerve guidance conduit is an artificial means of guiding axon growth to enable neuroregeneration, and is one of the many treatments used for different kinds of nerve injury.
Axon	958	Terminology	Some general dictionaries define "nerve fiber" as any neuronal process, including both axons and dendrites. However, medical sources generally use "nerve fiber" to refer to the axon only.
Axon	958	History	German anatomist Otto Friedrich Karl Deiters is generally credited with the discovery of the axon by distinguishing it from the dendrites. Swiss Rüdolf Albert von Kölliker and German Robert Remak were the first to identify and characterize the axon initial segment. Kölliker named the axon in 1896. Louis-Antoine Ranvier was the first to describe the gaps or nodes found on axons and for this contribution these axonal features are now commonly referred to as the nodes of Ranvier. Santiago Ramón y Cajal, a Spanish anatomist, proposed that axons were the output components of neurons, describing their functionality. Joseph Erlanger and Herbert Gasser earlier developed the classification system for peripheral nerve fibers, based on axonal conduction velocity, myelination, fiber size etc. Alan Hodgkin and Andrew Huxley also employed the squid giant axon (1939) and by 1952 they had obtained a full quantitative description of the ionic basis of the action potential, leading to the formulation of the Hodgkin–Huxley model. Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963. The formulae detailing axonal conductance were extended to vertebrates in the Frankenhaeuser–Huxley equations. The understanding of the biochemical basis for action potential propagation has advanced further, and includes many details about individual ion channels.
Axon	958	Other animals	The axons in invertebrates have been extensively studied. The longfin inshore squid, often used as a model organism has the longest known axon. The giant squid has the largest axon known. Its size ranges from 0.5 (typically) to 1 mm in diameter and is used in the control of its jet propulsion system. The fastest recorded conduction speed of 210 m/s, is found in the ensheathed axons of some pelagic Penaeid shrimps and the usual range is between 90 and 200 meters/s (cf 100–120 m/s for the fastest myelinated vertebrate axon.)
Axon	958	Other animals	In other cases as seen in rat studies an axon originates from a dendrite; such axons are said to have "dendritic origin". Some axons with dendritic origin similarly have a "proximal" initial segment that starts directly at the axon origin, while others have a "distal" initial segment, discernibly separated from the axon origin. In many species some of the neurons have axons that emanate from the dendrite and not from the cell body, and these are known as axon-carrying dendrites. In many cases, an axon originates at an axon hillock on the soma; such axons are said to have "somatic origin". Some axons with somatic origin have a "proximal" initial segment adjacent the axon hillock, while others have a "distal" initial segment, separated from the soma by an extended axon hillock.
Aramaic alphabet	960		The ancient Aramaic alphabet was used to write the Aramaic languages spoken by ancient Aramean pre-Christian tribes throughout the Fertile Crescent. It was also adopted by other peoples as their own alphabet when empires and their subjects underwent linguistic Aramaization during a language shift for governing purposes — a precursor to Arabization centuries later — including among the Assyrians and Babylonians who permanently replaced their Akkadian language and its cuneiform script with Aramaic and its script, and among Jews (but not Samaritans), who adopted the Aramaic language as their vernacular and started using the Aramaic alphabet even for writing Hebrew, displacing the former Paleo-Hebrew alphabet. (The modern Hebrew alphabet derives from the Aramaic alphabet, in contrast to the modern Samaritan alphabet, which derives from Paleo-Hebrew).
Aramaic alphabet	960		The letters in the Aramaic alphabet all represent consonants, some of which are also used as matres lectionis to indicate long vowels. Writing systems (like the Aramaic) that indicate consonants but do not indicate most vowels other than by means of matres lectionis or added diacritical signs, have been called abjads by Peter T. Daniels to distinguish them from alphabets such as the Greek alphabet that represent vowels more systematically. The term was coined to avoid the notion that a writing system that represents sounds must be either a syllabary or an alphabet, which would imply that a system like Aramaic must be either a syllabary (as argued by Ignace Gelb) or an incomplete or deficient alphabet (as most other writers had said before Daniels). Rather, Daniels put forward, this is a different type of writing system, intermediate between syllabaries and 'full' alphabets.
Aramaic alphabet	960		The Aramaic alphabet is historically significant since virtually all modern Middle Eastern writing systems can be traced back to it. That is primarily due to the widespread usage of the Aramaic language after it was adopted as both a lingua franca and the official language of the Neo-Assyrian and Neo-Babylonian Empires, and their successor, the Achaemenid Empire. Among the descendant scripts in modern use, the Jewish Hebrew alphabet bears the closest relation to the Imperial Aramaic script of the 5th century BC, with an identical letter inventory and, for the most part, nearly identical letter shapes. By contrast the Samaritan Hebrew script is directly descended from Proto-Hebrew/Phoenician script, which was in turn the ancestor of the Aramaic alphabet. The Aramaic alphabet was also an ancestor to the Nabataean alphabet, which in turn had the Arabic alphabet as a descendant.
Aramaic alphabet	960	History	The earliest inscriptions in the Aramaic language use the Phoenician alphabet. Over time, the alphabet developed into the Aramaic alphabet by the 8th century BC. It was used to write the Aramaic languages spoken by ancient Aramean pre-Christian tribes throughout the Fertile Crescent. It was also adopted by other peoples as their own alphabet when empires and their subjects underwent linguistic Aramaization during a language shift for governing purposes—a precursor to Arabization centuries later—including among Assyrians who permanently replaced their Akkadian language and its cuneiform script with Aramaic and its script, and among Jews (but not Samaritans), who adopted the Aramaic language as their vernacular and started using the Aramaic alphabet even for writing Hebrew, displacing the former Paleo-Hebrew alphabet. (The modern Hebrew alphabet derives from the Aramaic alphabet, in contrast to the modern Samaritan alphabet, which derives from Paleo-Hebrew).
Aramaic alphabet	960	History	Around 500 BC, following the Achaemenid conquest of Mesopotamia under Darius I, Old Aramaic was adopted by the Persians as the "vehicle for written communication between the different regions of the vast Persian empire with its different peoples and languages. The use of a single official language, which modern scholarship has dubbed as Official Aramaic, Imperial Aramaic or Achaemenid Aramaic, can be assumed to have greatly contributed to the astonishing success of the Achaemenid Persians in holding their far-flung empire together for as long as they did."
Aramaic alphabet	960	History	Imperial Aramaic was highly standardised; its orthography was based more on historical roots than any spoken dialect and was influenced by Old Persian. The Aramaic glyph forms of the period are often divided into two main styles, the "lapidary" form, usually inscribed on hard surfaces like stone monuments, and a cursive form whose lapidary form tended to be more conservative by remaining more visually similar to Phoenician and early Aramaic. Both were in use through the Achaemenid Persian period, but the cursive form steadily gained ground over the lapidary, which had largely disappeared by the 3rd century BC.
Aramaic alphabet	960	History	For centuries after the fall of the Achaemenid Empire in 331 BC, Imperial Aramaic, or something near enough to it to be recognisable, would remain an influence on the various native Iranian languages. The Aramaic script would survive as the essential characteristics of the Iranian Pahlavi writing system.
Aramaic alphabet	960	History	30 Aramaic documents from Bactria have been recently discovered, an analysis of which was published in November 2006. The texts, which were rendered on leather, reflect the use of Aramaic in the 4th century BC in the Persian Achaemenid administration of Bactria and Sogdiana.
Aramaic alphabet	960	History	The widespread usage of Achaemenid Aramaic in the Middle East led to the gradual adoption of the Aramaic alphabet for writing Hebrew. Formerly, Hebrew had been written using an alphabet closer in form to that of Phoenician, the Paleo-Hebrew alphabet.
Aramaic alphabet	960	History	Since the evolution of the Aramaic alphabet out of the Phoenician one was a gradual process, the division of the world's alphabets into the ones derived from the Phoenician one directly and the ones derived from Phoenician via Aramaic is somewhat artificial. In general, the alphabets of the Mediterranean region (Anatolia, Greece, Italy) are classified as Phoenician-derived, adapted from around the 8th century BC, and those of the East (the Levant, Persia, Central Asia, and India) are considered Aramaic-derived, adapted from around the 6th century BC from the Imperial Aramaic script of the Achaemenid Empire.
Aramaic alphabet	960	History	After the fall of the Achaemenid Empire, the unity of the Imperial Aramaic script was lost, diversifying into a number of descendant cursives.
Aramaic alphabet	960	History	The Hebrew and Nabataean alphabets, as they stood by the Roman era, were little changed in style from the Imperial Aramaic alphabet. Ibn Khaldun (1332–1406) alleges that not only the old Nabataean writing was influenced by the "Syrian script" (i.e. Aramaic), but also the old Chaldean script.
Aramaic alphabet	960	History	A cursive Hebrew variant developed from the early centuries AD, but it remained restricted to the status of a variant used alongside the noncursive. By contrast, the cursive developed out of the Nabataean alphabet in the same period soon became the standard for writing Arabic, evolving into the Arabic alphabet as it stood by the time of the early spread of Islam.
Aramaic alphabet	960	History	The development of cursive versions of Aramaic also led to the creation of the Syriac, Palmyrene and Mandaic alphabets, which formed the basis of the historical scripts of Central Asia, such as the Sogdian and Mongolian alphabets.
Aramaic alphabet	960	History	The Old Turkic script is generally considered to have its ultimate origins in Aramaic, in particular via the Pahlavi or Sogdian alphabets, as suggested by V. Thomsen, or possibly via Kharosthi (cf., Issyk inscription).
Aramaic alphabet	960	History	Brahmi script was also possibly derived or inspired by Aramaic. Brahmic family of scripts includes Devanagari.
Aramaic alphabet	960	Languages using the alphabet	Today, Biblical Aramaic, Jewish Neo-Aramaic dialects and the Aramaic language of the Talmud are written in the modern-Hebrew alphabet (distinguished from the Old Hebrew script). In classical Jewish literature, the name given to the modern-Hebrew script was "Ashurit" (the ancient Assyrian script), a script now known widely as the Aramaic script. It is believed that during the period of Assyrian dominion that Aramaic script and language received official status. Syriac and Christian Neo-Aramaic dialects are today written in the Syriac alphabet, which script has superseded the more ancient Assyrian script and now bears its name. Mandaic is written in the Mandaic alphabet. The near-identical nature of the Aramaic and the classical Hebrew alphabets caused Aramaic text to be typeset mostly in the standard Hebrew script in scholarly literature.
Aramaic alphabet	960	Languages using the alphabet	In Maaloula, one of few surviving communities in which a Western Aramaic dialect is still spoken, an Aramaic Language Institute was established in 2006 by Damascus University that teaches courses to keep the language alive.
Aramaic alphabet	960	Languages using the alphabet	Unlike Classical Syriac, which has a rich literary tradition in Syriac-Aramaic script, Western Neo-Aramaic was solely passed down orally for generations until 2006 and was not utilized in a written form.
Aramaic alphabet	960	Languages using the alphabet	Therefore, the Language Institute's chairman, George Rizkalla (Rezkallah), undertook the writing of a textbook in Western Neo-Aramaic. Being previously unwritten, Rizkalla opted for the Hebrew alphabet. However, in 2010, the institute's activities were halted due to concerns that the square Maalouli-Aramaic alphabet used in the program bore a resemblance to the square script of the Hebrew alphabet. As a result, all signs featuring the square Maalouli script were subsequently removed. The program stated that they would instead use the more distinct Syriac-Aramaic alphabet, although use of the Maalouli alphabet has continued to some degree. Al Jazeera Arabic also broadcast a program about Western Neo-Aramaic and the villages in which it is spoken with the square script still in use.
Aramaic alphabet	960	Letters	In Aramaic writing, waw and yodh serve a double function. Originally, they represented only the consonants w and y, but they were later adopted to indicate the long vowels ū and ī respectively as well (often also ō and ē respectively). In the latter role, they are known as matres lectionis or 'mothers of reading'.
Aramaic alphabet	960	Letters	Ālap, likewise, has some of the characteristics of a mater lectionis because in initial positions, it indicates a glottal stop (followed by a vowel), but otherwise, it often also stands for the long vowels ā or ē. Among Jews, the influence of Hebrew often led to the use of Hē instead, at the end of a word.
Aramaic alphabet	960	Letters	The practice of using certain letters to hold vowel values spread to Aramaic-derived writing systems, such as in Arabic and Hebrew, which still follow the practice.
Aramaic alphabet	960	Unicode	The Imperial Aramaic alphabet was added to the Unicode Standard in October 2009, with the release of version 5.2.
Aramaic alphabet	960	Unicode	The Unicode block for Imperial Aramaic is U+10840–U+1085F:
Aramaic alphabet	960	Unicode	The Syriac Aramaic alphabet was added to the Unicode Standard in September 1999, with the release of version 3.0.
Aramaic alphabet	960	Unicode	The Syriac Abbreviation (a type of overline) can be represented with a special control character called the Syriac Abbreviation Mark (U+070F). The Unicode block for Syriac Aramaic is U+0700–U+074F:
American shot	966		"American shot" or "cowboy shot" is a translation of a phrase from French film criticism, plan américain, and refers to a medium-long ("knee") film shot of a group of characters, who are arranged so that all are visible to the camera. The usual arrangement is for the actors to stand in an irregular line from one side of the screen to the other, with the actors at the end coming forward a little and standing more in profile than the others. The purpose of the composition is to allow complex dialogue scenes to be played out without changes in camera position. In some literature, this is simply referred to as a 3/4 shot.
American shot	966		One of the other main reasons why French critics called it "American shot" was its frequent use in the western genre. This was because a shot that started at knee level would reveal the weapon of a cowboy, usually holstered at their waist. It is actually the closest the camera can get to an actor while keeping both their face and their holstered gun in frame.
American shot	966		The French critics thought it was characteristic of American films of the 1930s or 1940s; however, it was mostly characteristic of cheaper American movies, such as Charlie Chan mysteries where people collected in front of a fireplace or at the foot of the stairs in order to explain what happened a few minutes ago.
American shot	966		Howard Hawks legitimized this style in his films, allowing characters to act, even when not talking, when most of the audience would not be paying attention. It became his trademark style.
American shot	966	References
Acute disseminated encephalomyelitis	967		Acute disseminated encephalomyelitis (ADEM), or acute demyelinating encephalomyelitis, is a rare autoimmune disease marked by a sudden, widespread attack of inflammation in the brain and spinal cord. As well as causing the brain and spinal cord to become inflamed, ADEM also attacks the nerves of the central nervous system and damages their myelin insulation, which, as a result, destroys the white matter. The cause is often a trigger such as from viral infection or vaccinations.

Apiaceae

957

Taxonomy

Apioideae is by far the largest subfamily with about 90% of the genera. Most subsequent studies have supported this division, although leaving some genera unplaced. A 2021 study suggested the relationships shown in the following cladogram.

Apiaceae

957

Taxonomy

The Platysace clade and the genera Klotzschia and Hermas fell outside the four subfamilies. It was suggested that they could be accommodated in subfamilies of their own. Phlyctidocarpa was formerly placed in the subfamily Apioideae, but if kept there makes Apioideae paraphyletic. It could be placed in an enlarged Saniculoideae, or restored to Apioideae if the latter were expanded to include Saniculoideae.

Apiaceae

957

Taxonomy

The subfamilies can be further divided into tribes and clades, with many clades falling outside formally recognized tribes.

Apiaceae

957

Taxonomy

The number of genera accepted by sources varies. As of December 2022, Plants of the World Online (PoWO) accepted 444 genera, while GRIN Taxonomy accepted 462. The PoWO genera are not a subset of those in GRIN; for example, Haloselinum is accepted by PoWO but not by GRIN, while Halosciastrum is accepted by GRIN but not by PoWO, which treats it as a synonym of Angelica. The Angiosperm Phylogeny Website had an "approximate list" of 446 genera.

Apiaceae

957

Ecology

The black swallowtail butterfly, Papilio polyxenes, uses the family Apiaceae for food and host plants for oviposition. The 22-spot ladybird is also commonly found eating mildew on these plants.

Apiaceae

957

Uses

Many members of this family are cultivated for various purposes. Parsnip (Pastinaca sativa), carrot (Daucus carota) and Hamburg parsley (Petroselinum crispum) produce tap roots that are large enough to be useful as food. Many species produce essential oils in their leaves or fruits and as a result are flavourful aromatic herbs. Examples are parsley (Petroselinum crispum), coriander (Coriandrum sativum), culantro, and dill (Anethum graveolens). The seeds may be used in cuisine, as with coriander (Coriandrum sativum), fennel (Foeniculum vulgare), cumin (Cuminum cyminum), and caraway (Carum carvi).

Apiaceae

957

Uses

Other notable cultivated Apiaceae include chervil (Anthriscus cerefolium), angelica (Angelica spp.), celery (Apium graveolens), arracacha (Arracacia xanthorrhiza), sea holly (Eryngium spp.), asafoetida (Ferula asafoetida), galbanum (Ferula gummosa), cicely (Myrrhis odorata), anise (Pimpinella anisum), lovage (Levisticum officinale), and hacquetia (Sanicula epipactis).

Apiaceae

957

Uses

Generally, all members of this family are best cultivated in the cool-season garden; they may not grow at all if the soils are too warm. Almost every widely cultivated plant of this group is a considered useful as a companion plant. One reason is that the tiny flowers, clustered into umbels, are well suited for ladybugs, parasitic wasps, and predatory flies, which drink nectar when not reproducing. They then prey upon insect pests on nearby plants. Some of the members of this family considered "herbs" produce scents that are believed to mask the odours of nearby plants, thus making them harder for insect pests to find.

Apiaceae

957

Uses

The poisonous members of the Apiaceae have been used for a variety of purposes globally. The poisonous Oenanthe crocata has been used as an aid in suicides, and arrow poisons have been made from various other family species.

Apiaceae

957

Uses

Daucus carota has been used as coloring for butter.

Apiaceae

957

Uses

Dorema ammoniacum, Ferula galbaniflua, and Ferula moschata (sumbul) are sources of incense.

Apiaceae

957

Uses

The woody Azorella compacta Phil. has been used in South America for fuel.

Apiaceae

957

Toxicity

Many species in the family Apiaceae produce phototoxic substances (called furanocoumarins) that sensitize human skin to sunlight. Contact with plant parts that contain furanocoumarins, followed by exposure to sunlight, may cause phytophotodermatitis, a serious skin inflammation. Phototoxic species include Ammi majus, Notobubon galbanum, the parsnip (Pastinaca sativa) and numerous species of the genus Heracleum, especially the giant hogweed (Heracleum mantegazzianum). Of all the plant species that have been reported to induce phytophotodermatitis, approximately half belong to the family Apiaceae.

Apiaceae

957

Toxicity

The family Apiaceae also includes a smaller number of poisonous species, including poison hemlock, water hemlock, spotted cowbane, fool's parsley, and various species of water dropwort.

Apiaceae

957

Toxicity

Some members of the family Apiaceae, including carrot, celery, fennel, parsley and parsnip, contain polyynes, an unusual class of organic compounds that exhibit cytotoxic effects.

Axon

958

An axon (from Greek ἄξων áxōn, axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons (pseudounipolar neurons), such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction can be the cause of many inherited and acquired neurological disorders that affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, and Type IV.

Axon

958

An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron; the other type is a dendrite. Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites receive signals whereas axons transmit them). Some types of neurons have no axon and transmit signals from their dendrites. In some species, axons can emanate from dendrites known as axon-carrying dendrites. No neuron ever has more than one axon; however in invertebrates such as insects or leeches the axon sometimes consists of several regions that function more or less independently of each other.

Axon

958

Axons are covered by a membrane known as an axolemma; the cytoplasm of an axon is called axoplasm. Most axons branch, in some cases very profusely. The end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendrite or cell body of another neuron forming a synaptic connection. Axons make contact with other cells – usually other neurons but sometimes muscle or gland cells – at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon closely adjoins the membrane of the target cell, and special molecular structures serve to transmit electrical or electrochemical signals across the gap. Some synaptic junctions appear along the length of an axon as it extends; these are called en passant ("in passing") synapses and can be in the hundreds or even the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches.

Axon

958

A single axon, with all its branches taken together, can target multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, and a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 200 million axons in the human brain.

Axon

958

Anatomy

Axons are the primary transmission lines of the nervous system, and as bundles they form nerves. Some axons can extend up to one meter or more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot. The diameter of axons is also variable. Most individual axons are microscopic in diameter (typically about one micrometer (µm) across). The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, which is specialized to conduct signals very rapidly, is close to 1 millimeter in diameter, the size of a small pencil lead. The numbers of axonal telodendria (the branching structures at the end of the axon) can also differ from one nerve fiber to the next. Axons in the central nervous system (CNS) typically show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain.

Axon

958

Anatomy

There are two types of axons in the nervous system: myelinated and unmyelinated axons. Myelin is a layer of a fatty insulating substance, which is formed by two types of glial cells: Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. Oligodendrocytes form the insulating myelin in the CNS. Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an especially rapid mode of electrical impulse propagation called saltatory conduction.

Axon

958

Anatomy

The myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures. The largest of these is the corpus callosum that connects the two cerebral hemispheres, and this has around 20 million axons.

Axon

958

Anatomy

The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, and the axonal region as the other.

Axon

958

Anatomy

The axonal region or compartment, includes the axon hillock, the initial segment, the rest of the axon, and the axon telodendria, and axon terminals. It also includes the myelin sheath. The Nissl bodies that produce the neuronal proteins are absent in the axonal region. Proteins needed for the growth of the axon, and the removal of waste materials, need a framework for transport. This axonal transport is provided for in the axoplasm by arrangements of microtubules and intermediate filaments known as neurofilaments.

Axon

958

Anatomy

The axon hillock is the area formed from the cell body of the neuron as it extends to become the axon. It precedes the initial segment. The received action potentials that are summed in the neuron are transmitted to the axon hillock for the generation of an action potential from the initial segment.

Axon

958

Anatomy

The axonal initial segment (AIS) is a structurally and functionally separate microdomain of the axon. One function of the initial segment is to separate the main part of an axon from the rest of the neuron; another function is to help initiate action potentials. Both of these functions support neuron cell polarity, in which dendrites (and, in some cases the soma) of a neuron receive input signals at the basal region, and at the apical region the neuron's axon provides output signals.

Axon

958

Anatomy

The axon initial segment is unmyelinated and contains a specialized complex of proteins. It is between approximately 20 and 60 µm in length and functions as the site of action potential initiation. Both the position on the axon and the length of the AIS can change showing a degree of plasticity that can fine-tune the neuronal output. A longer AIS is associated with a greater excitability. Plasticity is also seen in the ability of the AIS to change its distribution and to maintain the activity of neural circuitry at a constant level.

Axon

958

Anatomy

The AIS is highly specialized for the fast conduction of nerve impulses. This is achieved by a high concentration of voltage-gated sodium channels in the initial segment where the action potential is initiated. The ion channels are accompanied by a high number of cell adhesion molecules and scaffold proteins that anchor them to the cytoskeleton. Interactions with ankyrin-G are important as it is the major organizer in the AIS.

Axon

958

Anatomy

The axoplasm is the equivalent of cytoplasm in the cell. Microtubules form in the axoplasm at the axon hillock. They are arranged along the length of the axon, in overlapping sections, and all point in the same direction – towards the axon terminals. This is noted by the positive endings of the microtubules. This overlapping arrangement provides the routes for the transport of different materials from the cell body. Studies on the axoplasm has shown the movement of numerous vesicles of all sizes to be seen along cytoskeletal filaments – the microtubules, and neurofilaments, in both directions between the axon and its terminals and the cell body.

Axon

958

Anatomy

Outgoing anterograde transport from the cell body along the axon, carries mitochondria and membrane proteins needed for growth to the axon terminal. Ingoing retrograde transport carries cell waste materials from the axon terminal to the cell body. Outgoing and ingoing tracks use different sets of motor proteins. Outgoing transport is provided by kinesin, and ingoing return traffic is provided by dynein. Dynein is minus-end directed. There are many forms of kinesin and dynein motor proteins, and each is thought to carry a different cargo. The studies on transport in the axon led to the naming of kinesin.

Axon

958

Anatomy

In the nervous system, axons may be myelinated, or unmyelinated. This is the provision of an insulating layer, called a myelin sheath. The myelin membrane is unique in its relatively high lipid to protein ratio.

Axon

958

Anatomy

In the peripheral nervous system axons are myelinated by glial cells known as Schwann cells. In the central nervous system the myelin sheath is provided by another type of glial cell, the oligodendrocyte. Schwann cells myelinate a single axon. An oligodendrocyte can myelinate up to 50 axons.

Axon

958

Anatomy

The composition of myelin is different in the two types. In the CNS the major myelin protein is proteolipid protein, and in the PNS it is myelin basic protein.

Axon

958

Anatomy

Nodes of Ranvier (also known as myelin sheath gaps) are short unmyelinated segments of a myelinated axon, which are found periodically interspersed between segments of the myelin sheath. Therefore, at the point of the node of Ranvier, the axon is reduced in diameter. These nodes are areas where action potentials can be generated. In saltatory conduction, electrical currents produced at each node of Ranvier are conducted with little attenuation to the next node in line, where they remain strong enough to generate another action potential. Thus in a myelinated axon, action potentials effectively "jump" from node to node, bypassing the myelinated stretches in between, resulting in a propagation speed much faster than even the fastest unmyelinated axon can sustain.

Axon

958

Anatomy

An axon can divide into many branches called telodendria (Greek for 'end of tree'). At the end of each telodendron is an axon terminal (also called a synaptic bouton, or terminal bouton). Axon terminals contain synaptic vesicles that store the neurotransmitter for release at the synapse. This makes multiple synaptic connections with other neurons possible. Sometimes the axon of a neuron may synapse onto dendrites of the same neuron, when it is known as an autapse.

Axon

958

Anatomy

In the normally developed brain, along the shaft of some axons are located pre-synaptic boutons also known as axonal varicosities and these have been found in regions of the hippocampus that function in the release of neurotransmitters. However, axonal varicosities are also present in neurodegenerative diseases where they interfere with the conduction of an action potential. Axonal varicosities are also the hallmark of traumatic brain injuries. Axonal damage is usually to the axon cytoskeleton disrupting transport. As a consequence protein accumulations such as amyloid-beta precursor protein can build up in a swelling resulting in a number of varicosities along the axon.

Axon

958

Action potentials

Most axons carry signals in the form of action potentials, which are discrete electrochemical impulses that travel rapidly along an axon, starting at the cell body and terminating at points where the axon makes synaptic contact with target cells. The defining characteristic of an action potential is that it is "all-or-nothing" – every action potential that an axon generates has essentially the same size and shape. This all-or-nothing characteristic allows action potentials to be transmitted from one end of a long axon to the other without any reduction in size. There are, however, some types of neurons with short axons that carry graded electrochemical signals, of variable amplitude.

Axon

958

Action potentials

When an action potential reaches a presynaptic terminal, it activates the synaptic transmission process. The first step is rapid opening of calcium ion channels in the membrane of the axon, allowing calcium ions to flow inward across the membrane. The resulting increase in intracellular calcium concentration causes synaptic vesicles (tiny containers enclosed by a lipid membrane) filled with a neurotransmitter chemical to fuse with the axon's membrane and empty their contents into the extracellular space. The neurotransmitter is released from the presynaptic nerve through exocytosis. The neurotransmitter chemical then diffuses across to receptors located on the membrane of the target cell. The neurotransmitter binds to these receptors and activates them. Depending on the type of receptors that are activated, the effect on the target cell can be to excite the target cell, inhibit it, or alter its metabolism in some way. This entire sequence of events often takes place in less than a thousandth of a second. Afterward, inside the presynaptic terminal, a new set of vesicles is moved into position next to the membrane, ready to be released when the next action potential arrives. The action potential is the final electrical step in the integration of synaptic messages at the scale of the neuron.

Axon

958

Action potentials

Extracellular recordings of action potential propagation in axons has been demonstrated in freely moving animals. While extracellular somatic action potentials have been used to study cellular activity in freely moving animals such as place cells, axonal activity in both white and gray matter can also be recorded. Extracellular recordings of axon action potential propagation is distinct from somatic action potentials in three ways: 1. The signal has a shorter peak-trough duration (~150μs) than of pyramidal cells (~500μs) or interneurons (~250μs). 2. The voltage change is triphasic. 3. Activity recorded on a tetrode is seen on only one of the four recording wires. In recordings from freely moving rats, axonal signals have been isolated in white matter tracts including the alveus and the corpus callosum as well hippocampal gray matter.

Axon

958

Action potentials

In fact, the generation of action potentials in vivo is sequential in nature, and these sequential spikes constitute the digital codes in the neurons. Although previous studies indicate an axonal origin of a single spike evoked by short-term pulses, physiological signals in vivo trigger the initiation of sequential spikes at the cell bodies of the neurons.

Axon

958

Action potentials

In addition to propagating action potentials to axonal terminals, the axon is able to amplify the action potentials, which makes sure a secure propagation of sequential action potentials toward the axonal terminal. In terms of molecular mechanisms, voltage-gated sodium channels in the axons possess lower threshold and shorter refractory period in response to short-term pulses.

Axon

958

Development and growth

The development of the axon to its target, is one of the six major stages in the overall development of the nervous system. Studies done on cultured hippocampal neurons suggest that neurons initially produce multiple neurites that are equivalent, yet only one of these neurites is destined to become the axon. It is unclear whether axon specification precedes axon elongation or vice versa, although recent evidence points to the latter. If an axon that is not fully developed is cut, the polarity can change and other neurites can potentially become the axon. This alteration of polarity only occurs when the axon is cut at least 10 μm shorter than the other neurites. After the incision is made, the longest neurite will become the future axon and all the other neurites, including the original axon, will turn into dendrites. Imposing an external force on a neurite, causing it to elongate, will make it become an axon. Nonetheless, axonal development is achieved through a complex interplay between extracellular signaling, intracellular signaling and cytoskeletal dynamics.

Axon

958

Development and growth

The extracellular signals that propagate through the extracellular matrix surrounding neurons play a prominent role in axonal development. These signaling molecules include proteins, neurotrophic factors, and extracellular matrix and adhesion molecules. Netrin (also known as UNC-6) a secreted protein, functions in axon formation. When the UNC-5 netrin receptor is mutated, several neurites are irregularly projected out of neurons and finally a single axon is extended anteriorly. The neurotrophic factors – nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NTF3) are also involved in axon development and bind to Trk receptors.

Axon

958

Development and growth

The ganglioside-converting enzyme plasma membrane ganglioside sialidase (PMGS), which is involved in the activation of TrkA at the tip of neutrites, is required for the elongation of axons. PMGS asymmetrically distributes to the tip of the neurite that is destined to become the future axon.

Axon

958

Development and growth

During axonal development, the activity of PI3K is increased at the tip of destined axon. Disrupting the activity of PI3K inhibits axonal development. Activation of PI3K results in the production of phosphatidylinositol (3,4,5)-trisphosphate (PtdIns) which can cause significant elongation of a neurite, converting it into an axon. As such, the overexpression of phosphatases that dephosphorylate PtdIns leads into the failure of polarization.

Axon

958

Development and growth

The neurite with the lowest actin filament content will become the axon. PGMS concentration and f-actin content are inversely correlated; when PGMS becomes enriched at the tip of a neurite, its f-actin content is substantially decreased. In addition, exposure to actin-depolimerizing drugs and toxin B (which inactivates Rho-signaling) causes the formation of multiple axons. Consequently, the interruption of the actin network in a growth cone will promote its neurite to become the axon.

Axon

958

Development and growth

Growing axons move through their environment via the growth cone, which is at the tip of the axon. The growth cone has a broad sheet-like extension called a lamellipodium which contain protrusions called filopodia. The filopodia are the mechanism by which the entire process adheres to surfaces and explores the surrounding environment. Actin plays a major role in the mobility of this system. Environments with high levels of cell adhesion molecules (CAMs) create an ideal environment for axonal growth. This seems to provide a "sticky" surface for axons to grow along. Examples of CAMs specific to neural systems include N-CAM, TAG-1 – an axonal glycoprotein – and MAG, all of which are part of the immunoglobulin superfamily. Another set of molecules called extracellular matrix-adhesion molecules also provide a sticky substrate for axons to grow along. Examples of these molecules include laminin, fibronectin, tenascin, and perlecan. Some of these are surface bound to cells and thus act as short range attractants or repellents. Others are difusible ligands and thus can have long range effects.

Axon

958

Development and growth

Cells called guidepost cells assist in the guidance of neuronal axon growth. These cells that help axon guidance, are typically other neurons that are sometimes immature. When the axon has completed its growth at its connection to the target, the diameter of the axon can increase by up to five times, depending on the speed of conduction required.

Axon

958

Development and growth

It has also been discovered through research that if the axons of a neuron were damaged, as long as the soma (the cell body of a neuron) is not damaged, the axons would regenerate and remake the synaptic connections with neurons with the help of guidepost cells. This is also referred to as neuroregeneration.

Axon

958

Development and growth

Nogo-A is a type of neurite outgrowth inhibitory component that is present in the central nervous system myelin membranes (found in an axon). It has a crucial role in restricting axonal regeneration in adult mammalian central nervous system. In recent studies, if Nogo-A is blocked and neutralized, it is possible to induce long-distance axonal regeneration which leads to enhancement of functional recovery in rats and mouse spinal cord. This has yet to be done on humans. A recent study has also found that macrophages activated through a specific inflammatory pathway activated by the Dectin-1 receptor are capable of promoting axon recovery, also however causing neurotoxicity in the neuron.

Axon

958

Development and growth

Axons vary largely in length from a few micrometers up to meters in some animals. This emphasizes that there must be a cellular length regulation mechanism allowing the neurons both to sense the length of their axons and to control their growth accordingly. It was discovered that motor proteins play an important role in regulating the length of axons. Based on this observation, researchers developed an explicit model for axonal growth describing how motor proteins could affect the axon length on the molecular level. These studies suggest that motor proteins carry signaling molecules from the soma to the growth cone and vice versa whose concentration oscillates in time with a length-dependent frequency.

Axon

958

Classification

The axons of neurons in the human peripheral nervous system can be classified based on their physical features and signal conduction properties. Axons were known to have different thicknesses (from 0.1 to 20 µm) and these differences were thought to relate to the speed at which an action potential could travel along the axon – its conductance velocity. Erlanger and Gasser proved this hypothesis, and identified several types of nerve fiber, establishing a relationship between the diameter of an axon and its nerve conduction velocity. They published their findings in 1941 giving the first classification of axons.

Axon

958

Classification

Axons are classified in two systems. The first one introduced by Erlanger and Gasser, grouped the fibers into three main groups using the letters A, B, and C. These groups, group A, group B, and group C include both the sensory fibers (afferents) and the motor fibers (efferents). The first group A, was subdivided into alpha, beta, gamma, and delta fibers – Aα, Aβ, Aγ, and Aδ. The motor neurons of the different motor fibers, were the lower motor neurons – alpha motor neuron, beta motor neuron, and gamma motor neuron having the Aα, Aβ, and Aγ nerve fibers, respectively.

Axon

958

Classification

Later findings by other researchers identified two groups of Aa fibers that were sensory fibers. These were then introduced into a system (Lloyd classification) that only included sensory fibers (though some of these were mixed nerves and were also motor fibers). This system refers to the sensory groups as Types and uses Roman numerals: Type Ia, Type Ib, Type II, Type III, and Type IV.

Axon

958

Classification

Lower motor neurons have two kind of fibers:

Axon

958

Classification

Different sensory receptors innervate different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers and nociceptors and thermoreceptors by type III and IV sensory fibers.

Axon

958

Classification

The autonomic nervous system has two kinds of peripheral fibers:

Axon

958

Clinical significance

In order of degree of severity, injury to a nerve in the peripheral nervous system can be described as neurapraxia, axonotmesis, or neurotmesis. Concussion is considered a mild form of diffuse axonal injury. Axonal injury can also cause central chromatolysis. The dysfunction of axons in the nervous system is one of the major causes of many inherited and acquired neurological disorders that affect both peripheral and central neurons.

Axon

958

Clinical significance

When an axon is crushed, an active process of axonal degeneration takes place at the part of the axon furthest from the cell body. This degeneration takes place quickly following the injury, with the part of the axon being sealed off at the membranes and broken down by macrophages. This is known as Wallerian degeneration. Dying back of an axon can also take place in many neurodegenerative diseases, particularly when axonal transport is impaired, this is known as Wallerian-like degeneration. Studies suggest that the degeneration happens as a result of the axonal protein NMNAT2, being prevented from reaching all of the axon.

Axon

958

Clinical significance

Demyelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis.

Axon

958

Clinical significance

Dysmyelination is the abnormal formation of the myelin sheath. This is implicated in several leukodystrophies, and also in schizophrenia.

Axon

958

Clinical significance

A severe traumatic brain injury can result in widespread lesions to nerve tracts damaging the axons in a condition known as diffuse axonal injury. This can lead to a persistent vegetative state. It has been shown in studies on the rat that axonal damage from a single mild traumatic brain injury, can leave a susceptibility to further damage, after repeated mild traumatic brain injuries.

Axon

958

Clinical significance

A nerve guidance conduit is an artificial means of guiding axon growth to enable neuroregeneration, and is one of the many treatments used for different kinds of nerve injury.

Axon

958

Terminology

Some general dictionaries define "nerve fiber" as any neuronal process, including both axons and dendrites. However, medical sources generally use "nerve fiber" to refer to the axon only.

Axon

958

History

German anatomist Otto Friedrich Karl Deiters is generally credited with the discovery of the axon by distinguishing it from the dendrites. Swiss Rüdolf Albert von Kölliker and German Robert Remak were the first to identify and characterize the axon initial segment. Kölliker named the axon in 1896. Louis-Antoine Ranvier was the first to describe the gaps or nodes found on axons and for this contribution these axonal features are now commonly referred to as the nodes of Ranvier. Santiago Ramón y Cajal, a Spanish anatomist, proposed that axons were the output components of neurons, describing their functionality. Joseph Erlanger and Herbert Gasser earlier developed the classification system for peripheral nerve fibers, based on axonal conduction velocity, myelination, fiber size etc. Alan Hodgkin and Andrew Huxley also employed the squid giant axon (1939) and by 1952 they had obtained a full quantitative description of the ionic basis of the action potential, leading to the formulation of the Hodgkin–Huxley model. Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963. The formulae detailing axonal conductance were extended to vertebrates in the Frankenhaeuser–Huxley equations. The understanding of the biochemical basis for action potential propagation has advanced further, and includes many details about individual ion channels.

Axon

958

Other animals

The axons in invertebrates have been extensively studied. The longfin inshore squid, often used as a model organism has the longest known axon. The giant squid has the largest axon known. Its size ranges from 0.5 (typically) to 1 mm in diameter and is used in the control of its jet propulsion system. The fastest recorded conduction speed of 210 m/s, is found in the ensheathed axons of some pelagic Penaeid shrimps and the usual range is between 90 and 200 meters/s (cf 100–120 m/s for the fastest myelinated vertebrate axon.)

Axon

958

Other animals

In other cases as seen in rat studies an axon originates from a dendrite; such axons are said to have "dendritic origin". Some axons with dendritic origin similarly have a "proximal" initial segment that starts directly at the axon origin, while others have a "distal" initial segment, discernibly separated from the axon origin. In many species some of the neurons have axons that emanate from the dendrite and not from the cell body, and these are known as axon-carrying dendrites. In many cases, an axon originates at an axon hillock on the soma; such axons are said to have "somatic origin". Some axons with somatic origin have a "proximal" initial segment adjacent the axon hillock, while others have a "distal" initial segment, separated from the soma by an extended axon hillock.

Aramaic alphabet

960

The ancient Aramaic alphabet was used to write the Aramaic languages spoken by ancient Aramean pre-Christian tribes throughout the Fertile Crescent. It was also adopted by other peoples as their own alphabet when empires and their subjects underwent linguistic Aramaization during a language shift for governing purposes — a precursor to Arabization centuries later — including among the Assyrians and Babylonians who permanently replaced their Akkadian language and its cuneiform script with Aramaic and its script, and among Jews (but not Samaritans), who adopted the Aramaic language as their vernacular and started using the Aramaic alphabet even for writing Hebrew, displacing the former Paleo-Hebrew alphabet. (The modern Hebrew alphabet derives from the Aramaic alphabet, in contrast to the modern Samaritan alphabet, which derives from Paleo-Hebrew).

Aramaic alphabet

960

The letters in the Aramaic alphabet all represent consonants, some of which are also used as matres lectionis to indicate long vowels. Writing systems (like the Aramaic) that indicate consonants but do not indicate most vowels other than by means of matres lectionis or added diacritical signs, have been called abjads by Peter T. Daniels to distinguish them from alphabets such as the Greek alphabet that represent vowels more systematically. The term was coined to avoid the notion that a writing system that represents sounds must be either a syllabary or an alphabet, which would imply that a system like Aramaic must be either a syllabary (as argued by Ignace Gelb) or an incomplete or deficient alphabet (as most other writers had said before Daniels). Rather, Daniels put forward, this is a different type of writing system, intermediate between syllabaries and 'full' alphabets.

Aramaic alphabet

960

The Aramaic alphabet is historically significant since virtually all modern Middle Eastern writing systems can be traced back to it. That is primarily due to the widespread usage of the Aramaic language after it was adopted as both a lingua franca and the official language of the Neo-Assyrian and Neo-Babylonian Empires, and their successor, the Achaemenid Empire. Among the descendant scripts in modern use, the Jewish Hebrew alphabet bears the closest relation to the Imperial Aramaic script of the 5th century BC, with an identical letter inventory and, for the most part, nearly identical letter shapes. By contrast the Samaritan Hebrew script is directly descended from Proto-Hebrew/Phoenician script, which was in turn the ancestor of the Aramaic alphabet. The Aramaic alphabet was also an ancestor to the Nabataean alphabet, which in turn had the Arabic alphabet as a descendant.

Aramaic alphabet

960

History

The earliest inscriptions in the Aramaic language use the Phoenician alphabet. Over time, the alphabet developed into the Aramaic alphabet by the 8th century BC. It was used to write the Aramaic languages spoken by ancient Aramean pre-Christian tribes throughout the Fertile Crescent. It was also adopted by other peoples as their own alphabet when empires and their subjects underwent linguistic Aramaization during a language shift for governing purposes—a precursor to Arabization centuries later—including among Assyrians who permanently replaced their Akkadian language and its cuneiform script with Aramaic and its script, and among Jews (but not Samaritans), who adopted the Aramaic language as their vernacular and started using the Aramaic alphabet even for writing Hebrew, displacing the former Paleo-Hebrew alphabet. (The modern Hebrew alphabet derives from the Aramaic alphabet, in contrast to the modern Samaritan alphabet, which derives from Paleo-Hebrew).

Aramaic alphabet

960

History

Around 500 BC, following the Achaemenid conquest of Mesopotamia under Darius I, Old Aramaic was adopted by the Persians as the "vehicle for written communication between the different regions of the vast Persian empire with its different peoples and languages. The use of a single official language, which modern scholarship has dubbed as Official Aramaic, Imperial Aramaic or Achaemenid Aramaic, can be assumed to have greatly contributed to the astonishing success of the Achaemenid Persians in holding their far-flung empire together for as long as they did."

Aramaic alphabet

960

History

Imperial Aramaic was highly standardised; its orthography was based more on historical roots than any spoken dialect and was influenced by Old Persian. The Aramaic glyph forms of the period are often divided into two main styles, the "lapidary" form, usually inscribed on hard surfaces like stone monuments, and a cursive form whose lapidary form tended to be more conservative by remaining more visually similar to Phoenician and early Aramaic. Both were in use through the Achaemenid Persian period, but the cursive form steadily gained ground over the lapidary, which had largely disappeared by the 3rd century BC.

Aramaic alphabet

960

History

For centuries after the fall of the Achaemenid Empire in 331 BC, Imperial Aramaic, or something near enough to it to be recognisable, would remain an influence on the various native Iranian languages. The Aramaic script would survive as the essential characteristics of the Iranian Pahlavi writing system.

Aramaic alphabet

960

History

30 Aramaic documents from Bactria have been recently discovered, an analysis of which was published in November 2006. The texts, which were rendered on leather, reflect the use of Aramaic in the 4th century BC in the Persian Achaemenid administration of Bactria and Sogdiana.

Aramaic alphabet

960

History

The widespread usage of Achaemenid Aramaic in the Middle East led to the gradual adoption of the Aramaic alphabet for writing Hebrew. Formerly, Hebrew had been written using an alphabet closer in form to that of Phoenician, the Paleo-Hebrew alphabet.

Aramaic alphabet

960

History

Since the evolution of the Aramaic alphabet out of the Phoenician one was a gradual process, the division of the world's alphabets into the ones derived from the Phoenician one directly and the ones derived from Phoenician via Aramaic is somewhat artificial. In general, the alphabets of the Mediterranean region (Anatolia, Greece, Italy) are classified as Phoenician-derived, adapted from around the 8th century BC, and those of the East (the Levant, Persia, Central Asia, and India) are considered Aramaic-derived, adapted from around the 6th century BC from the Imperial Aramaic script of the Achaemenid Empire.

Aramaic alphabet

960

History

After the fall of the Achaemenid Empire, the unity of the Imperial Aramaic script was lost, diversifying into a number of descendant cursives.

Aramaic alphabet

960

History

The Hebrew and Nabataean alphabets, as they stood by the Roman era, were little changed in style from the Imperial Aramaic alphabet. Ibn Khaldun (1332–1406) alleges that not only the old Nabataean writing was influenced by the "Syrian script" (i.e. Aramaic), but also the old Chaldean script.

Aramaic alphabet

960

History

A cursive Hebrew variant developed from the early centuries AD, but it remained restricted to the status of a variant used alongside the noncursive. By contrast, the cursive developed out of the Nabataean alphabet in the same period soon became the standard for writing Arabic, evolving into the Arabic alphabet as it stood by the time of the early spread of Islam.

Aramaic alphabet

960

History

The development of cursive versions of Aramaic also led to the creation of the Syriac, Palmyrene and Mandaic alphabets, which formed the basis of the historical scripts of Central Asia, such as the Sogdian and Mongolian alphabets.

Aramaic alphabet

960

History

The Old Turkic script is generally considered to have its ultimate origins in Aramaic, in particular via the Pahlavi or Sogdian alphabets, as suggested by V. Thomsen, or possibly via Kharosthi (cf., Issyk inscription).

Aramaic alphabet

960

History

Brahmi script was also possibly derived or inspired by Aramaic. Brahmic family of scripts includes Devanagari.

Aramaic alphabet

960

Languages using the alphabet

Today, Biblical Aramaic, Jewish Neo-Aramaic dialects and the Aramaic language of the Talmud are written in the modern-Hebrew alphabet (distinguished from the Old Hebrew script). In classical Jewish literature, the name given to the modern-Hebrew script was "Ashurit" (the ancient Assyrian script), a script now known widely as the Aramaic script. It is believed that during the period of Assyrian dominion that Aramaic script and language received official status. Syriac and Christian Neo-Aramaic dialects are today written in the Syriac alphabet, which script has superseded the more ancient Assyrian script and now bears its name. Mandaic is written in the Mandaic alphabet. The near-identical nature of the Aramaic and the classical Hebrew alphabets caused Aramaic text to be typeset mostly in the standard Hebrew script in scholarly literature.

Aramaic alphabet

960

Languages using the alphabet

In Maaloula, one of few surviving communities in which a Western Aramaic dialect is still spoken, an Aramaic Language Institute was established in 2006 by Damascus University that teaches courses to keep the language alive.

Aramaic alphabet

960

Languages using the alphabet

Unlike Classical Syriac, which has a rich literary tradition in Syriac-Aramaic script, Western Neo-Aramaic was solely passed down orally for generations until 2006 and was not utilized in a written form.

Aramaic alphabet

960

Languages using the alphabet

Therefore, the Language Institute's chairman, George Rizkalla (Rezkallah), undertook the writing of a textbook in Western Neo-Aramaic. Being previously unwritten, Rizkalla opted for the Hebrew alphabet. However, in 2010, the institute's activities were halted due to concerns that the square Maalouli-Aramaic alphabet used in the program bore a resemblance to the square script of the Hebrew alphabet. As a result, all signs featuring the square Maalouli script were subsequently removed. The program stated that they would instead use the more distinct Syriac-Aramaic alphabet, although use of the Maalouli alphabet has continued to some degree. Al Jazeera Arabic also broadcast a program about Western Neo-Aramaic and the villages in which it is spoken with the square script still in use.

Aramaic alphabet

960

Letters

In Aramaic writing, waw and yodh serve a double function. Originally, they represented only the consonants w and y, but they were later adopted to indicate the long vowels ū and ī respectively as well (often also ō and ē respectively). In the latter role, they are known as matres lectionis or 'mothers of reading'.

Aramaic alphabet

960

Letters

Ālap, likewise, has some of the characteristics of a mater lectionis because in initial positions, it indicates a glottal stop (followed by a vowel), but otherwise, it often also stands for the long vowels ā or ē. Among Jews, the influence of Hebrew often led to the use of Hē instead, at the end of a word.

Aramaic alphabet

960

Letters

The practice of using certain letters to hold vowel values spread to Aramaic-derived writing systems, such as in Arabic and Hebrew, which still follow the practice.

Aramaic alphabet

960

Unicode

The Imperial Aramaic alphabet was added to the Unicode Standard in October 2009, with the release of version 5.2.

Aramaic alphabet

960

Unicode

The Unicode block for Imperial Aramaic is U+10840–U+1085F:

Aramaic alphabet

960

Unicode

The Syriac Aramaic alphabet was added to the Unicode Standard in September 1999, with the release of version 3.0.

Aramaic alphabet

960

Unicode

The Syriac Abbreviation (a type of overline) can be represented with a special control character called the Syriac Abbreviation Mark (U+070F). The Unicode block for Syriac Aramaic is U+0700–U+074F:

American shot

966

"American shot" or "cowboy shot" is a translation of a phrase from French film criticism, plan américain, and refers to a medium-long ("knee") film shot of a group of characters, who are arranged so that all are visible to the camera. The usual arrangement is for the actors to stand in an irregular line from one side of the screen to the other, with the actors at the end coming forward a little and standing more in profile than the others. The purpose of the composition is to allow complex dialogue scenes to be played out without changes in camera position. In some literature, this is simply referred to as a 3/4 shot.

American shot

966

One of the other main reasons why French critics called it "American shot" was its frequent use in the western genre. This was because a shot that started at knee level would reveal the weapon of a cowboy, usually holstered at their waist. It is actually the closest the camera can get to an actor while keeping both their face and their holstered gun in frame.

American shot

966

The French critics thought it was characteristic of American films of the 1930s or 1940s; however, it was mostly characteristic of cheaper American movies, such as Charlie Chan mysteries where people collected in front of a fireplace or at the foot of the stairs in order to explain what happened a few minutes ago.

American shot

966

Howard Hawks legitimized this style in his films, allowing characters to act, even when not talking, when most of the audience would not be paying attention. It became his trademark style.

American shot

966

References

Acute disseminated encephalomyelitis

967

Acute disseminated encephalomyelitis (ADEM), or acute demyelinating encephalomyelitis, is a rare autoimmune disease marked by a sudden, widespread attack of inflammation in the brain and spinal cord. As well as causing the brain and spinal cord to become inflamed, ADEM also attacks the nerves of the central nervous system and damages their myelin insulation, which, as a result, destroys the white matter. The cause is often a trigger such as from viral infection or vaccinations.