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Axon

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.

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.

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 usually make contact with other neurons at junctions called synapses but can also make contact with muscle or gland cells. 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 boutons ("in passing boutons") and can be in the hundreds or even the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches.

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.

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.

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.

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.

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.

Axonal region
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 hillock
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.

Axonal initial segment
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.

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.

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.

Axonal transport
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.

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.

Myelination
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.

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.

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.

Nodes of Ranvier
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 terminals
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.

Axonal varicosities
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.

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.

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.

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.

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.

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.

Development
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.

Extracellular signaling
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.

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.

Intracellular signaling
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.

Cytoskeletal dynamics
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.

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.

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.

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.

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.

Length regulation
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.

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.

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.

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.

Motor
Lower motor neurons have two kind of fibers:

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.

Autonomic
The autonomic nervous system has two kinds of peripheral fibers:

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.

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.

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

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

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.

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.

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.

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.

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.)

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.

title: Axon


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Aramaic alphabet

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, which they call "Square Script", 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.

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. Daniels put forward, this is a different type of writing system, intermediate between syllabaries and 'full' alphabets.

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 the ancestor of the Aramaic alphabet. The Aramaic alphabet was also an ancestor to the Nabataean alphabet, which had the Arabic alphabet as a descendant.

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.

These include 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.

Achaemenid Empire (The First Persian Empire)
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."

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.

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

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.

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-derived scripts
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. 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.

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

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.

A cursive Hebrew variant developed from the early centuries AD. 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.

The development of cursive versions of Aramaic 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.

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).

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

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.

Maaloula
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.

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. 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. 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.

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

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

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

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:

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Aramaic alphabet


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Talk:Agma

The format of the agma page makes it appear that the entire text of the entry was lifted from ANCIENT SCRIPTS AND PHONOLOGICAL KNOWLEDGE. If so, I would have questions about whether this is violating any intellectual property rights. I mean, sure, you can take brief quotations from works without permission--but this seems to be a quotation of an entire entry from a reference work. Even if that is legal, it doesn't seem ethical. I'm sure no one was intending to do anything unethical, but perhaps we should think harder about these things. Or maybe I'm just confused and you need to unconfuse me. --Larry Sanger

I don't have that book, and I can't find the quoted text on the net, so I can't verify whether or not this text was copied from that book. I can verify that if it was, it is clearly a copyright violation not covered by fair use. Small excerpts can be used in a larger work for commentary, criticism, parody, and the like; copies may also be made for classroom use. But this doesn't qualify as any of those: the paragraph isn't being taken so that we can comment on itin a larger work--it's being taken merely to reproduce it as is, for exactly the same use as the original copyrighted work, which is clearly infringement. --LDC

With apologies to the author if, e.g., the author was simply citing ANCIENT SCRIPTS as a bibliographic source, but without further information from the person who put this information up, it just looks to me like it was swiped from the book. So I'm taking it down--though I will be very happy to put it back immediately if the author can assure me it wasn't copied out of a book. --LMS

naw, that was only quoting, well, paraphrasing actually. certainly not a violation of intellectual property. Well, I rre-introduced some of the old info; i hope, it won't disappear again... anyway, no quotes in it, so everybody should be happy, and no info that needs a bibliography. Wathiik

well, i think now it's obvious that it isn't plagiarism (reminds me of my first year of studying english when everybody warned about plagiarism... Wathiik

Please don't confuse copyright violation with plagiarism--they are different things entirely. Plagiarism is misrepresenting someone else's work as your own; if one simply copies a paragraph from a book making no mention of the source and present it as if you had written it yourself, that's plagiarism. If you copy a paragraph from a book, giving full credit to the correct author, that's not plagiarism, but it is probably still a copyright violation. If you rewrite the information from a text in your own words, that's called research :-) --LDC - I'm assuming the content of agma is no longer a copyright violation. Another happy ending!  --LMS

Well, it never WAS a copyright violation, even though it probably did LOOK like a violation. Anway, it's true, a happy ending... anyway, plagiarism is probably a form of copyright violation in its own right. You can sue someone for plagiarism, and you can sue someone for other kinds of copyright violations; although no one would sue a student, I guess... as long as s/he doesn't publish it on the internet ... Wathiik

Shouldn't this page be merged with velar nasal? Eng, afterall, redirects there, not here. Should we also have a page for eng (letter) (Ŋŋ)? --Vlad

Link suggestions
An automated Wikipedia link suggester has some possible wiki link suggestions for the Agma article, and they have been placed on this page for your convenience. Tip: Some people find it helpful if these suggestions are shown on this talk page, rather than on another page. To do this, just add to this page. — LinkBot 10:40, 17 Dec 2004 (UTC)

title: Talk:Agma


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American shot

"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.

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.

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.

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.

title: American shot


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Acute disseminated encephalomyelitis

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.

ADEM's symptoms resemble the symptoms of multiple sclerosis (MS), so the disease itself is sorted into the classification of the multiple sclerosis borderline diseases. However, ADEM has several features that distinguish it from MS. Unlike MS, ADEM occurs usually in children and is marked with rapid fever, although adolescents and adults can get the disease too. ADEM consists of a single flare-up whereas MS is marked with several flare-ups (or relapses), over a long period of time. Relapses following ADEM are reported in up to a quarter of patients, but the majority of these 'multiphasic' presentations following ADEM likely represent MS. ADEM is also distinguished by a loss of consciousness, coma and death, which is very rare in MS, except in severe cases.

It affects about 8 per 1,000,000 people per year. Although it occurs in all ages, most reported cases are in children and adolescents, with the average age around 5 to 8 years old. The disease affects males and females almost equally. ADEM shows seasonal variation with higher incidence in winter and spring months which may coincide with higher viral infections during these months. The mortality rate may be as high as 5%; however, full recovery is seen in 50 to 75% of cases with increase in survival rates up to 70 to 90% with figures including minor residual disability as well. The average time to recover from ADEM flare-ups is one to six months.

ADEM produces multiple inflammatory lesions in the brain and spinal cord, particularly in the white matter. Usually these are found in the subcortical and central white matter and cortical gray-white junction of both cerebral hemispheres, cerebellum, brainstem, and spinal cord, but periventricular white matter and gray matter of the cortex, thalami and basal ganglia may also be involved.

When a person has more than one demyelinating episode of ADEM, the disease is then called recurrent disseminated encephalomyelitis or multiphasic disseminated encephalomyelitis (MDEM). Also, a fulminant course in adults has been described.

Signs and symptoms
ADEM has an abrupt onset and a monophasic course. Symptoms usually begin 1–3 weeks after infection. Major symptoms include fever, headache, nausea and vomiting, confusion, vision impairment, drowsiness, seizures and coma. Although initially the symptoms are usually mild, they worsen rapidly over the course of hours to days, with the average time to maximum severity being about four and a half days. Additional symptoms include hemiparesis, paraparesis, and cranial nerve palsies.

ADEM in COVID-19
Neurological symptoms were the main presentation of COVID-19, which did not correlate with the severity of respiratory symptoms. The high incidence of ADEM with hemorrhage is striking. Brain inflammation is likely caused by an immune response to the disease rather than neurotropism. CSF analysis was not indicative of an infectious process, neurological impairment was not present in the acute phase of the infection, and neuroimaging findings were not typical of classical toxic and metabolic disorders. The finding of bilateral periventricular relatively asymmetrical lesions allied with deep white matter involvement, that may also be present in cortical gray-white matter junction, thalami, basal ganglia, cerebellum, and brainstem suggests an acute demyelination process. Additionally, hemorrhagic white matter lesions, clusters of macrophages related to axonal injury and ADEM-like appearance were also found in subcortical white matter.

Causes
Since the discovery of the anti-MOG specificity against multiple sclerosis diagnosis it is considered that ADEM is one of the possible clinical causes of anti-MOG associated encephalomyelitis.

About how the anti-MOG antibodies appear in the patients serum there are several theories:
 * A preceding antigenic challenge can be identified in approximately two-thirds of people. Some viral infections thought to induce ADEM include influenza virus, dengue, enterovirus, measles, mumps, rubella, varicella zoster, Epstein–Barr virus, cytomegalovirus, herpes simplex virus, hepatitis A, coxsackievirus and COVID-19. Bacterial infections include Mycoplasma pneumoniae, Borrelia burgdorferi, Leptospira, and beta-hemolytic Streptococci.
 * Exposure to vaccines: The only vaccine proven related to ADEM is the Semple form of the rabies vaccine, but hepatitis B, pertussis, diphtheria, measles, mumps, rubella, pneumococcus, varicella, influenza, Japanese encephalitis, and polio vaccines have all been implicated. The majority of the studies that correlate vaccination with ADEM onset use only small samples or are case studies. Large-scale epidemiological studies (e.g., of MMR vaccine or smallpox vaccine) do not show increased risk of ADEM following vaccination.       An upper bound for the risk of ADEM from measles vaccination, if it exists, can be estimated to be 10 per million, which is far lower than the risk of developing ADEM from an actual measles infection, which is about 1 per 1,000 cases. For a rubella infection, the risk is 1 per 5,000 cases. Some early vaccines, later shown to have been contaminated with host animal CNS tissue, had ADEM incidence rates as high as 1 in 600.
 * In rare cases, ADEM seems to follow from organ transplantation.

Diagnosis
The term ADEM has been inconsistently used at different times. Currently, the commonly accepted international standard for the clinical case definition is the one published by the International Pediatric MS Study Group, revision 2007.

Given that the definition is clinical, it is currently unknown if all the cases of ADEM are positive for anti-MOG autoantibody; in any case, it appears to be strongly related to ADEM diagnosis.

Multiple sclerosis
While ADEM and MS both involve autoimmune demyelination, they differ in many clinical, genetic, imaging, and histopathological aspects. Some authors consider MS and its borderline forms to constitute a spectrum, differing only in chronicity, severity, and clinical course, while others consider them discretely different diseases.

Typically, ADEM appears in children following an antigenic challenge and remains monophasic. Nevertheless, ADEM does occur in adults, and can also be clinically multiphasic.

Problems for differential diagnosis increase due to the lack of agreement for a definition of multiple sclerosis. If MS were defined only by the separation in time and space of the demyelinating lesions as McDonald did, it would not be enough to make a difference, as some cases of ADEM satisfy these conditions. Therefore, some authors propose to establish the dividing line as the shape of the lesions around the veins, being therefore "perivenous vs. confluent demyelination". The pathology of ADEM is very similar to that of MS with some differences. The pathological hallmark of ADEM is perivenous inflammation with limited "sleeves of demyelination". Nevertheless, MS-like plaques (confluent demyelination) can appear

Plaques in the white matter in MS are sharply delineated, while the glial scar in ADEM is smooth. Axons are better preserved in ADEM lesions. Inflammation in ADEM is widely disseminated and ill-defined, and finally, lesions are strictly perivenous, while in MS they are disposed around veins, but not so sharply.

Nevertheless, the co-occurrence of perivenous and confluent demyelination in some individuals suggests pathogenic overlap between acute disseminated encephalomyelitis and multiple sclerosis and misclassification even with biopsy or even postmortem ADEM in adults can progress to MS

Multiphasic disseminated encephalomyelitis
When the person has more than one demyelinating episode of ADEM, the disease is then called recurrent disseminated encephalomyelitis or multiphasic disseminated encephalomyelitis (MDEM).

It has been found that anti-MOG auto-antibodies are related to this kind of ADEM

Another variant of ADEM in adults has been described, also related to anti-MOG auto-antibodies, has been named fulminant disseminated encephalomyelitis, and it has been reported to be clinically ADEM, but showing MS-like lesions on autopsy. It has been classified inside the anti-MOG associated inflammatory demyelinating diseases.

Acute hemorrhagic leukoencephalitis
Acute hemorrhagic leukoencephalitis (AHL, or AHLE), acute hemorrhagic encephalomyelitis (AHEM), acute necrotizing hemorrhagic leukoencephalitis (ANHLE), Weston-Hurst syndrome, or Hurst's disease, is a hyperacute and frequently fatal form of ADEM. AHL is relatively rare (less than 100 cases have been reported in the medical literature ), it is seen in about 2% of ADEM cases, and is characterized by necrotizing vasculitis of venules and hemorrhage, and edema. Death is common in the first week and overall mortality is about 70%, but increasing evidence points to favorable outcomes after aggressive treatment with corticosteroids, immunoglobulins, cyclophosphamide, and plasma exchange. About 70% of survivors show residual neurological deficits, but some survivors have shown surprisingly little deficit considering the extent of the white matter affected.

This disease has been occasionally associated with ulcerative colitis and Crohn's disease, malaria, sepsis associated with immune complex deposition, methanol poisoning, and other underlying conditions. Also anecdotal association with MS has been reported

Laboratory studies that support diagnosis of AHL are: peripheral leukocytosis, cerebrospinal fluid (CSF) pleocytosis associated with normal glucose and increased protein. On magnetic resonance imaging (MRI), lesions of AHL typically show extensive T2-weighted and fluid-attenuated inversion recovery (FLAIR) white matter hyperintensities with areas of hemorrhages, significant edema, and mass effect.

Treatment
No controlled clinical trials have been conducted on ADEM treatment, but aggressive treatment aimed at rapidly reducing inflammation of the CNS is standard. The widely accepted first-line treatment is high doses of intravenous corticosteroids, such as methylprednisolone or dexamethasone, followed by 3–6 weeks of gradually lower oral doses of prednisolone. Patients treated with methylprednisolone have shown better outcomes than those treated with dexamethasone. Oral tapers of less than three weeks duration show a higher chance of relapsing, and tend to show poorer outcomes. Other anti-inflammatory and immunosuppressive therapies have been reported to show beneficial effect, such as plasmapheresis, high doses of intravenous immunoglobulin (IVIg), mitoxantrone and cyclophosphamide. These are considered alternative therapies, used when corticosteroids cannot be used or fail to show an effect.

There is some evidence to suggest that patients may respond to a combination of methylprednisolone and immunoglobulins if they fail to respond to either separately In a study of 16 children with ADEM, 10 recovered completely after high-dose methylprednisolone, one severe case that failed to respond to steroids recovered completely after IV Ig; the five most severe cases – with ADAM and severe peripheral neuropathy – were treated with combined high-dose methylprednisolone and immunoglobulin, two remained paraplegic, one had motor and cognitive handicaps, and two recovered. A recent review of IVIg treatment of ADEM (of which the previous study formed the bulk of the cases) found that 70% of children showed complete recovery after treatment with IVIg, or IVIg plus corticosteroids. A study of IVIg treatment in adults with ADEM showed that IVIg seems more effective in treating sensory and motor disturbances, while steroids seem more effective in treating impairments of cognition, consciousness and rigor. This same study found one subject, a 71-year-old man who had not responded to steroids, that responded to an IVIg treatment 58 days after disease onset.

Prognosis
Full recovery is seen in 50 to 70% of cases, ranging to 70 to 90% recovery with some minor residual disability (typically assessed using measures such as mRS or EDSS), average time to recover is one to six months. The mortality rate may be as high as 5–10%. Poorer outcomes are associated with unresponsiveness to steroid therapy, unusually severe neurological symptoms, or sudden onset. Children tend to have more favorable outcomes than adults, and cases presenting without fevers tend to have poorer outcomes. The latter effect may be due to either protective effects of fever, or that diagnosis and treatment is sought more rapidly when fever is present.

ADEM can progress to MS. It will be considered MS if some lesions appear in different times and brain areas

Motor deficits
Residual motor deficits are estimated to remain in about 8 to 30% of cases, the range in severity from mild clumsiness to ataxia and hemiparesis.

Neurocognitive
Patients with demyelinating illnesses, such as MS, have shown cognitive deficits even when there is minimal physical disability. Research suggests that similar effects are seen after ADEM, but that the deficits are less severe than those seen in MS. A study of six children with ADEM (mean age at presentation 7.7 years) were tested for a range of neurocognitive tests after an average of 3.5 years of recovery. All six children performed in the normal range on most tests, including verbal IQ and performance IQ, but performed at least one standard deviation below age norms in at least one cognitive domain, such as complex attention (one child), short-term memory (one child) and internalizing behaviour/affect (two children). Group means for each cognitive domain were all within one standard deviation of age norms, demonstrating that, as a group, they were normal. These deficits were less severe than those seen in similar aged children with a diagnosis of MS.

Another study compared nineteen children with a history of ADEM, of which 10 were five years of age or younger at the time (average age 3.8 years old, tested an average of 3.9 years later) and nine were older (mean age 7.7y at time of ADEM, tested an average of 2.2 years later) to nineteen matched controls. Scores on IQ tests and educational achievement were lower for the young onset ADEM group (average IQ 90) compared to the late onset (average IQ 100) and control groups (average IQ 106), while the late onset ADEM children scored lower on verbal processing speed. Again, all groups means were within one standard deviation of the controls, meaning that while effects were statistically reliable, the children were as a whole, still within the normal range. There were also more behavioural problems in the early onset group, although there is some suggestion that this may be due, at least in part, to the stress of hospitalization at a young age.

Research
The relationship between ADEM and anti-MOG associated encephalomyelitis is currently under research. A new entity called MOGDEM has been proposed.

About animal models, the main animal model for MS, experimental autoimmune encephalomyelitis (EAE) is also an animal model for ADEM. Being an acute monophasic illness, EAE is far more similar to ADEM than MS.

title: Acute disseminated encephalomyelitis


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Ataxia

Ataxia (from Greek α- [a negative prefix] + -τάξις [order] = "lack of order") is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements, that indicates dysfunction of parts of the nervous system that coordinate movement, such as the cerebellum.

These nervous system dysfunctions occur in several different patterns, with different results and different possible causes. Ataxia can be limited to one side of the body, which is referred to as hemiataxia. Friedreich's ataxia has gait abnormality as the most commonly presented symptom. Dystaxia is a mild degree of ataxia.

Cerebellar
The term cerebellar ataxia is used to indicate ataxia due to dysfunction of the cerebellum. The cerebellum is responsible for integrating a significant amount of neural information that is used to coordinate smoothly ongoing movements and to participate in motor planning. Although ataxia is not present with all cerebellar lesions, many conditions affecting the cerebellum do produce ataxia. People with cerebellar ataxia may have trouble regulating the force, range, direction, velocity, and rhythm of muscle contractions. This results in a characteristic type of irregular, uncoordinated movement that can manifest itself in many possible ways, such as asthenia, asynergy, delayed reaction time, and dyschronometria. Individuals with cerebellar ataxia could also display instability of gait, difficulty with eye movements, dysarthria, dysphagia, hypotonia, dysmetria, and dysdiadochokinesia. These deficits can vary depending on which cerebellar structures have been damaged, and whether the lesion is bi- or unilateral.

People with cerebellar ataxia may initially present with poor balance, which could be demonstrated as an inability to stand on one leg or perform tandem gait. As the condition progresses, walking is characterized by a widened base and high stepping, as well as staggering and lurching from side to side. Turning is also problematic and could result in falls. As cerebellar ataxia becomes severe, great assistance and effort are needed to stand and walk. Dysarthria, an impairment with articulation, may also be present and is characterized by "scanning" speech that consists of slower rate, irregular rhythm, and variable volume. Also, slurring of speech, tremor of the voice, and ataxic respiration may occur. Cerebellar ataxia could result with incoordination of movement, particularly in the extremities. Overshooting (or hypermetria) occurs with finger-to-nose testing and heel to shin testing; thus, dysmetria is evident. Impairments with alternating movements (dysdiadochokinesia), as well as dysrhythmia, may also be displayed. Tremor of the head and trunk (titubation) may be seen in individuals with cerebellar ataxia.

Dysmetria is thought to be caused by a deficit in the control of interaction torques in multijoint motion. Interaction torques are created at an associated joint when the primary joint is moved. For example, if a movement required reaching to touch a target in front of the body, flexion at the shoulder would create a torque at the elbow, while extension of the elbow would create a torque at the wrist. These torques increase as the speed of movement increases and must be compensated and adjusted for to create coordinated movement. This may, therefore, explain decreased coordination at higher movement velocities and accelerations.
 * Dysfunction of the vestibulocerebellum (flocculonodular lobe) impairs balance and the control of eye movements. This presents itself with postural instability, in which the person tends to separate his/her feet upon standing, to gain a wider base and to avoid titubation (bodily oscillations tending to be forward-backward ones). The instability is, therefore, worsened when standing with the feet together, regardless of whether the eyes are open or closed. This is a negative Romberg's test, or more accurately, it denotes the individual's inability to carry out the test, because the individual feels unstable even with open eyes.
 * Dysfunction of the spinocerebellum (vermis and associated areas near the midline) presents itself with a wide-based "drunken sailor" gait (called truncal ataxia), characterised by uncertain starts and stops, lateral deviations, and unequal steps. As a result of this gait impairment, falling is a concern in patients with ataxia. Studies examining falls in this population show that 74–93% of patients have fallen at least once in the past year and up to 60% admit to fear of falling.
 * 'Dysfunction of the cerebrocerebellum' (lateral hemispheres) presents as disturbances in carrying out voluntary, planned movements by the extremities (called appendicular ataxia). These include:
 * Intention tremor (coarse trembling, accentuated over the execution of voluntary movements, possibly involving the head and eyes, as well as the limbs and torso)
 * Peculiar writing abnormalities (large, unequal letters, irregular underlining)
 * A peculiar pattern of dysarthria (slurred speech, sometimes characterised by explosive variations in voice intensity despite a regular rhythm)
 * Inability to perform rapidly alternating movements, known as dysdiadochokinesia, occurs, and could involve rapidly switching from pronation to supination of the forearm. Movements become more irregular with increases of speed.
 * Inability to judge distances or ranges of movement happens. This dysmetria is often seen as undershooting, hypometria, or overshooting, hypermetria, the required distance or range to reach a target. This is sometimes seen when a patient is asked to reach out and touch someone's finger or touch his or her own nose.
 * The rebound phenomenon, also known as the loss of the check reflex, is also sometimes seen in patients with cerebellar ataxia, for example, when patients are flexing their elbows isometrically against a resistance. When the resistance is suddenly removed without warning, the patients' arms may swing up and even strike themselves. With an intact check reflex, the patients check and activate the opposing triceps to slow and stop the movement.
 * Patients may exhibit a constellation of subtle to overt cognitive symptoms, which are gathered under the terminology of Schmahmann's syndrome.

Sensory
The term sensory ataxia is used to indicate ataxia due to loss of proprioception, the loss of sensitivity to the positions of joint and body parts. This is generally caused by dysfunction of the dorsal columns of the spinal cord, because they carry proprioceptive information up to the brain. In some cases, the cause of sensory ataxia may instead be dysfunction of the various parts of the brain that receive positional information, including the cerebellum, thalamus, and parietal lobes.

Sensory ataxia presents itself with an unsteady "stomping" gait with heavy heel strikes, as well as a postural instability that is usually worsened when the lack of proprioceptive input cannot be compensated for by visual input, such as in poorly lit environments.

Physicians can find evidence of sensory ataxia during physical examination by having patients stand with their feet together and eyes shut. In affected patients, this will cause the instability to worsen markedly, producing wide oscillations and possibly a fall; this is called a positive Romberg's test. Worsening of the finger-pointing test with the eyes closed is another feature of sensory ataxia. Also, when patients are standing with arms and hands extended toward the physician, if the eyes are closed, the patients' fingers tend to "fall down" and then be restored to the horizontal extended position by sudden muscular contractions (the "ataxic hand").

Vestibular
The term vestibular ataxia is used to indicate ataxia due to dysfunction of the vestibular system, which in acute and unilateral cases is associated with prominent vertigo, nausea, and vomiting. In slow-onset, chronic bilateral cases of vestibular dysfunction, these characteristic manifestations may be absent, and dysequilibrium may be the sole presentation.

Causes
The three types of ataxia have overlapping causes, so can either coexist or occur in isolation. Cerebellar ataxia can have many causes despite normal neuroimaging.

Focal lesions
Any type of focal lesion of the central nervous system (such as stroke, brain tumor, multiple sclerosis, inflammatory [such as sarcoidosis], and "chronic lymphocytyc inflammation with pontine perivascular enhancement responsive to steroids syndrome" [CLIPPERS ]) will cause the type of ataxia corresponding to the site of the lesion: cerebellar if in the cerebellum; sensory if in the dorsal spinal cord...to include cord compression by thickened ligamentum flavum or stenosis of the boney spinal canal...(and rarely in the thalamus or parietal lobe); or vestibular if in the vestibular system (including the vestibular areas of the cerebral cortex).

Exogenous substances (metabolic ataxia)
Exogenous substances that cause ataxia mainly do so because they have a depressant effect on central nervous system function. The most common example is ethanol (alcohol), which is capable of causing reversible cerebellar and vestibular ataxia. Chronic intake of ethanol causes atrophy of the cerebellum by oxidative and endoplasmic reticulum stresses induced by thiamine deficiency.

Other examples include various prescription drugs (e.g. most antiepileptic drugs have cerebellar ataxia as a possible adverse effect), Lithium level over 1.5mEq/L, synthetic cannabinoid HU-211 ingestion and various other medical and recreational drugs (e.g. ketamine, PCP or dextromethorphan, all of which are NMDA receptor antagonists that produce a dissociative state at high doses). A further class of pharmaceuticals which can cause short term ataxia, especially in high doses, are benzodiazepines. Exposure to high levels of methylmercury, through consumption of fish with high mercury concentrations, is also a known cause of ataxia and other neurological disorders.

Radiation poisoning
Ataxia can be induced as a result of severe acute radiation poisoning with an absorbed dose of more than 30 grays. Furthermore, those with ataxia telangiectasia may have a high sensitivity towards gamma rays and x-rays.

Vitamin B12 deficiency
Vitamin B12 deficiency may cause, among several neurological abnormalities, overlapping cerebellar and sensory ataxia. Neuropsychological symptoms may include sense loss, difficulty in proprioception, poor balance, loss of sensation in the feet, changes in reflexes, dementia, and psychosis, can be reversible with treatment. Complications may include a neurological complex known as subacute combined degeneration of spinal cord, and other neurological disorders.

Hypothyroidism
Symptoms of neurological dysfunction may be the presenting feature in some patients with hypothyroidism. These include reversible cerebellar ataxia, dementia, peripheral neuropathy, psychosis and coma. Most of the neurological complications improve completely after thyroid hormone replacement therapy.

Causes of isolated sensory ataxia
Peripheral neuropathies may cause generalised or localised sensory ataxia (e.g. a limb only) depending on the extent of the neuropathic involvement. Spinal disorders of various types may cause sensory ataxia from the lesioned level below, when they involve the dorsal columns.

Non-hereditary cerebellar degeneration
Non-hereditary causes of cerebellar degeneration include chronic alcohol use disorder, head injury, paraneoplastic and non-paraneoplastic autoimmune ataxia,  high-altitude cerebral edema, celiac disease, normal-pressure hydrocephalus, and infectious or post-infectious cerebellitis.

Hereditary ataxias
Ataxia may depend on hereditary disorders consisting of degeneration of the cerebellum or of the spine; most cases feature both to some extent, and therefore present with overlapping cerebellar and sensory ataxia, even though one is often more evident than the other. Hereditary disorders causing ataxia include autosomal dominant ones such as spinocerebellar ataxia, episodic ataxia, and dentatorubropallidoluysian atrophy, as well as autosomal recessive disorders such as Friedreich's ataxia (sensory and cerebellar, with the former predominating) and Niemann Pick disease, ataxia-telangiectasia (sensory and cerebellar, with the latter predominating),autosomal recessive spinocerebellar ataxia-14 and abetalipoproteinaemia. An example of X-linked ataxic condition is the rare fragile X-associated tremor/ataxia syndrome or FXTAS.

Arnold–Chiari malformation (congenital ataxia)
Arnold–Chiari malformation is a malformation of the brain. It consists of a downward displacement of the cerebellar tonsils and the medulla through the foramen magnum, sometimes causing hydrocephalus as a result of obstruction of cerebrospinal fluid outflow.

Succinic semialdehyde dehydrogenase deficiency
Succinic semialdehyde dehydrogenase deficiency is an autosomal-recessive gene disorder where mutations in the ALDH5A1 gene results in the accumulation of gamma-Hydroxybutyric acid (GHB) in the body. GHB accumulates in the nervous system and can cause ataxia as well as other neurological dysfunction.

Wilson's disease
Wilson's disease is an autosomal-recessive gene disorder whereby an alteration of the ATP7B gene results in an inability to properly excrete copper from the body. Copper accumulates in the liver and raises the toxicity levels in the nervous system causing demyelination of the nerves. This can cause ataxia as well as other neurological and organ impairments.

Gluten ataxia
Gluten ataxia is an autoimmune disease derived from celiac disease, which is triggered by the ingestion of gluten. Early diagnosis and treatment with a gluten-free diet can improve ataxia and prevent its progression. The effectiveness of the treatment depends on the elapsed time from the onset of the ataxia until diagnosis, because the death of neurons in the cerebellum as a result of gluten exposure is irreversible. It accounts for 40% of ataxias of unknown origin and 15% of all ataxias. Less than 10% of people with gluten ataxia present any gastrointestinal symptom and only about 40% have intestinal damage. This entity is classified into primary auto-immune cerebellar ataxias (PACA). There is a continuum between presymptomatic ataxia and immune ataxias with clinical deficits.

Potassium pump
Malfunction of the sodium-potassium pump may be a factor in some ataxias. The - pump has been shown to control and set the intrinsic activity mode of cerebellar Purkinje neurons. This suggests that the pump might not simply be a homeostatic, "housekeeping" molecule for ionic gradients; but could be a computational element in the cerebellum and the brain. Indeed, an ouabain block of - pumps in the cerebellum of a live mouse results in it displaying ataxia and dystonia. Ataxia is observed for lower ouabain concentrations, dystonia is observed at higher ouabain concentrations.

Cerebellar ataxia associated with anti-GAD antibodies
Antibodies against the enzyme glutamic acid decarboxylase (GAD: enzyme changing glutamate into GABA) cause cerebellar deficits. The antibodies impair motor learning and cause behavioral deficits. GAD antibodies related ataxia is part of the group called immune-mediated cerebellar ataxias. The antibodies induce a synaptopathy. The cerebellum is particularly vulnerable to autoimmune disorders. Cerebellar circuitry has capacities to compensate and restore function thanks to cerebellar reserve, gathering multiple forms of plasticity. LTDpathies gather immune disorders targeting long-term depression (LTD), a form of plasticity.

Diagnosis

 * Imaging studies - A CT scan or MRI of the brain might help determine potential causes. An MRI can sometimes show shrinkage of the cerebellum and other brain structures in people with ataxia. It may also show other treatable findings, such as a blood clot or benign tumour, that could be pressing on the cerebellum.
 * Lumbar puncture (spinal tap) - A needle is inserted into the lower back (lumbar region) between two lumbar vertebrae to obtain a sample of cerebrospinal fluid for testing.
 * Genetic testing - Determines whether the mutation that causes one of the hereditary ataxic conditions is present. Tests are available for many but not all of the hereditary ataxias.

Treatment
The treatment of ataxia and its effectiveness depend on the underlying cause. Treatment may limit or reduce the effects of ataxia, but it is unlikely to eliminate them entirely. Recovery tends to be better in individuals with a single focal injury (such as stroke or a benign tumour), compared to those who have a neurological degenerative condition. A review of the management of degenerative ataxia was published in 2009. A small number of rare conditions presenting with prominent cerebellar ataxia are amenable to specific treatment and recognition of these disorders is critical. Diseases include vitamin E deficiency, abetalipoproteinemia, cerebrotendinous xanthomatosis, Niemann–Pick type C disease, Refsum's disease, glucose transporter type 1 deficiency, episodic ataxia type 2, gluten ataxia, glutamic acid decarboxylase ataxia. Novel therapies target the RNA defects associated with cerebellar disorders, using in particular anti-sense oligonucleotides.

The movement disorders associated with ataxia can be managed by pharmacological treatments and through physical therapy and occupational therapy to reduce disability. Some drug treatments that have been used to control ataxia include: 5-hydroxytryptophan (5-HTP), idebenone, amantadine, physostigmine, L-carnitine or derivatives, trimethoprim/sulfamethoxazole, vigabatrin, phosphatidylcholine, acetazolamide, 4-aminopyridine, buspirone, and a combination of coenzyme Q10 and vitamin E.

Physical therapy requires a focus on adapting activity and facilitating motor learning for retraining specific functional motor patterns. A recent systematic review suggested that physical therapy is effective, but there is only moderate evidence to support this conclusion. The most commonly used physical therapy interventions for cerebellar ataxia are vestibular habituation, Frenkel exercises, proprioceptive neuromuscular facilitation (PNF), and balance training; however, therapy is often highly individualized and gait and coordination training are large components of therapy.

Current research suggests that, if a person is able to walk with or without a mobility aid, physical therapy should include an exercise program addressing five components: static balance, dynamic balance, trunk-limb coordination, stairs, and contracture prevention. Once the physical therapist determines that the individual is able to safely perform parts of the program independently, it is important that the individual be prescribed and regularly engage in a supplementary home exercise program that incorporates these components to further improve long term outcomes. These outcomes include balance tasks, gait, and individual activities of daily living. While the improvements are attributed primarily to changes in the brain and not just the hip or ankle joints, it is still unknown whether the improvements are due to adaptations in the cerebellum or compensation by other areas of the brain.

Decomposition, simplification, or slowing of multijoint movement may also be an effective strategy that therapists may use to improve function in patients with ataxia. Training likely needs to be intense and focused—as indicated by one study performed with stroke patients experiencing limb ataxia who underwent intensive upper limb retraining. Their therapy consisted of constraint-induced movement therapy which resulted in improvements of their arm function. Treatment should likely include strategies to manage difficulties with everyday activities such as walking. Gait aids (such as a cane or walker) can be provided to decrease the risk of falls associated with impairment of balance or poor coordination. Severe ataxia may eventually lead to the need for a wheelchair. To obtain better results, possible coexisting motor deficits need to be addressed in addition to those induced by ataxia. For example, muscle weakness and decreased endurance could lead to increasing fatigue and poorer movement patterns.

There are several assessment tools available to therapists and health care professionals working with patients with ataxia. The International Cooperative Ataxia Rating Scale (ICARS) is one of the most widely used and has been proven to have very high reliability and validity. Other tools that assess motor function, balance and coordination are also highly valuable to help the therapist track the progress of their patient, as well as to quantify the patient's functionality. These tests include, but are not limited to:
 * The Berg Balance Scale
 * Tandem Walking (to test for Tandem gaitability)
 * Scale for the Assessment and Rating of Ataxia (SARA)
 * tapping tests – The person must quickly and repeatedly tap their arm or leg while the therapist monitors the amount of dysdiadochokinesia.
 * finger-nose testing – This test has several variations including finger-to-therapist's finger, finger-to-finger, and alternate nose-to-finger.

Other uses
The term "ataxia" is sometimes used in a broader sense to indicate lack of coordination in some physiological process. Examples include optic ataxia (lack of coordination between visual inputs and hand movements, resulting in inability to reach and grab objects) and ataxic respiration (lack of coordination in respiratory movements, usually due to dysfunction of the respiratory centres in the medulla oblongata).

Optic ataxia may be caused by lesions to the posterior parietal cortex, which is responsible for combining and expressing positional information and relating it to movement. Outputs of the posterior parietal cortex include the spinal cord, brain stem motor pathways, pre-motor and pre-frontal cortex, basal ganglia and the cerebellum. Some neurons in the posterior parietal cortex are modulated by intention. Optic ataxia is usually part of Balint's syndrome, but can be seen in isolation with injuries to the superior parietal lobule, as it represents a disconnection between visual-association cortex and the frontal premotor and motor cortex.

title: Ataxia


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Talk:Aal

I did not write this entry; it came from an old public domain source. Feel free to update or expand it. --KQ

A minimal bit of editing, Wikification, and the addition of an easily found fact or two from the web increases the value of these articles tremendously. EB11 is a great source, but it's just that--one source. Always add some value to a copied article to make Wikipedia something worth searching. --LDC

It was my intention to, but it's easier for me to add the text now and update it later. If you'd prefer me not to, I won't. --KQ

Oh please do add more. Your adding it gives me a great incentive to modify it. I hate copying, but I love editing; you and I should make a good team. I wouldn't automatically create the "Talk" pages though just to mention the source. It doesn't matter where it came from, once you've entered it here it's ours. --LDC

I put the "talk" pages there to disclaim ownership of it; people have already asked me about two articles I moved from another OddlyCapitalize page, but did not write. I will add more info to the articles I added today. --KQ

This page is an orphan. Could you find some articles to point to it? Kingturtle 02:28 Apr 16, 2003 (UTC)

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