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07-SEPTEMBER-2008 03:17:44 - Axon For the fictional extraterrestrial species in scifi drama Doctor Who, see The Claws of Axos. Structure of a typical neuron Neuron Dendrite Soma Axon Nucleus Node of Ranvier Axon Terminal Schwann cell Myelin sheath An axon or nerve fiber is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma. Contents 1 Anatomy 2 Physiology 2.1 Types 2.1.1 Motor 2.1.2 Sensory 3 Growth and development 4 History 5 Concussion 6 See also 7 References 8 External links Anatomy Axons are in effect the primary transmission lines of the nervous system, and as bundles they help make up nerves. Individual axons are microscopic in diameter typically about 1μm across, but may be up to multiple feet long. The longest axons in the human body, for example, are those of the sciatic nerve, which run from the base of the spine to the big toe of each foot. These single-cell fibers of the sciatic nerve may extend a meter or even longer. In vertebrates, only the axons of many neurons are sheathed in myelin, which is formed by either of two types of glial cells: Schwann cells ensheathing peripheral neurons and oligodendrocytes insulating those of the central nervous system. Along myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier occur at evenly-spaced intervals, enabling an especially rapid mode of electrical impulse propagation called saltation. The demyelination of axons is what causes the multitude of neurological symptoms found in the disease Multiple Sclerosis. The axons of some neurons branch to form axon collaterals, that can be divided into a number of smaller branches called telodendria. Along these the bifurcated impulse travels simultaneously to signal more than one other cell. Physiology The physiology can be described by the Hodgkin-Huxley Model, extended to vertebrates in Frankenhaeuser-Huxley equations. Types Peripheral nerve fibers can be classified based on axonal conduction velocity, mylenation, fiber size etc. For example, there are slow-conducting unmyelinated C fibers and faster-conducting myelinated Aδ fibers. More complex mathematical modeling continues to be done today. There are several types of sensory- as well as motorfibers. Other fibers not mentioned in table are e.g. fibers of the autonomic nervous system Motor Lower motor neurons have two kind of fibers: Motor fiber types Type Diameter Conduction velocity Associated muscle fibers α Extrafusal muscle fibers γ 4-24 m/s12 Intrafusal muscle fibers Sensory Different sensory receptors are innervated by different types of nerve fibers. Muscles and associated sensory receptors are innvervated by type I and II sensory fibers, while cutaneous receptors acivate Aβ, Aδ and C fibers. Sensory fiber types Type Diameter Conduction velocity Associated sensory receptors Ia Receptors of muscle spindle Ib Golgi tendon organ AβII 6-12 µm diameter 33-75 m/s All cutaneous mechanoreceptors Aδ 1-5 µm 3-30 m/s Free nerve endings of touch and pressure Cold thermoreceptors Nociceptors of neospinothalamic tract C 0.2-1.5 µm 0.5-2.0 m/s Nociceptors of paleospinothalamic tract warmth receptors Growth and development Axon of 9 day old mouse with growth cone visible. Axon of 9 day old mouse with growth cone visible. 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 lamellipodia 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 or CAM's create an ideal environment for axonal growth. This seems to provide a sticky surface for axons to grow along. Examples of CAM's specific to neural systems include N-CAM, neuroglial CAM or NgCAM, TAG-1, MAG, and DCC, 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 are typically other, sometimes immature, neurons. History Some of the first intracellular recordings in a nervous system were made in the late 1930s by K. Cole and H. Curtis. 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 the formulation of the Hodgkin-Huxley Model. Hodgkin and Huxley were awarded jointly the Nobel Prize for this work in 1963. The formulas detailing axonal conductance were extended to vertebrates in the Frankenhaeuser-Huxley equations. Erlanger and Gasser later developed the classification system for peripheral nerve fibers, based on axonal conduction velocity, mylenation, fiber size etc. Even recently our understanding of the biochemical basis for action potential propagation has advanced, and now includes many details about individual ion channels. Concussion Concussion is considered a mild form of diffuse axonal injury 3. See also Neuron Dendrite Synapse Axon guidance Pioneer axon Electrophysiology References January 2008 ^ Andrew BL, Part NJ 1972 Properties of fast and slow motor units in hind limb and tail muscles of the rat. Q J Exp Physiol Cogn Med Sci 57:213-225. ^ Russell NJ 1980 Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat. J Physiol 298:347-360. ^ eMedicine - Traumatic Brain Injury: Definition, Epidemiology, Pathophysiology : Article by Segun T Dawodu, MD, FAAPMR, FAANEM, CIME, DipMIRCSed External links Histology at OU 3_09 - Slide 3 Spinal cord v d e Histology: nervous tissue Neurons gray matter soma - axon axon hillock, axoplasm, axolemma, neurofibril/neurofilament dendrite Nissl body, dendritic spine, apical dendrite, basal dendrite types: bipolar - pseudounipolar - multipolar - pyramidal - Purkinje - granule Afferent nerve/Sensory nerve/Sensory neuron GSA - GVA - SSA - SVA - fibers Ia, Ib or Golgi, II or Aβ, III or Aδ or fast pain, IV or C or slow pain Efferent nerve/Motor nerve/Motor neuron GSE - GVE - SVE - Upper motor neuron - Lower motor neuron α motorneuron, γ motorneuron Synapses neuropil - synaptic vesicle - neuromuscular junction - electrical synapse - Interneuron Renshaw Sensory receptors Meissner's corpuscle - Merkel nerve ending - Pacinian corpuscle - Ruffini ending - Muscle spindle - Free nerve ending - Olfactory receptor neuron - Photoreceptor cell - Hair cell - Taste bud Glial cells Astrocyte Radial glia - Oligodendrocyte - Ependymal cells Tanycytes - Microglia Myelination white matter CNS: Oligodendrocyte PNS: Schwann cell - Neurolemma - Nodes of Ranvier/Internode - Schmidt-Lanterman incisures Related connective tissues epineurium - perineurium - endoneurium - nerve fascicle - meninges Retrieved from http://en..org/wiki/Axon Categories: Neurons | Neurophysiology | NeuroanatomyHidden category: Articles needing additional references from January 2008 Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page Languages БългарÑ?ки Català Česky Dansk Deutsch Ελληνικά Español Euskara Ù?ارسی Français Hrvatski Bahasa Indonesia Italiano עברית Latina Lietuvių МакедонÑ?ки Nederlands 日本語 ‪Norsk bokmÃ¥l‬ Polski Português Română РуÑ?Ñ?кий SlovenÅ¡Ä?ina Suomi Svenska ไทย Тоҷикӣ Türkçe اردو ä¸æ–‡ This page was last modified on 26 August 2008, at 10:27
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