Axon
958
225839063
2008-07-15T17:14:56Z
J.delanoy
2372780
Reverted edits by [[Special:Contributions/68.163.37.208|68.163.37.208]] to last version by Numbo3-bot (using [[WP:HG|Huggle]])
{{for|the fictional extraterrestrial species in scifi drama Doctor Who|The Claws of Axos}}
{{Neuron map|Neuron}}
An '''axon''' or '''nerve fiber''' is a long, slender projection
of a nerve cell, or [[neuron]], that conducts [[action potential|electrical impulses]]
away from the neuron's [[cell body]] or soma.
== Anatomy ==
Axons are in effect the primary transmission lines of the [[nervous system]], and as bundles they help make up [[nerve]]s. Individual axons are microscopic in diameter (typically about 1[[micrometre|μm]] 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 (anatomy)|spine]] to the big toe of each foot. These single-cell fibers of the sciatic nerve may extend a meter or even longer.
In [[vertebrate|vertebrates]], only the axons of many neurons are sheathed in [[myelin]], which is formed by either of two types of [[glia|glial cells]]: [[Schwann cell]]s ensheathing [[PNS|peripheral]] neurons and [[oligodendrocyte]]s 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 [[saltatory conduction|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=== <!--Fiber types and Aβ fiber redirect here-->
Peripheral nerve fibers can be classified based on axonal conduction velocity, mylenation, fiber size etc. For example, there are slow-conducting unmyelinated [[C fiber]]s and faster-conducting myelinated [[A delta fiber|Aδ fiber]]s. 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==== <!-- Motor fiber types redirects here-->
[[Lower motor neurons]] have two kind of fibers:
{| class="wikitable"
|+Motor fiber types
|-
! Type !! Diameter || Conduction velocity !! Associated [[muscle fiber]]s
|-
! [[α-motorneuron|α]]
| || || [[Extrafusal muscle fibers]]
|-
! [[γ-motoneuron|γ]]
| || 4-24 m/s<ref>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.</ref><ref>Russell NJ (1980) Axonal conduction velocity changes following muscle tenotomy or deafferentation during development in the rat. J Physiol 298:347-360.</ref> || [[Intrafusal muscle fibers]]
|}
====Sensory==== <!--Sensory fiber types redirects here-->
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]] are innervated by Aβ, Aδ and C fibers.
{| class="wikitable"
|+Sensory fiber types
|-
! Type !! Diameter || Conduction velocity !! Associated [[sensory receptor]]s
|-
! [[Type Ia sensory fiber|Ia]]
| || || Receptors of [[muscle spindle]]
|-
! Ib
| || || [[Golgi tendon organ]]
|-
! [[Aβ fibers|Aβ(II)]]
| 6-12 [[µm]] diameter || 33-75 m/s || All [[cutaneous mechanoreceptor]]s
|-
! [[A delta fiber|Aδ]]
| 1-5 [[µm]] || 3-30 m/s || [[Free nerve ending]]s of touch and pressure <BR> Cold [[thermoreceptors]] <BR> [[Nociceptors]] of [[neospinothalamic tract]]
|-
! [[Group C nerve fiber|C]]
| 0.2-1.5 [[µm]] || 0.5-2.0 m/s || [[Nociceptors]] of [[paleospinothalamic tract]] <BR> [[warmth receptors]]
|}
==Growth and development==
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 molecule]]s 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 [[Neural Cell Adhesion Molecule|N-CAM]], neuroglial CAM or [[NgCAM]], [[TAG-1]], [[MAG (neural)|MAG]], and [[Dicyclohexylcarbodiimide|DCC]], all of which are part of the [[immunoglobulin]] superfamily. Another set of molecules called [[extracellular matrix adhesion molecule]]s 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 1930's 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 in Physiology or Medicine|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 channel]]s.
==Concussion==
Concussion is considered a mild form of diffuse axonal injury <ref>[http://www.emedicine.com/pmr/topic212.htm eMedicine - Traumatic Brain Injury: Definition, Epidemiology, Pathophysiology : Article by Segun T Dawodu, MD, FAAPMR, FAANEM, CIME, DipMI(RCSed)<!-- Bot generated title -->]</ref>.
==See also==
*[[Neuron]]
*[[Dendrite]]
*[[Synapse]]
*[[Axon guidance]]
*[[Electrophysiology]]
==References==
{{Refimprove|date=January 2008}}
{{reflist}}
== External links ==
* {{OklahomaHistology|3_09}} - "Slide 3 [[Spinal cord]]"
{{Nervous tissue}}
[[Category:Neurons]]
[[Category:Neurophysiology]]
[[Category:Neuroanatomy]]
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