Everything about Axon totally explained
» For the fictional extraterrestrial species in scifi drama Doctor Who, see The Claws of Axos
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.
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 1
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 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 for example fibers of the
autonomic nervous system
Motor
Lower motor neurons have two kind of 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 are innervated by Aβ, Aδ and C fibers.
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 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 1930's by K. Cole and H. Curtis.
Alan Hodgkin and
Andrew Huxley also employed the
squid giant axon (1939) and by 1952 they'd 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 .
Further Information
Get more info on 'Axon'.
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