Axon
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
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Motor
Lower motor neurons have two kind of fibers:
| Type | Diameter | Conduction velocity | Associated muscle fibers |
|---|---|---|---|
| α | Extrafusal muscle fibers | ||
| γ | 4-24 m/s[1][2] | Intrafusal muscle fibers |
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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.
| Type | Diameter | Conduction velocity | Associated sensory receptors |
|---|---|---|---|
| Ia | Receptors of muscle spindle | ||
| Ib | Golgi tendon organ | ||
| Aβ(II) | 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 |
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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.
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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 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.
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Concussion
Concussion is considered a mild form of diffuse axonal injury [3].
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See also
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References
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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (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, DipMI(RCSed)
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External links
- Histology at OU 3_09 - "Slide 3 Spinal cord"
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