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Neurons - Anatomy & Physiology (view source)
Revision as of 09:40, 25 August 2011
, 09:40, 25 August 2011→Myelinated Axons
====Myelinated Axons====
====Myelinated Axons====
The addition of the myelin sheath to nerve axons greatly enhances the speed with which they are able to conduct nerve impulses. Unmyelinated nerves are able to conduct up to speeds to 25m/s whilst myelinated nerves are capable of up to 100m/s. For an unmyelinated nerve to conduct impulses at the same rate, it would have to have a diameter of approximately 100 times that of the myelinated nerve.
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As with unmyelinated axons, an area of locally positive charge flows through the cytosol from the activated area within the axon. The major difference with myelinated nerves is that where the current effectively leaks away in unmyelinated nerves, here the current is only able to leak away through '''nodes of Ranvier''' (see above). Therefore even a weak current is able to depolarise the axon membrane to the threshold level. There is a small loss of current but as these nodes are relatively small the local current is able to travel much further in myelinated axons resulting in a much higher conduction velocity.
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The influx of sodium ions via voltage-gated channels is only possible at these nodes and therefore the nerve impulse is able to effectively 'jump' from node to node. This type of impulse propogation is called '''saltatory conduction'''. Myelination of axons in mammals means that the nervous system can sustain a large number of high velocity axons within a relatively small space.
==Astrocytes==
==Astrocytes==