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Monique
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#3
Nov15-12, 01:34 PM
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Neurons in a single organism can already look very different, but a clear structural example is that Drosophila neurons don't have a myelin sheath and thus no nodes of Ranvier. Glia do wrap the neurons though:
In the absence of a myelin sheath and nodes of Ranvier, Drosophila axons are wrapped by peripheral (inner) glial cells, which in turn are wrapped by much larger perineurial (outer) glial cells (Bellen et al., 1998). The inner glial cell processes wrap the axons, which in turn are ensheathed by the outer glial membrane. This insulation protects the neural microenvironment from the high potassium levels of the hemolymph (Hoyle, 1952).

..

The fundamental basis of axonal ensheathment in any species is to faithfully transmit neuronal signals along the nerve fibers and optimize desired cellular responses. To maximize the speed of conduction and/or to minimize the loss of nerve signals, many species evolved mechanisms in which axonal lengths remained short (as seen in insects) by increasing the diameter of the axons or by clustering voltage-gated Na+ channels to discrete unmyelinated regions of the axon, the node of Ranvier, as seen in myelinated nerve fibers of vertebrates. Most invertebrate species use some type of glial cells to ensheath their axons without generating a myelin sheath. The insulation is contiguous without any breaks, suggesting that primitive nodal structures or clustering of voltage-gated Na+ channels may not exist in invertebrates.
http://www.jneurosci.org/content/26/12/3319.full