| Essentially, the astrocytes and the oligodendrocytes comprise the neuroglial structures
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| In the cortex, or gray matter, one typically finds a protoplasmic astrocyte with one set of processes surrounding the endothelial cells, thereby helping to 'filter' materials from the blood, and a separate set of processes surrounding the neurons, which are thereby being 'fed' selected substances that have been extracted from the blood for passage to the neurons. In the white matter, the astrocytes have a rather more fibrous appearance, and have more of a structural role. Under pathologic conditions in which there is injury to the CNS, astrocytes can play a major part in the reaction, synthesizing large amounts of the glial fibrillary acid protein (GFAP). This is the cellular equivalent of scar tissue, and is found in diseases such as multiple sclerosis, in which it is the major constituent of the characteristic plaques. Astrocytes are not found in the PNS.
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| The oligodendrocytes of the CNS can wrap round as many as 20 axons, forming the myelin sheath that insulates these neuronal processes from one another, and stops cross-talk
between neurons. There is also intense oligodendrocyte mitochondrial activity at the nodes of Ranvier, which are parallel to the sites of depolarization within the underlying axon. In the PNS, the Schwann cells form the myelin and, typically, wrap round only a single axon. As noted previously, the chemical constituents of the myelin sheath are different when produced by Schwann cells rather than by oligodendrocytes.
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