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(also glia), the group of interstitial cells whose cell bodies and outgrowths fill the spaces in the brain and spinal cord between the capillary blood vessels and the nerve cells, or neurons.
Each neuron is surrounded by several neuroglial cells. The neuroglia is evenly distributed over the entire brain and accounts for approximately 40 percent of the brain’s volume. There are about 140 billion neuroglial cells within the mammalian central nervous system (CNS); they differ from neurons in size (neuroglial cells are three to four times smaller) and in morphological and biochemical characteristics. In contrast to neurons, the cells of the neuroglia retain the capacity to divide. This is why the number of neurons in the CNS decreases with age, while the number of neuroglial cells increases. The neuroglia acts as a protective layer for the neurons and forms part of the blood-brain barrier between the bloodstream and the encephalic neurons. This barrier regulates the passage of matter between the blood and the CNS. The neuroglia also helps to maintain the reactive properties of nerve tissue in such conditions as posttraumatic scarring, inflammatory reactions, and oncogenesis. The neuroglia comprises the astroglia (also called macroglia), oligoglia (also called oligodendroglia), and the ependyma. The microglia occupies a special position among neuroglial cells as the “scavenger” of the CNS.
Astrocytes (the cells of the astroglia) account for about 60 percent of the total number of neuroglial cells. They are star-shaped cells with numerous slender outgrowths that entwine the neurons and the walls of the capillary blood vessels. The astroglia regulates the water-salt metabolism of nervous tissue and is the principal element of the blood-brain barrier. About 25 to 30 percent of neuroglial cells are contained in the oligoglia. Oligodendrocytes (the cells of the oligoglia) are rounded cells with short outgrowths and are smaller than astrocytes. They surround the cell body and axon (the conducting portion) of a neuron. Oligodendrocytes are characterized by a highly active protein and nuclein metabolism and are also responsible for the transport of matter to the neurons. The myelin sheath that surrounds an axon mostly consists of oligodendrocytes. The ependyma consists of cylindrical cells that line the ventricles of the brain and the central lumen of the spinal cord. The ependyma is the barrier between the blood and cerebrospinal fluid and also appears to have a secretory function.
The neuroglia, especially the oligoglia, participates in the generation of the slow, spontaneous bioelectric activity that is characterized by α waves on an electroencephalogram. Neurons and neuroglial cells form a unified functional and metabolic complex that operates in cycles and has an adaptive function. The complex has the capacity to shift certain metabolic processes predominantly to the neuronal or to the neuroglial elements, depending on the nature and intensity of the physiological and pathological condition of the CNS.
REFERENCESHidden, H. “Kletki-satellity ν nervnoi sisteme.” In the collection Struktura i funktsiia kletki. Moscow, 1964. (Translated from English.)
Pevzner, L. Z. Funktsional’naia biokhimiia neiroglii. Leningrad, 1972.
Kuffler, S. W., and J. G. Nicholls. “The Physiology of Neuroglial Cells.” In the collection Ergebnisse der Physiologic, biologischen Chemie und experimentellen Pharmakologie, vol. 57. Berlin-Heidelberg-New York, 1966.
L. Z. PEVZNER