Neurons account for only about 10 percent of the cells in the central nervous system. The remainder are glial cells (also called neuroglia). The neurons branch more extensively than glia do, however, and therefore neurons occupy about 50 percent of the volume of the brain and spinal cord.
Glial cells physically and metabolically support neurons and, as noted earlier, some glia, the oligo-dendroglia, form the myelin covering of CNS axons. A second type of glial cell, the astroglia, helps
Neural Control Mechanisms CHAPTER EIGHT
regulate the composition of the extracellular fluid in the central nervous system by removing potassium ions and neurotransmitters around synapses. Astroglia sustain the neurons metabolically—for example, by providing glucose and removing ammonia. In development of the embryo, astroglia guide neurons as they migrate, and they stimulate the neurons' growth by secreting growth factors. In addition, astroglia have many neuron-like characteristics, for example, they have ion channels, receptors for certain neurotrans-mitters and the enzymes for processing them, and the capability of generating weak electrical responses. Thus, in addition to all their other roles, it is speculated that astroglia may take part in information signaling in the brain. A third type of glia, the microglia, perform immune functions in the central nervous system.
Schwann cells, the glial cells of the peripheral nervous system, have most of the properties of the central nervous system glia. As mentioned earlier, Schwann cells produce the myelin sheath of peripheral nerve fibers.
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