Supportive Cells

According to most estimates, neurons are outnumbered, perhaps by an order of magnitude, by various supportive cells: the macroglia, the microglia, the cells of the vascular elements comprising the intracerebral vasculature and cerebrospinal fluid (CSF)-forming cells of the choroid plexus found within the intracerebral ventricular system, and the meninges, which cover the brain surface and comprise the CSF-containing envelope. Macroglia are the most abundant supportive cells; some are categorized as astrocytes (cells interposed between the vasculature and the neurons, often surrounding individual compartments of synaptic complexes), which play a variety of metabolic support roles including furnishing energy intermediates and supplementary removal of extracellular neurotransmitter secretions. The oligodendroglia, a second category of macroglia, are the myelin-producing cells. Myelin, made up of multiple layers of their compacted membranes, insulates segments of long axons bioelectrically and accelerates action potential conduction velocity. Microglia are derived from mesoderm and are related to the macrophage/monocyte lineage. Some microglia reside within the brain, while additional microglial cells may be recruited to the brain by inflammation following microbial infection or other brain injury (see Chapter 52).

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