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Page 28

are present in the central nervous system (CNS), providing structural support, transporting nutrients, and phagocytizing cellular debris. They transport glucose and also perform a metabolic service by converting it to lactate before it is transported to the neurons. Astrocytes also store glucose in the form of glycogen, serving as a small metabolic reservoir.

Oligodendrocytes are present in the CNS as well and wrap around axons to form a myelin sheath. Myelin wraps into concentric layers that spiral around the axon. Gaps in the oligodendrocytes are the nodes of Ranvier, where the membrane maintains contact with extracellular fluid. The nodes serve to propagate the action potential in myelinated axons. Schwann cells perform an analogous function, myelinating axons in the peripheral nervous system. Not all neurons are myelinated, but myelination increases the metabolic efficiency of action potentials. Demyelination of neurons produces deficits in neuronal conduction, as is seen in multiple sclerosis.

Microglia, in contrast, serve as macrophages in the central nervous system. They are relatively inactive during normal conditions, but rapidly proliferate during inflammatory or degenerative processes.

Brain Permeability Barriers

The brain is protected from potentially toxic substances in the blood by a semipermeable barrier called the blood-brain barrier. This barrier is created by tight junctions between endothelial cells forming the wall of capillaries in the brain and fatty astrocytes that wrap the capillaries. Whereas in peripheral tissues the endothelial junctions contain gaps that freely allow substances to pass through, brain capillaries do not contain such gaps. Penetration of substances across the blood-brain barrier is determined by the molecular size, its lipid-solubility, and the presence of endothelial carrier transport to bring certain substances through. The epithelium of the choroid plexus and ventricles form a blood-cerebrospinal fluid (CSF) barrier, which is a small fraction of the size of the blood-brain barrier. However, it mediates a limited transport of certain circulating peptides, such as leptin.

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