Blood Brain Barrier

In most tissues, blood capillaries have pores in their walls that consist of a single layer of cells large enough to permit the passage of most drug molecules not bound to plasma proteins. Because of these pores, the entry of drugs into tissue extracellular fluid is not influenced by the lipid solubility of the drug, but rather by the rate of blood flow through the given tissue. This situation is different in the brain, where the capillary walls contain no pores and are further surrounded by the basement membrane, a type of extracellular matrix, and sheaths formed by the processes of astrocytes, star-shaped glia cells. Thus, access to the neurons is strongly associated with lipid solubility of the drug, and therefore psychoactive drugs must be lipid soluble.

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