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==Structure and Function==

==Structure and Function==

Endothelial cells are the structural basis of the blood brain barrier and are joined by tight cellular junctions. These junctions are impermeable to proteins and also restrict the passage of non-lipid soluble molecules. Within the BBB, most molecules use carrier systems. The properties of the BBB are induced and probably maintained by molecules secreted by [[Astrocytes|astrocytes]]. The BBB prevents circulating antibodies reaching the central nervous system (CNS) and thus is a component of the immunological privilege manifest by the CNS. The BBB is not considered to affect the movement of inflammatory cells into the CNS; activated lymphocytes can enter the normal CNS.

The Blood Brain Barrier refers to the mechanisms in place around the microvasculature of the brain to ensure optimal neural functioning. Endothelial cells are the structural basis of the blood brain barrier and are joined by tight cellular junctions formed by the transmembrane proteins the occludins and the claudins. These junctions form a physical barrier are impermeable to proteins and also restrict the passage of non-lipid soluble molecules. Oxygen and carbon dioxide can diffuse across the endothelial cells. Within the BBB, most molecules use carrier systems to cross the endothelial cells as the tight junctions prevent molecules from crossing via the paracellular route (between cells). Larger hydrophilic molecules can be transported across the endothelial cells from the lumen of the blood vessels into the interstitial fluid by a variety of ways such as the specific receptor mediated endocytosis or transcytosis or the less specific adsorptive mediated transcytosis. The endothelial cells possess transporters for glucose, amino acids, purine bases, nucleosides, choline and other substances. The properties of the BBB are induced and probably maintained by molecules secreted by [[Astrocytes|astrocytes]]. The astrocyte endfeet (perivascular endfeet) surround the endothelial cells and have K+ channels and aquaporins and so are likely to be involved in ion and water volume regulation within the neural environment. As well as the transporters mentioned, there are also efflux pumps present, such as p-glycoprotein, responsible for transporting potentially toxic molecules back into the lumen of the blood vessels. Along with this 'physical barrier', a 'metabolic barrier' also exists which involves the presence of intracellular and extracellular enzymes. Peptidases and nucleotidases are capable of metabolizing peptides and ATP where as the intracellular enzymes such monoamine oxidase and cytochrome P450 can break down neuroactive and potentially toxic compounds.  The BBB prevents circulating antibodies reaching the central nervous system (CNS) and thus is a component of the immunological privilege manifest by the CNS. The BBB is not considered to affect the movement of inflammatory cells into the CNS; activated lymphocytes can enter the normal CNS.

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Inflammatory responses in the CNS are different to elsewhere in the body. Injections of lipopolysaccaride, which would result in massive influx of [[Blood Cells - Overview|granulocytes]] (polymorphonuclear leukocytes (PML)) in peripheral tissue result in no influx into the brain substance, however influx into CSF does occur.

Inflammatory responses in the CNS are different to elsewhere in the body. Injections of lipopolysaccaride, which would result in massive influx of [[Blood Cells - Overview|granulocytes]] (polymorphonuclear leukocytes (PML)) in peripheral tissue result in no influx into the brain substance, however influx into CSF does occur.

==Defects of the Blood Brain Barrier==

==Defects of the Blood Brain Barrier==