Blood–brain barrier

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Part of a network of capillaries supplying brain cells
A cortical microvessel stained for blood–brain barrier protein ZO-1

The blood–brain barrier (BBB) is a highly selective permeability barrier. It separates the circulating blood from the brain extracellular fluid in the central nervous system (CNS).

The BBB was one of Paul Ehrlich's discoveries. He injected dye into the bloodstream of mice, and it got to every organ except the brain. A student of his injected the dye directly into the brain: the brain turned blue, but other organs did not.[1]

The blood-brain barrier is formed by capillary endothelial cells.[2] It allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion. It also allows the selective transport of molecules such as glucose and amino acids which are crucial to nerve function.[3] On the other hand, the blood-brain barrier may prevent the entry of neurotoxins by means of an active transport mechanism.

A few small regions in the brain do not have a blood-brain barrier.

Function[change | change source]

The blood–brain barrier protects the brain from many common bacterial infections. Infections of the brain are rare. If they do occur they are often serious and difficult to treat. Antibodies are too large to cross the blood–brain barrier, and only some antibiotics are able to pass.[4]

One way to get drugs into the central nervous system is to inject them into the cerebrospinal fluid (CSF).[5] However, drugs delivered directly to the CSF still do not penetrate well into the brain tissue itself.[4][6]

Some spirochaete bacteria do cause brain infections. Examples are Borrelia, which causes Lyme disease, and Treponema pallidum, which causes syphilis. These harmful bacteria may get through the blood–brain barrier by physically tunnelling through the blood vessel walls.

There are also some biochemical poisons that are made up of large molecules that are too big to pass through the blood–brain barrier. Neurotoxins such as botulinum in the food might affect peripheral nerves, but the blood–brain barrier usually prevents them from reaching the central nervous system, where they could cause serious or fatal damage.[6]

References[change | change source]

  1. Interlandi, Jeneen 2013. Messing with the blood-brain barrier may be key to treating a host of diseases. Scientific American 308, #6. [1]
  2. de Vries, Helga E. et al (1997). "The blood-brain barrier in neuroinflammatory diseases". Pharmacological Reviews 49 (2): 143–156. PMID 9228664.
  3. "About". Blood brain barrier. Johns Hopkins University. Retrieved 7 May 2013.
  4. 4.0 4.1 Raza M.W. et al (2005). "Penetration and activity of antibiotics in brain abscess". Journal of the College of Physicians and Surgeons--Pakistan : JCPSP 15 (3): 165–7. PMID 15808097.
  5. The injection is done through the back of the spine between vertebrae
  6. 6.0 6.1 Tortora, Gerard J; Berdell R. Funke; Christine L. Case (2010). Microbiology: an introduction. San Francisco: Benjamin Cummings. p. 439, 611; 616–618. ISBN 0-321-55007-2.