Chapters 3- Neurophysiology basics 08. The blood-brain barrier

08. The blood-brain barrier

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The human brain weighs about 2% [ 137 ] body mass requires more than 20% [ 138 ] of the energy intake of the whole body, so he alone receives 16% [ 5 ] of blood irrigation.   The SNC has no real energy reserves or oxygen [ 109 ] and neurons are not able to function anaerobically [ 52 ]. This is a body entirely dependent on good blood supply and a sufficient supply of oxygen and nutrients. Thus, after 10 seconds without oxygen and lose consciousness after a few minutes the neurons begin to die [ 105 ]!   In return, the nervous system requires a stable environment for optimal functioning. He can not tolerate huge and sudden fluctuations in molecular and ionic constitution at interstitial spaces. For this, the nervous system is almost completely isolated from the blood through a barrier filter which is obligatory passage and highly selective between the content of the blood capillaries and the extracellular medium of the nervous tissue. We call this barrier, the blood-brain barrier [ 41 , 70 ] (BBB).   Before the discovery of microglia and immune role in the CNS, it was considered that he had only the blood-brain barrier as the sole means of passive defense against the toxic and infectious attacks.

1. Anatomy of the blood-brain barrier:

The BBB consists of three essential elements which are [ 70 ]:

  • Tight endothelial cells that line the inside of blood capillary junctions. In the brain, these junctions have more particularly a tight structure at the level of the rest of the body and the number of mitochondria in these endothelial cells is 5 to 10 times higher than elsewhere [ 91 ] This is in response to the very high active transport processes at this level energy demand.
  • The arterial capillary basement membrane.
  • Endings and astrocytes (astrocytic feet) of type I astrocytes [ 41 , 70 ], which (by joining with each other) form a shield against any passage of undesirable elements within the nervous tissue.

2. Physiology of the blood-brain barrier:

In addition to their relatively passive participation in the formation of the blood brain barrier, the astrocytes are also able to control the contraction and dilation of blood vessels [ 96 ], thereby controlling the blood flow in order to manage the removal of the substances according needs.   The blood-brain barrier plays a physiological role as well qu'anatomique, gases (oxygen and carbon dioxide) and soluble substances and alcohol [ 91 ] can freely pass through depending on their concentration gradient (from most to less concentrated). While polar molecules (ionized, hydrophilic) can broadcast only through mechanisms of active transport rely on specific channels and pumps and which are only as needed.

3. Blood-brain barrier and therapeutic:

The blood-brain barrier is a serious obstacle to the passage of drugs [ 41 ] that target neurological disorders such as brain tumors. Several studies are underway to address this problem.   This constraint can be overcome either by the injection of high doses of drugs, the administration of an agonist or precursor passes the barrier (for example the case of L-Dopa vs. dopamine [ 1 , 41 ]), or by intrathecal injection [ 42 ] the medicament (directly in CSF).

4. Blood-brain barrier and disease:

In newborns and infants, the blood-brain barrier is not as effective as in adults, it allows the passage of certain neurotoxic molecules such as bile pigments which can cause brain damage (kernicterus) [ 42 ] .   In certain disease such as meningitis cases, there is a breakdown of the BBB, which happily promotes the passage of antibiotics such as penicillin within the central nervous system [ 42 ].