The blood brain barrier (BHE) is a protection system against the entry of foreign substances formed by endothelial cells that line the capillaries of the brain.
Functions of the Blood-brain Barrier
It serves to control and restrict the passage of toxic substances between blood circulation and cerebral fluid. Participate in the regulation of the volume and composition of the cerebrospinal fluid It surrounds the brain through specific transport processes, and therefore contributes to the central nervous system homoeostasis.
The blood brain barrier (BHE)protects nervous tissue from variations in the composition of blood and toxins. In other parts of the body extracellular concentrations of hormones, amino acids and potassium experience frequent fluctuations, especially after meals, exercise or stressful moments. Since many of these molecules regulate neuronal excitability, a similar change in the composition of the interstitial fluid in the SNC It could generate uncontrolled brain activity. The endothelial cells that form the blood brain barrier are highly specialized to exert control over the entry and exit of these substances into the brain.
Not all areas of the brain have a blood brain barrier. The structures located in the midline of the ventricular system lack BHE, and are collectively referred to as circumventricular organs. In these regions the tight junctions between the endothelial cells are discontinuous, which allows the entry of molecules. Many of these areas participate in hormonal control.
Properties of the BHE
The blood brain barrier (BHE) has unique properties, these CNS vessels are actually continuous vessels that do not have permeability holes, but also contain a series of additional properties that allow them to closely regulate the flow of molecules, ions and cells between the blood and the SNC. This ability to highly restrictive barrier allows cells to control CNS homeostasis, which is essential to allow adequate neuronal function, as well as to protect the CNS from toxins, pathogens, inflammation, lesions and diseases.
BHE is a selectively permeable barrier, since it allows the passage of small molecules such as ions or water, but instead does not allow the passage of large molecules such as proteins.
BHE responds to the peculiar structure of blood vessels throughout the body; the cells that form the walls of the blood vessels are not linked together in an absolutely hermetic way, but leave small openings that allow the free exchange of most substances between the blood plasma and the fluid outside the blood vessels surrounding the cells.
In the SNC; The capillaries do not have these openings, and therefore, many substances cannot leave the blood. That is to say, in the CNS the blood vessel wall cells are very close and constitute a barrier to the passage of many molecules.
It is important to note that BHE does not prevent the passage of all large molecules. Some of these, essential for normal brain functioning such as glucose, are actively transported through the walls of blood vessels by special proteins that act as transporters.
BHE is not uniform throughout the SN. There are places where it is relatively permeable, which allow some substances, which in other places could not pass through it, can pass freely through these areas. These areas are in contact with the walls of the cerebral ventricle and are called circumventricular. For example, in an area of the brain called the last area, BHE is much weaker, and it increases the sensitivity of this region to the toxic substances found in the blood.
Hemotoencephalic barrier discontinuity
As we have already mentioned, there are areas that do not have this barrier of protection. Most of those areas are around the brain ventricles, also called circumventricular organs and they include the choroid plexus, the vascular organ of the terminal lamina, the subfornical organ, the subcomisural organ, the middle eminence, the pineal gland, the neurohypophysis, and the dessertma area.
In these areas without BHE there is a free bidirectional exchange between blood molecules and neurons, and contribute to regulate the autonomic nervous system and the endocrine glands.
BHE dysfunction can lead to increased infiltration of cells in brain tissue, this is associated with a multitude of neurological disorders that include the Alzheimer's disease, Parkinson's disease and multiple sclerosis.
The weakening of the blood brain barrier can precede, accelerate or contribute to a series of neurodegenerative disorders. There are studies that suggest a leaking blood-brain barrier allows too many white blood cells to pass to the brain in people with multiple sclerosis. With free access to the brain, these cells attack myelin, the insulating layer of nerve cells, leading to devastating symptoms of the disease.
When the blood brain barrier is broken, as is the case with some brain cancers, brain infections or when there are ruptures in the blood vesselsSome substances that are normally kept out of the brain can penetrate and cause problems.
There are other neuropathological conditions in which the normal functioning of BHE is modified, such as hypoxia and ischemia. The stress it is also an important factor that affects the functioning and development of the BHE; in the adult mammal Acute stress increases the permeability of BHE to circulating macromolecules in the blood.
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