The Blood brain barrier (BBB) is a semipermeable wall that lies between the blood and the brain. It consists of the cells that form the walls of the cerebral blood capillaries. This barrier allows the neurons of the central nervous system to be chemically isolated from the rest of the body.
Paul Ehrlich , The German physician who won the Nobel Prize for Medicine in 1908, demonstrated the existence of the blood-brain barrier.
Barrier hematoencephalic and astrocytes.
In 1878 he made a thesis on histological staining. Ehrlich tried to inject a blue dye called aniline into the bloodstream of a mouse. He found that all tissues were stained blue except brain Y spinal cord .
However, when injecting the same colorant into the cerebrospinal fluid of the Cerebral ventricles All the Central Nervous System It was dyed blue.
This experiment stated that there is a barrier between blood and fluid in the brain cells (extracellular fluid): the blood-brain barrier.
The brain is the only organ that has its own security system. Thanks to the blood-brain barrier, essential nutrients can be reached while blocking the entry of other substances.
This barrier serves to maintain the proper functioning of neurons by controlling the entry and exit of chemicals in the brain. Unfortunately, however, this barrier works so effectively by blocking the passage of foreign substances into the brain that it normally also prevents drugs from reaching the brain.
In any case, the investigations continue to design drugs that have the necessary requirements to penetrate this barrier.
However, there are some regions of the body where there is no blood-brain barrier. These are known as circuventricular organs.
Finally, there are certain conditions that produce an opening of the blood-brain barrier. This allows the exchange of substances freely, so that the Brain function Can be altered. Some of them are inflammation, trauma or diseases such as multiple sclerosis .
Structure of the blood-brain barrier
Some substances can cross this barrier, but others can not. Which means it is a selectively permeable barrier.
In much of the body, the cells that form the blood capillaries do not attach tightly. These are called endothelial cells, and they have clefts between them through which various substances can enter and exit. Thus, elements are exchanged between the blood plasma and the fluid surrounding the cells of the body (extracellular fluid).
However, in the central nervous system, the capillaries do not have these clefts. In contrast, the cells are tightly bound. This prevents many substances from leaving the blood.
It is true that there are some concrete substances that can cross this barrier. They do this by special proteins that transport them from the walls of the capillaries.
For example, glucose transporters allow this substance to enter the brain to give it fuel. In addition, these carriers prevent toxic waste products from remaining in the brain.
Nail Glial cells (Support) calls Astrocytes , Are clustered around the blood vessels of the brain and appear to play an important role in the development of the blood-brain barrier. These also appear to contribute to the transport of ions from the brain to the blood.
On the other hand, there are areas of the nervous system that have a more permeable blood-brain barrier than others. The following section explains what this is for.
Functions
For a good brain function, it is imperative that a balance be maintained between substances inside the neurons and in the extracellular fluid that is around them. This allows messages to be transmitted properly between cells.
If the components of the extracellular fluid change, even slightly, that transmission will be altered resulting in alterations in brain function.
Therefore, the blood-brain barrier acts by regulating the composition of this liquid. For example, many of the foods we eat contain chemicals that could modify the exchange of information between neurons. The blood-brain barrier prevents these substances from reaching the brain, maintaining a good functioning.
It is important to note that the blood-brain barrier does not have a uniform structure throughout the nervous system. There are places where it has more permeability than others. This is useful to allow passage of substances that are not welcome in other places.
An example is the Postrema area of the Trunk of the brain . This region controls vomiting, and has a much more permeable blood-brain barrier. Its purpose is that neurons in that area can quickly detect toxic substances in the blood.
Thus, when some poison that comes from the stomach reaches the circulatory system, it stimulates the cerebral postrema area causing vomiting. In this way, the organism can expel poisonous contents from the stomach before it begins to be harmful.
In summary, the three main functions of the blood-brain barrier are:
- Protects the brain from foreign substances that are potentially dangerous or that could impair brain function.
- Protects and separates the central nervous system from hormones and neurotransmitters that are in the rest of the body, avoiding unwanted effects.
- Maintains a constant chemical balance in our brain.
What substances cross the blood-brain barrier?
There are substances more susceptible than others to cross the blood-brain barrier. Substances with the following characteristics enter more easily than others:
- Small molecules pass the blood-brain barrier much more easily than large molecules.
- Fat-soluble substances easily cross the blood-brain barrier, while those that are not, do it more slowly or fail to cross it. One type of liposoluble drug that easily reaches our brain is the Barbiturates . Other examples are the ethanol , Nicotine, caffeine or heroin .
- The molecules that have less electric charge pass the barrier faster than those with high charge.
Some substances can cross the blood-brain barrier. Above all, they pass glucose, oxygen and amino acids molecules that are fundamental for the proper functioning of the brain.
Amino acids such as tyrosine, tryptophan, phenylalanine, valine, or leucine enter the blood-brain barrier very rapidly. Many of these are precursors of neurotransmitters that are synthesized in the brain.
However, this barrier excludes virtually all large molecules and 98% of all drugs that are composed of small molecules.
That is why there are difficulties for the treatment of cerebral diseases, since the medicines do not usually cross the barrier or they do not in the necessary amounts. In certain cases, therapeutic agents may be injected directly into the brain to avoid the blood-brain barrier.
At the same time, it prevents the entry of neurotoxins and lipophilic via a carrier regulated by the call P-glycoprotein .
Circulventricular organs
As mentioned, there are several regions of the brain where the blood-brain barrier is weaker and more permeable. This causes substances to reach these regions easily.
Thanks to these areas, the brain can control blood composition. Within the circuventricular organs are:
- Pineal gland : Is a structure located in the interior of our brain, between the eyes. It relates to our biological rhythms and important hormonal functions. It releases melatonin and neuroactive peptides.
- Neurohypophysis: is the posterior lobe of the pituitary gland . Store substances from the Hypothalamus , Mainly neurohormones as Oxytocin And vasopressin.
- Area postrema: as mentioned above, produces vomiting to prevent us intoxicate.
- Subfornical organ: it is essential in the regulation of body fluids. For example, it plays an important role in the sensation of thirst.
- Vascular organ of the terminal lamina: also contributes to thirst and fluid balance by releasing vasopressin. Detects peptides and other molecules.
- Middle Eminence: is an area of the hypothalamus that regulates the anterior pituitary through interactions between stimulating and inhibitory hypothalamic hormones.
Conditions affecting the blood-brain barrier
It is possible that the blood-brain barrier is altered due to different diseases. In addition, when this barrier weakens, it may increase the likelihood or accelerate the occurrence of neurodegenerative disorders.
- Hypertension or high tension: can cause this barrier to be altered, becoming permeable, which can be dangerous for our body.
- Radiation: Long exposure to radiation can weaken the blood-brain barrier.
- Infections: the inflammation of some part of the central nervous system weakens this barrier. An example is the meningitis , A disease in which Cerebral meninges (Layers that envelop the brain and spinal cord) by various viruses and bacteria.
- Injuries, ischemia, ictus ... can cause direct damage to the brain, affecting the blood-brain barrier.
- Brain abscess . It is due to inflammation and accumulation of pus inside the brain. The infection usually comes from the ear, mouth, sinuses, etc. Although it can be a result of trauma or surgery. In most cases 8 to 12 weeks of antibacterial therapy is required.
- Multiple sclerosis: it seems that people with this disease leak in the blood brain barrier. This causes too many white blood cells to reach the brain, where they mistakenly attack myelin.
The Myelin Is a substance that covers nerve cells and allows nerve impulses to travel quickly and effectively. If it is destroyed, a Cognitive impairment And progressive motor.
References
- Blood Brain barrier. (S.f.). Retrieved on April 22, 2017, from Wikipedia: en.wikipedia.org.
- The Blood Brain Barrier ("Keep Out"). (S.f.). Retrieved on April 22, 2017, from Neuroscience for kids: faculty.washington.edu.
- The Blood-Brain Barrier. (July 2, 2014). Retrieved from BrainFacts: brainfacts.org.
- Carlson, N.R. (2006). Physiology of behavior 8ª Ed. Madrid: Pearson.