In the brain and the nervous system as a whole, it is a fundamental organ for human beings. Therefore, it is strongly protected by the bones (the skull and the spine) and by a three-layer system of membranes called the meninges. The safety of different parts of the brain has been enhanced by millions of years of evolution.
However, while all of these may be essential in protecting your skull from a blow or trauma, they may not be enough to protect your brain from other dangers such as viral infections that could lead through it. blood. To avoid such dangers as much as possible, we have another type of protection: the blood-brain barrier (BBB).
The discovery of the BHE
Although the existence of something separating the content of the blood present in the blood system and the nervous system had previously been suspected, the discovery of this fact would not come until 1885. A researcher named Paul Ehrlich would introduce a tincture. an animal and would later observe that the only point that did not stain was the central nervous system, and in particular the brain. The reason for this must have been linked to a protective system that surrounded this area as if it were a membrane.
Later, another researcher, Edwin Goldman, will attempt the reverse process by dyeing the cerebrospinal fluid, noting that the only colored parts corresponded to the nerve tissue. These experiences reflect the existence of which produces a high level of blockage between the nervous system and the rest of the bodySomething that, years later, would be called a blood brain barrier by Lewandowski and explored by a large number of experts.
Protection between blood and brain
The blood brain barrier is a small layer of endothelial cells, cells that are part of the lining of blood vessels, Located along most of the capillaries that supply the brain. The main characteristic of this layer is its high level of impermeability, which does not allow a large number of substances to pass from the blood to the brain and vice versa.
In this way, the BHE it acts as a filter between the blood and the nervous system. However, some substances such as water, oxygen, glucose, carbon dioxide, amino acids and a few additional molecules may pass, impermeability being relative.
Its action as a filter is carried out both by its structure, by restricting the union between the cells that make up the passage to different substances, and by the metabolism of substances to achieve this through the use of enzymes and carriers. In other words, it has a physical facet and another which is chemical.
While the blood brain barrier is itself a layer of endothelial cells, its proper functioning also depends on other types of cell structures. Specifically, it is supported by cells called pericytes, which provide structural support and surround the endothelial cells by keeping the blood vessel wall stable, as well as the microglia.
The blind spots of the BHE
Despite its importance in protecting the nervous system, the blood-brain barrier it does not cover the whole brain, because it needs to receive and be able to emit certain substances, Such as hormones and neurotransmitters. The existence of this class of blind spots is necessary for the proper functioning of the body, since it is not possible to keep the brain completely isolated from what is happening to the rest of the body.
The areas not protected by this barrier are located around the third cerebral ventricle and are called circumventricular organs. In these areas, the capillaries have a fenestrated endothelium, with a few openings or accesses that allow the flow of substances from one side of the membrane to the other.
The locations without a blood-brain barrier are mainly in the neuroendocrine system and the autonomic nervous system, forming part of the structures of this group of circumventricular organs the neurohypophysis, the pineal gland, some areas of the hypothalamus, the area after the vascular organ of the terminal lamina and the subfornical organ (under the fornix).
Crossing the blood-brain barrier
As we have seen, the blood-brain barrier is permeable, but in a relative way, because it allows the passage of certain substances. Apart from places where the blood-brain barrier is not present, they exist a series of mechanisms by which the components essential to the functioning of cells can.
The most common and frequently used mechanism in this regard is the use of conveyors, Wherein the element or substance to be transported binds to a receptor which then enters the cytoplasm of the endothelial cell. Once there, the substance separates from the receptor and is excreted on the other side by the endothelial cell itself.
Another mechanism by which substances cross the blood-brain barrier is transcytosis, A process in which a series of vesicles form in the barrier through which substances can pass from one side to the other.
Transmembrane diffusion allows ions of different charge to travel across the blood-brain barrier, acting on the electronic charge and the concentration gradient so that substances on either side of the barrier are attracted to each other.
Finally, a fourth mechanism by which certain substances pass into the brain without the blood-brain barrier intervening is to jump directly. One way to do this is to use sensory neurons, forcing reverse transmission through the neuron’s axon to its soma. This is the mechanism used by diseases as well known as rabies.
As we have already seen some of the properties that make the blood brain barrier an essential part of the nervous system, since this layer of endothelial cells mainly performs the following functions.
The main function of the blood-brain barrier is that of protect the brain from the arrival of substances outside it, Prevent the passage of these elements. Thus, the vast majority of molecules external to the nervous system itself cannot affect it, preventing a large part of viral and bacterial infections from affecting the brain.
In addition to this defensive function by blocking the entry of harmful elements, their presence also allows the good maintenance of the neuronal environment by maintaining constant the composition of the interstitial fluid which bathes and maintains the lule cells.
A final function of the blood-brain barrier is to metabolize or modify elements in order to pass them between the blood and the nervous tissues without adversely affecting the functioning of the nervous system. Of course, some substances escape this control mechanism.
Therapeutically problematic protection
The fact that the blood-brain barrier is so impermeable and does not allow entry of most elements is beneficial when its brain is functioning properly and no medical or psychiatric intervention is required. But in cases where an external action is necessary at the medical or pharmacological level, this barrier poses a difficulty which is difficult to manage.
And is that many medically applied drugs that would be used to treat disease or infection in another part of the body are not effective in treating the brain problem, largely because of the action. blocking the blood-brain barrier. Examples of this can be found in drugs dedicated to the fight against tumors, Parkinson’s disease or dementia.
To resolve in many cases it becomes necessary to inject the substance directly into the interstitial fluid, Use circumventricular organs as an access route, temporarily break the barrier using microbubbles guided to specific points by ultrasound or use chemical compositions that can cross the blood-brain barrier by some of the mechanisms described above.
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