Blood-Brain Barrier

The brain is a vital organ that controls all the important functions of the body. However, for it to perform, there are various ways in which it needs to be protected. Just as the skull protects the brain from any physical damage, it's the blood brain barrier that protects the brain from unwanted and toxic substances. Thus, protection from brain injury is the major function of these two features. The barrier is like the gatekeeper that allows and denies entry of substance to the brain. This makes the environment in the brain stable and also protects it from infections. Its definition can be stated as, "the selective barrier that separates the blood from the parenchyma of the central nervous system".

About the Blood Brain Barrier

This barrier is found in the capillaries which supply blood to the brain. A normal blood vessel is permeable to various water soluble substances. However, it is observed that the capillaries leading to the brain are dense and compact and are not as permeable when compared to other capillaries. The walls of capillaries are made of endothelial cells. These cells in the capillaries of the brain are connected by tight junctions that have a high electrical resistance that provide a barrier against various ions. There is no transcellular movement of substances or molecules in the endothelial cells of the brain as seen in peripheral endothelial cells. The capillaries of the brain are in contact with a process of the astrocytes which separate the capillaries from neurones. The barrier readily allows the diffusion of molecules such as oxygen, carbon dioxide, glucose and specific hormones. There are areas of the brain that are not regulated or protected by the barrier. These are the third and fourth ventricles, pineal gland.

Discovery
The discovery of the blood brain barrier can be attributed to Paul Ehrlich, who studied differential staining of tissues. He discovered that when a dye was injected into the blood stream, it stained all the other organs except the brain, this led to the development of the hypothesis that, there is compartmentalization of the brain or presence of special structures that prevent the brain from getting stained.

Function
The main function of this barrier is protection. It protects the brain from various infections as well. It protects from the frequent ionic fluctuations that occur and only supplies vital nutrients to the brain. Thus, another main function is the maintenance of homeostasis in the brain. Contradictory to earlier beliefs, it is now proven that this barrier functions as efficiently in babies, as it does in adults.

Pathophysiology
The blood brain barrier is protective against infections, however there are certain pathogens that do cross the barrier. In this case, treatment becomes quite difficult, as the drugs such as antibiotics and antibodies do not cross this barrier. However, if there is an inflammation there are certain antibodies that do pass through the barrier. Viruses and certain bacteria like spirochetes easily pass through it. There are various diseases which disrupt the barrier. In certain conditions like meningitis caused due to inflammation of the meninges, the barrier may get disrupted. Certain studies have shown that in epilepsy a compromised blood brain barrier triggers seizures. Other disease such as multiple sclerosis, neuromyelitis optica, progressive mutltifocal leukoencephalopathy, Alzheimer's disease also damage this barrier.

There are various new techniques that are being developed to treat infections of the brain. Drug targeting using liposomes is a way that is being adopted as a method of treatment. This is the latest application of nanotechnology.

Thus the blood brain barrier is an important feature in order to protect the brain. However, sometimes due to its impermeable nature, treatment becomes complicated. With the advent of science and technology, hopefully this difficulty can be overcome as well.