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, prevention of brain injury is the major function of these two features.
The barrier is like the gatekeeper that allows and denies entry of any substance to the brain. This brings and maintains stability in the brain environment 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 provides 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 brain are in contact with a process of the astrocytes, which separate capillaries from neurones. The barrier readily allows diffusion of molecules like oxygen, carbon dioxide, glucose and specific hormones. Some areas of the brain that aren't regulated or protected by the barrier are the third and fourth ventricles and the pineal gland.
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 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.
The main function of this barrier is protection; it protects brain from various infections and frequent ionic fluctuations. It only supplies vital nutrients to the brain. Thus, another main function is maintaining homeostasis in the brain. Contradictory to earlier belief, it is proven that the barrier functions as efficiently in babies as it does in adults.
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 diseases, such as, multiple sclerosis, neuromyelitis optica, progressive multifocal 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 for the protection of 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 very soon.