What is the blood-brain barrier?
The blood-brain barrier separates the environmental conditions in the brain and in the bloodstream. In the brain very complex and finely tuned processes take place, the disturbance of which would have unforeseeable consequences. The blood-brain barrier therefore protects the CNS against pathogens, toxins, antibodies, leukocytes, the influence of the neurotransmitters in the blood and changes in the pH value.
At the same time, it must be ensured that the CNS is supplied with the basic nutrients and substances necessary for its function. The same applies to the removal of breakdown products of the brain metabolism. Therefore, the barrier is not completely hermetically sealed, but selectively permeable. The transport of important substances between the bloodstream and the brain is regulated by passive and active diffusion processes as well as selective chemical processes. Vital molecules such as water, oxygen, and essential nutrients can pass the blood-brain barrier without restriction.
Anatomy & Construction
The blood-brain barrier consists of the endothelial cells, the pericytes and the astrocytes. The endothelial cells form the innermost wall layer of the capillaries. Among other things, these cells regulate the exchange of substances between tissue and blood.
In the blood-brain barrier, the endothelial cells have tight junctions. These are narrow bands of membrane proteins that bind the endothelial cells together in such a way that they form an impermeable layer for many substances. Only very small molecules can diffuse through this layer. The mass transfer between cell and cell space is thus largely suppressed. The pericytes, in turn, are located on the outer wall of the capillaries and are connective tissue cells. Are connected via cell-cell channels, the gap junctions, with the endothelial cells.
The interaction of both cell types via these channels controls the membrane potential, which is responsible for the selective diffusion of substances. The astrocytes, called spider cells, represent the majority of the glial cells in the CNS. They provide the neurons with nutrients via the contacts to the blood vessel. Its membrane contains receptors for neurotransmitters. In addition, they induce and maintain the blood-brain barrier through the membrana limitans glialis perivascularis (a boundary membrane surrounding the blood vessels of the brain).
Function & Tasks
In addition to its protective function for the CNS, the blood-brain barrier regulates harmful influences and also the transport processes between the bloodstream and the brain. So there are different physical and chemical processes that control this transport. Most soluble substances that can even overcome this barrier, they pass through diffusion. Since the blood-brain barrier is tightly closed by tight junctions, the diffusion can not take place, as with other organs, via intercellular gaps.
About the capillaries of the brain, the substances can be forwarded only by a transmembrane transport. Free diffusion is the simplest form of this transport. Small lipophilic molecules can passively diffuse through the cell membranes of the epithelia and even through the tight junctions. Small polar molecules, such as water, are subject to channel-mediated permeability. Certain channel proteins, the aquaporins, mediate the transport of water through the blood-brain barrier, thereby regulating the water balance of the brain. For large and polar, but vital nutrient molecules, such as glucose or many amino acids, there are certain transport molecules, which facilitate the diffusion of the corresponding substances.
Since no energy is needed in these forms of diffusion, they are passive diffusions. However, there are also substances that can only be transported using ATP, ie by supplying energy. Active transporters are so-called "pumps", which transport the substrates under energy expenditure also against the concentration gradient. Selected molecules also cross the blood-brain barrier with the help of special receptors that are responsible for their transport.
Disorders of the blood-brain barrier can lead to various neurological diseases. Initial illnesses, such as diabetes mellitus, inflammation in the brain or brain tumors, often damage this barrier.
Long-term consequences are brain damage. Certain pathogens can cross the blood-brain barrier. This includes, among others, the HI virus. Some bacteria, such as Escherichia coli, sometimes overcome the protective mechanisms of the barrier by releasing specific toxins. When cells for the body's own immune defense cross the blood-brain barrier, the clinical picture of multiple sclerosis can develop. Studies have shown that even neurodegenerative diseases, such as Alzheimer's, make the barrier between the brain and the bloodstream permeable.
Perhaps this is the starting point for the extensive destruction of the brain cells. A major risk factor for neurological disorders is known to be alcohol abuse. Chronic alcohol consumption damages the blood-brain barrier with unpredictable consequences. Barrier dysfunction promotes bacterial infections and autoimmune-induced inflammatory responses in the brain. Nicotine abuse also represents a risk factor for damage to the blood-brain barrier. Nicotine favors cardiovascular diseases, which in turn have a major impact on brain performance.
Smokers have a higher risk of developing bacterial meningitis. Studies have shown that the structure of the blood-brain barrier is altered by nicotine. The tight-junction proteins are distributed differently and can no longer fully perceive their function. The influence of electromagnetic radiation on the blood-brain barrier is also discussed. Their negative health effects for the mega- to gigahertz range for high energy densities are proven. The high energy density of the electromagnetic radiation leads to a measurable warming in the affected tissue. To what extent warming damages the blood-brain barrier remains to be investigated.
Typical & common brain diseases
- Creutzfeldt-Jakob disease
- memory lapses
- cerebral hemorrhage