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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss the blood-brain barrier.
The blood-brain barrier is a complex that surrounds most of the blood vessels in the
brain.
It acts as a barrier between the bloodstream and the extracellular space of the brain,
allowing only certain substances like water, oxygen, and small lipid-soluble substances
to easily cross from the blood into the brain.
This prevents toxins, pathogens, and other potentially dangerous substances from crossing
from the circulatory system into the brain.
It is thought that the central components of the structure of the blood-brain barrier
are the tight junctions of endothelial cells, the cells that make up the interior surface
of blood vessels.
In other blood vessels throughout the body, there are small spaces between these endothelial
cells; small blood-borne substances can pass through these spaces and into surrounding
tissues.
The endothelial cells that make up the blood-brain barrier, however, are fused tightly together
to form tight junctions that restrict diffusion across the blood vessel lining.
Glial cells called astrocytes also have projections called astrocytic end-feet that extend to
the walls of blood vessels that are part of the blood-brain barrier.
Astrocytic end-feet often completely surround blood vessels in the brain and are thought
to play critical roles in the formation of the blood-brain barrier.
For example, they seem to be involved with signaling that prompts endothelial cells to
form the tight junctions necessary to create the blood-brain barrier and they are believed
to have multiple functions involving the maintenance and regulation of the blood-brain barrier.
The blood-brain barrier protects most of the blood vessels in the brain, but there are
some areas that lack a blood-brain barrier, allowing substances to pass from the circulatory
system to the brain and back.
For example, the circumventricular organs are a group of structures lacking a blood-brain
barrier that are centered around the ventricles of the brain.
These structures are thought to be lacking a blood-brain barrier because their functions
require access to the bloodstream.
The posterior pituitary gland, for example, has to be able to release hormones directly
into the bloodstream.