Blood Brain Barrier (BBB).:
The term blood-brain barrier denotes the barrier separating the brain tissues from the blood circulation.
The structural basis consists of three parts:
Layer of endothelial- cells :interconnected through tight junctions and not containing fenestrations (Gaps).
Basal membrane : consisting of basal lamina of astrocytes and basal lamina of endothelial cells.
Protrusions of astrocytes : so called pedicles, which packs on the basal membrane.
Structure:
The BBB is:
High permeability for: water, carbon dioxide, oxygen, most lipid-soluble substances (e.g.: alcohol and anesthetics)
Little permeability for: electrolytes (e.g.: Na+, Cl-, K+)
Almost totally impermeable to: plasma proteins and most non-lipid-soluble large organic molecules.
Some small CNS regions are out of the BBB:
Area postrema of the fourth ventricle.
Subfornical organ.
Epiphysis.
Neurohypophysis.
Functions of the BBB
Protection of the nervous system from pathogens.
Precise local environment regulation.
Retaining of certain factors within the brain (neurotransmitters, neuromodulators).
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