Blood Brain Barrier
BLOOD-BRAIN BARRIER
- Physiological barrier to movement of substances in & out of brain interstitial tissues.
ANATOMICAL BASIS:
- Depends mainly on special structure in brain capillaries & their endothelial cells.
- Endothelial cells are joined by ‘Tight-Junction’, reducing capillary permeability.
- Capillaries are surrounded by glial cell foot process (especially astrocytes).
- Expands forming complete sheath around capillaries.
FUNCTIONS:
- Maintains environment in & around brain.
- Prevents escape of neurotransmitters.
- Protects neurons from harmful substances present in blood.
PERMEABILITY OF BBB:
- Highly permeable – Water, CO2, O2, Lipid-soluble substances (alcohol & most anesthetics).
- Slightly permeable – Electrolytes – Na+2, K+, Cl–.
- Totally impermeable – Plasma proteins and large insoluble lipid & organic molecules.
BLOOD-CSF BARRIER:
- Formed by tight junction between capillary endothelial cells & tight junction between choroidal epithelial cells.
- Restricts movement of substances in & out of CSF.
STRUCTURAL DIFFERENCE IN BBB & BLOOD CSF BARRIER:
BBB:
- One effective barrier –
- Tight function between endothelium.
BLOOD-CSF BARRIER:
- Two effective barriers –
- Tight junction between endothelial cells & tight junction between choroidal epithelial cells.
STRUCTURES OUTSIDE BBB:
- Have fenestrated capillaries & hence their permeability; Hence, outside BBB.
Areas included:
- Circum-ventricular organs –
- Areas are located around third ventricle.
- Although circumventricular organs are BBB deficient, blood-CSF barrier is effective.
- Due to “Tancytes” – Tight-junctions between modified ependymal cells of third ventricle.
- Posterior pituitary (neurohypophysis)& adjacent ventral part of median eminence of hypothalamus.
- Area postrema
- Organ Vasculusum of Lamina Terminalis (OVLT).
- Subfornical organs (SFO).
Exam Question
BLOOD-BRAIN BARRIER
ANATOMICAL BASIS:
- Endothelial cells are joined by ‘Tight-Junction’, reducing capillary permeability.
- Capillaries are surrounded by glial cell foot process (especially astrocytes).
PERMEABILITY OF BBB:
- Highly permeable – Water, CO2, O2, Lipid-soluble substances (alcohol & most anesthetics).
- Totally impermeable – Plasma proteins.
STRUCTURES OUTSIDE BBB:
- Circum-ventricular organs.
- Posterior pituitary (neurohypophysis)
- Area postrema
- Organ Vasculusum of Lamina Terminalis (OVLT).
- Subfornical organs (SFO).
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