Describe the blood brain barrier. (50% of marks) What characteristics does a drug need to effectively penetrate into the central nervous system? (50% of marks)

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College Answer

Candidates were expected to state the purpose of the blood brain barrier (BBB), define what
constituted the BBB, what its function is, and what parts of the brain lie outside of this barrier
(and why). Further candidates should have mentioned what substances cross the BBB, and
how this is achieved.
Common omissions included not mentioning the presence of astrocyte foot processes in
addition to the tight junctions between the capillary endothelial cells, the presence of active
pumps for sugars amines, and some ions, and what parts of the brain lay outside of the BBB.
Characteristics of a drug that will penetrate the BBB included low molecular weight, good
lipid solubility, a low volume of distribution, low potency and low protein binding. Also
expected was a discussion of Fick’s Law and the drug features that would allow a high
concentration gradient in the cerebral blood vessels to be achieved and a high diffusion
coefficient and mention of drugs that resemble natural ligands for active transport
mechanisms.
Syllabus - G12d G2a
References: Pharmacology, Rang and Dale 6th edition page 476, Principles of physiology for
the anaesthetist, Kam page39-40, Pharmacology and Physiology in Anaesthetic Practice,
Stoelting, Pg 681, Textbook of medical physiology, Guyton and Hal11th edition page 766.
Foundations of Anesthesia. Hemmings and Hopkins. 2nd edition.