What important function does the blood-brain barrier serve? How might this barrier be problematic at times?

The blood-brain barrier serves to restrict the passage of toxic substances from the blood circulatory system and into the cerebral fluid.

This barrier has a downside: the majority of potential drug treatments do not cross it, this is an impediment to treating mental and neurological disorders.

Explanation:

The blood-brain barrier is a protection system against foreign substances that could cause brain infections, and is formed by endothelial cells that line the brain blood vessels.

The barrier allows only small molecules to pass through the capillary wall and into brain tissue. Larger molecules can gain entry through transporter proteins, such is the case of the glucose needed by the brain.

A dysfunction in the barrier may involve increased infiltration of cells in brain tissue, and this has been associated with neurological disordes, such as Parkinson's, Alzheimer's, and multiple sclerosis.

A weakening of the blood-brain barrier can precede, accelerate or contribute to a series of neurodegenerative disorders.