The blood-brain barrier

A) permits passage of foreign substances from the blood to the neurons.
B) prohibits the transport of amino acids and glucose to the neurons.
C) prohibits the removal of waste materials from the neurons.
D) protects neurons from toxic substances in the blood.
E) does not prevent fluctuations in the composition of the blood from affecting the functions of the brain.

The blood-brain barrier (BBB) is a selective barrier that protects the brain from potentially harmful substances in the blood while allowing essential nutrients to pass through. It does not permit most foreign substances to pass, nor does it prohibit the transport of necessary nutrients like amino acids and glucose. It also allows for the removal of waste materials from neurons. The BBB helps maintain a stable environment for the brain, contrary to option E, which suggests it does not prevent fluctuations in blood composition from affecting brain functions.