More than 100 years ago, scientists discovered that when a blue dye was injected into an animal's bloodstream, all the tissues in its body would turn blue -- except the brain and spinal cord. They concluded that the brain had a special barrier to protect its cells from damaging substances. Today this is known as the blood/brain barrier. It is formed by more than 400 miles of narrow capillaries that run throughout the brain. These are filled with tightly packed cells that carry blood into the brain.
Elsewhere in the body, these vessel cells (called endothelials) are not as densely packed. There are small spaces between each individual cell so that substances can easily move between the inside and outside of the vessel. But in the brain, they fit tightly together so that substances cannot pass out of the bloodstream. This protects the brain from any foreign substances like toxins that may cause injury, and from the hormones and transmitters that cause so many changes in the rest of the body. The brain needs a calm, constant environment.
Large molecules cannot pass through the blood/brain barrier, but some light molecules can: alcohol, caffeine and nicotine, for example, as well as nutritional molecules such as oxygen and glucose. The problem is that the barrier can't differentiate between harmful toxins, and life-saving chemicals. Large-molecule drugs could possibly cure patients with neurological disorders, but none of them can cross the blood/brain barrier. So scientists are finding new ways to "trick" the barrier into letting them pass by devising a molecular "Trojan Horse": attaching them water-soluble molecules, for example; the barrier makes an exception for those types of larger molecules.