blood-brain barrier
A mechanism that prevents some substances in the blood from reaching the brain. It is achieved by brain capillaries, which unlike other capillaries elsewhere in the body, are composed of endothelial cells sealed together in continuous tight junctions and surrounded by astrocytes that contribute to the selective passage of substances. Lipid-soluble substances such as alcohol, caffeine, nicotine and most anaesthetics, as well as glucose, oxygen and water, pass rapidly into brain cells, whereas proteins, most antibiotics and ions do not enter or enter very slowly. The mechanism protects brain cells against harmful substances and pathogens. See central nervous system.

central nervous system (CNS)
Etymology: Gk, kentron + L, nervus, nerve; Gk, systema one of the two main divisions of the nervous system, consisting of the brain and the spinal cord. The central nervous system processes information to and from the peripheral nervous system and is the main network of coordination and control for the entire body. The brain controls many functions and sensations, such as sleep, sexual activity, muscular movement, hunger, thirst, memory, and emotions. The spinal cord extends various types of nerve fibers from the brain and acts as a switching and relay terminal for the peripheral nervous system. The 12 pairs of cranial nerves emerge directly from the brain. Sensory nerves and motor nerves of the peripheral system leave the spinal cord separately between the vertebrae but unite to form 31 pairs of spinal nerves containing sensory fibers and motor fibers. More than 10 billion neurons constitute but one tenth of the brain cells; the other cells consist of neuroglia that support the neurons. The neurons and the neuroglia form the soft, jellylike substance of the brain, which is supported and protected by the skull. The brain and the spinal cord are