The central nervous system consists of the brain and spinal cord. It is the integrating portion of the nervous system, where sensory information is received and processed, thoughts are generated, and responses are directed. The central nervous system consists primarily of association neurons-somewhere between 10 and 100 billion of them!

The brain and spinal cord are protected in three ways. The first line of defense is a bony armor, consisting of the skull that surrounds the brain and the vertebral column that protects the spinal cord. Beneath the bones lies a triple layer of connective tissue called meninges. Between the layers of the meninges, a clear lymph-like liquid, the cerebrospinal fluid, cushions the brain and spinal cord.