The nervous system functions as the control center for all body activities and responds to changes inside and outside the body. It has two main divisions - the central nervous system and the peripheral nervous system. The central nervous system is made up of the brain and spinal cord. It contains neurons which are microscopic nerve cells that transmit electrical and chemical signals. The peripheral nervous system includes nerves that connect the central nervous system to other parts of the body and allow for sensation and movement. Common problems of the nervous system include concussions, paralysis, Parkinson's disease, Alzheimer's disease, epilepsy, schizophrenia, and depression.