Physiological description of the neuron and the human nervous system

In order to maintain equilibrium, the human body is constantly reacting to and adjusting to changes in the outside environment, as well as within the body (internal environment). Such environmental changes, or stimuli, initiate impulses in sensory receptors. These impulses (or inputs) pass via the nerves to the brain and spinal cord, where they are analyzed, combined, compared, and coordinated. After being sorted out, impulses are conveyed by nerves to the muscles and glands of the body. The nervous system expresses itself through muscles and glands, causing muscles to contract or relax, and glands to secrete or not secrete their products. Stimuli received by the nervous system are processed rapidly through a combination of electrical impulses and chemical substances called neurotransmitters for communication between a nerve cell and a muscle cell, or between a nerve cell and glandular cells. Although the nervous system is a single, unified communications network, it is divided on a gross anatomical basis into the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord; the peripheral nervous system includes all the nerve cells and fibers that lie outside the brain and spinal cord. The peripheral nervous system connects the brain and spinal cord with the rest of the body.