THE NERVOUS SYSTEM
Consisting of the brain, spinal cord, sense organs, and nerves, the nervous system is the principal means of communication between the body and the external environment.

The human nervous system is divided into two parts. The central nervous system (CNS), housed entirely within the protective structures of the skull and vertebral column, consists of the brain and spinal cord. The peripheral nervous system (PNS), distributed throughout the body, activates all parts of the body and transmits information to and from the CNS.

These two systems interact constantly. The CNS acts upon signals provided to it by the PNS. A resulting action carries "instructions" as part of an electrochemical stimulus passed from the CNS to a muscle, gland, or organ, which responds with an action.

The instructions that travel away from the CNS may perform their function as part of the somatic nervous system (SNS), which controls organs under voluntary control (mainly muscles). However, other reactions occur through the autonomic nervous system (ANS), which regulates individual organ function and homeostasis. For the most part, the ANS, also known as the visceral or automatic system, is not subject to voluntary control.