The primary function of the nervous system is to send electrical and chemical signals throughout the body. The nervous system senses changes within the body and the external environment. When this information is received, processed, and integrated, it is transmitted as a neural signal within the body, causing other body systems to produce a specific response. The nervous system also works with other body systems to respond to these internal and external changes.The nervous system is divided into two parts: the central nervous system (CNS) and peripheral nervous system (PNS). Both divisions are determined according to the anatomical location of each system in the body. The central nervous system (CNS) consists of the brain and spinal cord. It receives and processes information about the external and internal environments from sensory organs such as the nose and eyes. All nerves outside the CNS make up the peripheral nervous system (PNS), which is divided into the autonomic and somatic nervous systems. The somatic nervous system transmits neural signals from the CNS to the skeletal muscles in the lower and upper limbs of the body. The autonomic nervous system is used to involuntarily control smooth muscles found in the walls of organs and glands. This system is divided into the sympathetic and parasympathetic systems. The sympathetic nervous system activates the body to move and stimulates the fight-or-flight response. The parasympathetic nervous system calms the body and helps the body "rest and digest."