Brains, Biology, and Behavior

Peripheral Nervous System

The peripheral nervous system, made up of neurons outside the brain and spinal cord, transmits messages throughout the body. The somatic nervous system allows voluntary control of skeletal muscles, whereas the autonomic nervous system automatically controls internal organs and glands.
The nervous system is divided into two broad categories: central and peripheral. The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is the system of neurons outside the brain and spinal cord that transmits messages throughout the body. This system is also divided into two categories: the somatic and autonomic nervous systems.

Divisions of the Nervous System

The nervous system is subdivided into the central and peripheral nervous systems. The peripheral nervous system is further divided into the somatic nervous system, which allows voluntary control of muscles, and the autonomic nervous system, which handles automatic processes. In the autonomic nervous system, the sympathetic division controls the body's fight-or-flight (threat) response, and the parasympathetic division controls homeostasis.
The somatic nervous system is responsible for voluntary control of skeletal muscles. Motor neurons (also called efferent neurons) carry messages from the brain to muscles. This allows for intentional control of movements. A circuit linking motor neurons to the spinal cord makes fast reflexes possible. The somatic nervous system also includes sensory neurons (also called afferent neurons). These neurons carry messages about sensations from the body to the brain. The autonomic nervous system controls automatic, involuntary bodily functions. It regulates the activity of internal organs and glands, controls breathing and heart rate, and manages digestion. The autonomic nervous system has sympathetic and parasympathetic divisions. The sympathetic and parasympathetic divisions balance one another. The sympathetic nervous system increases arousal (the fight-or-flight response). It prepares the body for action by increasing breathing, heart rate, and blood pressure. It promotes sweating to cool the body and dilates the pupils to take in more visual information. In contrast the parasympathetic nervous system calms the body and conserves energy. It promotes digestion and helps to maintain the body in a normal resting state.

Autonomic Nervous System

The sympathetic nervous system activates the body to deal with emergencies. The parasympathetic nervous system manages resting state activities, such as digestion.