The peripheral nervous system (PNS) is the network of nerves that send information from all parts of the body to the brain and spinal cord, a group called the central nervous system (CNS). There are many functions of the peripheral nervous system, all serving a general purpose of transferring information for processing by the body. This information can come from sources outside or inside the body, depending on the position and purpose of any given nerve in the system. The human body contains thousands of nerves, and every one is vital to seamless functioning of the body.
There are two main functions of the peripheral nervous system, which are handled by two different types of cells called neurons. Sensory neurons register a change in the environment and pass it to the central nervous system for analysis. Once the CNS figures out what to do in response to the stimulus, motor neurons execute the change. It takes a fraction of a second for this loop to be completed, so the body can respond to changes within itself or in its environment very fast. The part of the PNS that is under voluntary control is called the somatic nervous system.
Certain functions of the peripheral nervous system are automatic. These nerves make up the autonomic nervous system, and connect the internal organs and glands to the CNS. They monitor activities like digestion and hormone secretion, and are controlled by a tightly regulated feedback system. Cardiac muscle in the heart is also controlled by this system, so it plays a major role in regulating heartbeat and circulation. Every organ and gland in the body is mostly or completely controlled by the autonomic nervous system, so any problems that arise can have a devastating effect on the body.
The peripheral nervous system also can control inflammation and prepare the body for emergencies. To suppress inflammation, signals are sent through the nerves that suppress chemicals which enhance the inflammatory response. Sometimes, these signals can completely control inflammation, but in the cases that they cannot, it doesn't necessarily mean something is wrong with the nerves, just that they need help. To prepare the body for emergencies, the PNS triggers the classic "fight or flight" reflex, which increases adrenaline in the blood and increases heart rate. These functions of the peripheral nervous system are diverse, but each one aids the body in a different situation.