The Peripheral Nervous System (PNS) is the nervous system outside of the brain and spinal cord, largely consisting of nerves that travel from these areas to other parts of the body. It's function is to connect limbs and organs to the Central Nervous System (CNS). 

The PNS is broken into the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of skeletal muscle, peripheral nerve fibers, and motor nerve fibers- it connects body movements and external stimuli. The autonomic nervous system controls the internal organs and glands. 

Since it is not protected by the blood-brain barrier like the CNS, the PNS is left exposed to injury and toxins.