The nervous system can be subdivided into the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of peripheral nerves that pass signals from the skin to the brain, or from the brain to the muscles and organs, such as the heart and intestines. Several subtypes of nerve fibres exist that differ in thickness and in the presence or absence of an isolating layer (myelin). Each subtype has its own function. Large nerve fibres are sensory and motor nerve fibers. Small nerve fibres are the terminal branches of the nerves, which terminate in the skin, and these are responsible for the sensation of pain and temperature.
Besides these, there are small nerve fibres that form a connection between the brain and several organs. These small nerve fibres are responsible for autonomic functions, such as regulation of blood pressure, gastrointestinal functions and perspiration. The small nerve fibres are also called Aδ- and C-fibres. The Aδ-fibres contain a thin layer of myelin, and the C-fibres are unmyelinated.