Nerve fibers, also known as axons, are very thin transmission lines that run from each nerve cell in the body to receptors in the skin, muscles, and internal organs. They are the main information lines of the central nervous system. Though nerve fibers are microscopic in diameter, they can be very long. One of the longest fibers runs from the sciatic nerve at tip of the spine down the leg to the tip of the big toe.

There are three main classes of nerve fibers: A, B, and C. Class A fibers transmit impulses related to muscle, tendon, and articular movement and situation. These fibers are myelinated, or sheathed in an electrically insulated material. Class B nerve fibers transmit autonomic or involuntary impulses and are also myelinated. Class C nerve fibers relay pain and temperature impulses and are not myelinated.

The first type of nerve fibers, Class A, are basically sensory nerve fibers and broken down into four main sub-classes: A-alpha, A-beta, A-gamma, and A-delta. Measuring in at about one-fifth of the diameter of a strand of human hair, the A class fibers are the thickest fibers and therefore the transmit impulses the fastest. A-alpha can transmit information as fast as 299 to 394 feet (about 70 to 120 meters) per second.

Class A-beta nerve fibers transmit information pertaining to touch and muscle movement at 131 to 299 feet (40 to 70 meters) per second. Information pertaining to touch and pressure is transmitted by Class A-gamma fibers at 49 to 131 feet (about 15 to 40 meters) per second. Class A-delta fibers transmit pain, touch, pressure, and temperature impulses at 16 to 49 feet (about 5 to 15 meters) per second.

Class B fibers carry preganglionic messages from the central nervous system to the ganglion, bundles of nerve cells that act as relay points, as part of the autonomic nervous system. The impulses traveling along Class C fibers, or the peripheral nerves, can move as slow as 7 inches (about .2 meters) per second. These impulses also relate to the autonomic nervous system and carry postganglionic information from the ganglion to organs concerning body functions that are beyond a persons control like pupil dilation, perspiration, and digestion

When the myelin sheath covering class A and B nerve fibers becomes damaged through heredity, hypoxia, nutritional deficiencies, or viral invasion of the central nervous system, impulse signals can not travel normally. This demyelination is part of the base cause of many diseases including Guillain-Barré syndrome and multiple sclerosis. In most cases, little can be done to reverse demyelination.