of Aa fibers that were sensory fibers. These were then introduced into a system that only included sensory fibers (though some of these were mixed nerves and were also motor fibers). This system refers to the sensory groups as Types and uses Roman numerals: Type Ia, Type Ib, Type II, Type III, and Type IV. Motor Lower motor neurons have two kind of fibers: Different sensory receptors innervate different types of nerve fibers. Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers and nociceptors and thermoreceptors by type III and IV sensory fibers. Autonomic The autonomic nervous system has two kinds of peripheral fibers: Clinical significance In order of degree of severity, injury to a nerve can be described as neurapraxia, axonotmesis, or neurotmesis. Concussion is considered a mild form of diffuse axonal injury. Axonal injury can also cause central chromatolysis. The dysfunction of axons in the nervous system is one of the major causes of many inherited neurological disorders that affect both peripheral and central neurons. When an axon is crushed, an active process of axonal degeneration takes place at the part of the axon furthest from the cell body. This degeneration takes place quickly following the injury, with the part of the axon being sealed off at the membranes and broken down by macrophages. This is known as Wallerian degeneration. Dying back of an axon can also take place in many neurodegenerative diseases, particularly when axonal transport is impaired, this is known as Wallerian-like degeneration. Studies suggest that the degeneration happens as a result of the axonal protein NMNAT2, being prevented from reaching all of the axon. Demyelination of axons causes the multitude of neurological symptoms found in the disease multiple sclerosis. Dysmyelination is the abnormal formation of the myelin sheath. This is implicated in several leukodystrophies, and also in schizophrenia. A severe traumatic brain injury can result in widespread lesions to nerve tracts damaging the axons in a condition known as diffuse axonal injury. This can lead to a persistent vegetative state. It has been shown in studies on the rat that axonal damage from a single mild traumatic brain injury, can leave a susceptibility to further damage, after repeated mild traumatic brain injuries. A nerve guidance conduit is an artificial means of guiding axon growth to enable neuroregeneration, and is one of the many treatments used