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 for different kinds of nerve injury. terminology some general dictionaries define "nerve fiber" as any neuronal process, including both axons and dendrites. however, medical sources generally use "nerve fiber" to refer to the axon only. history german anatomist otto friedrich karl deiters is generally credited with the discovery of the axon by distinguishing it from the dendrites. swiss rüdolf albert von kölliker and german robert remak were the first to identify and characterize the axon initial segment. kölliker named the axon in 1896. louis-antoine ranvier was the first to describe the gaps or nodes found on axons and for this contribution these axonal features are now commonly referred to as the nodes of ranvier. santiago ramón y cajal, a spanish anatomist, proposed that axons were the output components of neurons, describing their functionality. joseph erlanger and herbert gasser earlier developed the classification system for peripheral nerve fibers, based on axonal conduction velocity, myelination, fiber size etc. alan hodgkin and andrew huxley also employed the squid giant axon (1939) and by 1952 they had obtained a full quantitative description of the ionic basis of the action potential, leading to the formulation of the hodgkin–huxley model. hodgkin and huxley were awarded jointly the nobel prize for this work in 1963. the formulae detailing axonal conductance were extended to vertebrates in the frankenhaeuser–huxley equations. the understanding of the biochemical basis for action potential propagation has advanced further, and includes many details about individual ion channels. other animals the axons in invertebrates have been extensively studied. the longfin inshore squid, often used as a model organism has the longest known axon. the giant squid has the largest axon known. its size ranges from 0.5 (typically) to 1 mm in diameter and is used in the control of its jet propulsion system. the fastest recorded conduction speed of 210 m/s, is found in the ensheathed axons of some pelagic penaeid shrimps and the usual range is between 90 and 200 meters/s (cf 100–120 m/s for the fastest myelinated vertebrate axon.) in other cases as seen in rat studies an axon originates from a dendrite; such axons are said to have "dendritic origin". some axons with dendritic origin similarly have a "proximal" initial segment that starts directly at the axon origin, while others have a "distal" initial segment, discernibly separated from the axon origin. in many species some of the neurons have axons that emanate from the dendrite and not from the cell body, and these are known as axon-carrying dendrites. in many cases, an axon originates at an axon hillock on the soma; such axons are said to have "somatic origin". some axons with somatic origin have a "proximal" initial segment adjacent the axon hillock, while others have a "distal" initial segment, separated from the soma by an extended axon hillock.