<P> The nerve begins the process by destroying the nerve distal to the site of injury allowing Schwann cells, basal lamina, and the neurilemma near the injury to begin producing a regeneration tube . Nerve growth factors are produced causing many nerve sprouts to bud . When one of the growth processes finds the regeneration tube, it begins to grow rapidly towards its original destination guided the entire time by the regeneration tube . Nerve regeneration is very slow and can take up to several months to complete . While this process does repair some nerves, there will still be some functional deficit as the repairs are not perfect . </P> <P> A neuron is called identified if it has properties that distinguish it from every other neuron in the same animal--properties such as location, neurotransmitter, gene expression pattern, and connectivity--and if every individual organism belonging to the same species has exactly one neuron with the same set of properties . In vertebrate nervous systems, very few neurons are "identified" in this sense . Researchers believe humans have none--but in simpler nervous systems, some or all neurons may be thus unique . </P> <P> In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish . Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right . Each Mauthner cell has an axon that crosses over, innervating (stimulating) neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes . The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C - shape, then straightens, thereby propelling itself rapidly forward . Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish . Mauthner cells are not the only identified neurons in fish--there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus . Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response . </P> <P> Mauthner cells have been described as command neurons . A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself . Such neurons appear most commonly in the fast escape systems of various species--the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid . The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances . </P>

Which is not a characteristic of nerve tissue