advexontube.com Seosan TV DOCUMENTARY HD ( WORLD BEST DOCUMENTARIES AROUND THE WORLD ) br br During the development of the vertebrate nervous system, many neurons become redundant (because they have died, failed to connect to target cells, etc.) and are eliminated. At the same time, developing neurons send out axon outgrowths that contact their target cells. Such cells control their degree of innervation (the number of axon connections) by the secretion of various specific neurotrophic factors that are essential for neuron survival. One of these is nerve growth factor (NGF), which is involved in the survival of some classes of embryonic neuron (e.g., peripheral sympathetic neurons). NGF is mostly found outside the central nervous system (CNS), but slight traces have been detected in adult CNS tissues, although a physiological role for this is unknown ; it has also been found in several snake venoms . Proteins similar to NGF include brain-derived neurotrophic factor (BDNF) and neurotrophins 3 to 7, all of which demonstrate neuron survival and outgrowth activities.br br In contrast to mammalian NGFs, which exist as multimeric complexes of alpha, beta and gamma subunits, snake venom NGFs exist almost exclusively as beta-chains [PMID: 8843580].