In the peripheral and central neurons, neurotrophins are important regulators for survival, differentiation, and maintenance of nerve cells. They are small proteins that secrete into the nervous system to help keep nerve cells alive. There are two distinct classes of glycosylated receptors that can bind to neurotrophins. These two proteins are p75 (NTR), which binds to all neurotrophins, and subtypes of Trk, which are only specific for each different neurotrophins. The signaling pathways activated by neurotrophins through Trk receptors result in many neuronal functions, such as cell survival, differentiation, dendritic arborization, synapse formation, plasticity, axonal growth, and axonal guidance. Functions ascribed to the p75NTR receptor are diverse, complex, and sometimes contradictory. p75NTR has been implicated in both promoting survival and inducing apoptosis, enhancing neurite outgrowth and facilitating growth-cone collapse, and mediating differentiation and enhancing proliferation. Moreover, p75NTR may also play a role in myelination.
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 or beta-NGF), a vertebrate protein that stimulates division and differentiation of sympathetic and embryonic sensory 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.