Neurotrophins induce the survival, development and function of neurons. During nerve development, neurotrophins help nerve cells to decide whether they are going to live or die. Neurotrophins are small proteins secreted in the nervous system at extremely low concentrations. Low levels of a neurotrophic factor are needed to keep nerve cells alive. However, in some cases, presence of a neurotrophin can have the opposite effect, initiating cell-death. During development of the nervous system, levels of neurotrophins control unwanted nerve cells. Later, neurotrophins stimulate the growth of new dendrites and to cause unwanted dendrites to die back from crowded areas. Neurotrophins also induce differentiation of progenitor cells, to form neurons.