Neurotrophins are a family of neurotrophic factors including nerve growth factor (NGF) and brain-derived nerve growth factor (BDNF). One key characteristic of neurotrophins, and neurotrophic factors in general, is that synaptic targets release just enough of these chemicals to maintain the amount of innervation they need. If there are too many neurons synapsed to the target structure, some neurons won't receive enough neurotrophins to survive and will die out, and vice versa.