The development of synapses and the long-term survival of neurons are mediated by trophic factors that are secreted by the target nerve cells, bind to specific receptors, and signal to the nearby developing synapse. Within the nervous system, the most extensively studied of these factors is the family of neurotrophins. This accomplishment earned them the Nobel Prize in Medicine in 1986. Scientists have discovered several simple rules that describe how neurotrophins influence the growth of neurons. First, neurons require trophic factors to survive.
Neurons compete for the minute amounts of trophic factors that are produced.
Experiments have shown that when NGF is added into tissue cultures or injected directly into the CNS, the number of neurons, as well as their axons and dendrites, increases dramatically. Antibodies that bind to and thereby inactivate NGF have the opposite effect and lead to neuronal death. The brain grows dramatically over the first decade of life. The increase in brain size is due primarily to an increase in the number and the complexity of neuronal processes rather than to an increase in the total number of neurons. The neurotrophic hypothesis states that trophic factors are secreted by a neuron and diffuse to a nearby process of another neuron.