We then turn to building the brain, which requires a combination of pathfinding, refining connections, and maintaining those that result in appropriate function by an additional, activity-dependent mechanism. We briefly discuss the embryology of brain development, and then consider how neurons can be precisely mapped onto a specific region of the brain, using development of the visual system as the best understood example. The pathfinding mechanisms discussed previously can bring neurons into the vicinity of target cells. For a neuron to survive and synapse successfully, it must be stimulated by a neurotrophic factor released from the target cell, These factors are specific for different neuronal types. They bind to a class of RTK receptors, encoded by genes of the trk family, named for a proto-oncogene that was the first family member to be identified.
Innervation is a competitive process.
In general, neurons are produced in excess, and more innervate a target tissue than needed in the adult. The target tissues produce only limited amounts of the required neurotrophic factors. This is presumably a way to insure that all target cells become innervated. In vertebrates, about half of all neurons in both the PNS and CNS die shortly after their axons reach the vicinity of their targets. The best understood example of how ordered connections to a specific brain region are established and refined comes from study of how axons from vertebrate retinal neurons find their way to and synapse with the correct cells in the optic tectum of the visual cortex to produce an accurate visual map in the brain.