Figure 2

A) Neurons throughout the nervous system send axonal projections over distances ranging from microns to meters. For large or anatomically specialized animals such as the giraffe or the whale, more than 5 meters may separate the neuron cell body from the distal axon terminal. B) During development, neurons establish trophic interactions with target tissues. As an organism develops, the strength and maintenance of these trophic interactions determine whether neurons survive or die. Soluble protein trophic factors released by the target tissue (1) bind to transmembrane receptors on the presynaptic axon terminal (2), inducing receptor activation and the induction of intracellular signaling cascades (3). These signals must travel from the site of initiation to the distant cell body (4) in order to enter the nucleus and elicit transcriptional changes that determine the survival of the cell. This long-distance information transfer is a universal theme in neurodevelopment.