Axon guidance

The nervous system has the job of controlling and monitoring events throughout the whole body. But, unlike endocrine cells, which control their targets through long-range chemical cues, neurons communicate through direct cellular contacts, or synapses. As the information content of the electrical signals used by neurons is limited, much of the meaning in neural communication is conveyed by the specificity of the connections between the origin of the signal and its recipient. A failure to make or maintain the appropriate connections can therefore alter the interpretation of neural signals. Thus, precise connectivity is one of the most important issues in constructing a functional neural architecture. The establishment of correct neural connections begins with a neuron at some distance from its target. Neural connections are formed by means of specialized cellular processes: axons that send signals, and dendrites that receive them. Most neurons send out only one axon. In order to reach the correct target, this axon must be headed in the right direction. Because many neurons are far from their targets, the navigation of an axon to the correct destination often requires a series of decisions that will bring the axon closer and closer to its goal. The process that determines each correct decision is called axon guidance. Despite the immense complexity of the nervous system, axon guidance choices are achieved with remarkable accuracy in vivo.