axon guidance

Axon guidance is the process by which a growing nerve fiber finds its way through the developing body and plugs into exactly the right target. A neuron (nerve cell) sends out one long cable, the axon, that carries its signals away — and that axon may have to travel a very long distance, past thousands of other cells, to reach the muscle or brain region it is meant to connect to. Axon guidance is the set of road signs and traffic rules that keep it from getting lost. Picture a traveler crossing an unfamiliar country with no map, relying only on signposts planted along the way: turn here, keep going, do not enter. That is what a growing axon does, and axon guidance is the system of signposts that makes the journey come out right.

The signposts are molecules — guidance cues — scattered through the tissue the axon grows across. Some cues are attractive and say come closer, while others are repulsive and say keep away; some are glued to the surface of cells the axon crawls over, and others drift loose, forming a faint chemical scent that is strong in one spot and weak in another. At the very tip of the axon sits a restless, hand-shaped explorer called the growth cone, studded with sensors that read these cues. When the growth cone meets an attractive cue it steers toward it; when it meets a repulsive one it turns away. Famous families of these molecules — netrins, slits, semaphorins, and ephrins — work in pairs of cue and matching sensor, like a lock and its key, so that each axon responds only to the signs meant for it.

This careful steering is how the staggering wiring of the nervous system, with its billions of precisely placed connections, gets laid down in the first place. When axon guidance goes wrong, fibers wander to the wrong destinations, and the miswiring can show up as developmental disorders of the brain and eyes. The same guidance molecules can also reawaken after injury, which is part of why severed nerves in the adult brain and spinal cord struggle to find their way back — understanding the original road map is a first step toward helping damaged wiring regrow.