Science writer Carl Zimmer's new column in Discover is a fascinating exploration of how neurons find each other in the brain to form synapses. Neurons receive signals on one end through branch-like dentrites and transmit the signal via a long snaking outgrowth called an axon. But how does an axon know which neurons it's supposed to find in the vast forest of the brain?
The first order of business for new nerve cells is finding where, among the 100 billion neurons of the nervous system, their partners are waiting. They do so by following a chemical trail. The tip of the axon, called a growth cone, senses chemicals drifting by. Responding to these cues, the axon grows like a vine toward attractive chemicals and away from repellent ones. The chemical cocktail has a different flavor from one part of the body (or brain) to the next.
The nervous system further directs these wandering axons by placing guide cells along their path. Some guide cells release navigational chemicals. Others become part of the path itself, as migrating axons grab the cells and climb them like ropes. Guide cells even babysit axons that arrive at a destination early, before a partner cell is available to connect. Without a viable partner the axon would die; the guide cells form temporary synapses with the axons until the intended target is found.