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Axons find their way to the terminus by responding to axon guidance molecules (AGMs) that attract and repel growth cones or make them stir. This I understand.

Through a very specific combination of AGMs, a precise path can be layed out that an axon cone will follow even across half the brain. This I also understand.

What I don't understand is that in developing brain, there are millions of axons moving to their termini at the same time, and millions of AGMs released all over the brain. When an axon departs on a journey to reach an AGM at the opposite end of the brain, it will probably pass by thousands of other AGMs and yet ignore them all.

When an AGM is released, there are like thousands of neurons nearby, and yet they all will ignore it but some single neuron across the brain will reach for it.

I can think of two possible explanations:

  1. There are unique AGMs for every neuron that no other neurons will follow (but it doesn't seem to be the case).
  2. There are somehow very specific instructions wired into EACH neuron aka ignore five netrins, then follow the sixth, then ignore two more, and then follow the ninth one till you reach it (but how is it possible to program every neuron in the brain like that?).

So, how does it work in reality?

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