Now let’s consider a growing neuron. It extends a projection that has branched into two scout arms (“growth cones”) heading toward two neurons. Simplifying brain development to a single mechanism, each target neuron is attracting the growth cone by secreting a gradient of “attractant” molecules. One target is “better,” thus secreting more of the attractant, resulting in a growth cone reaching it first—which causes a tubule inside that growing neuron’s projection to bend in that direction, to be attracted to that direction. Which makes the parallel tubule adjacent to it more likely to do the
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