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Neural Development

The brain is not built once from a blueprint and shipped. Its storyline is overproduce, then cut back according to what actually gets used, then lock the survivors into the structure — which is why critical periods exist, and why they close.

The main line: proliferation → migration → differentiation → wiring

In the embryo, neural stem cells divide furiously along the ventricles. The new neurons don't stay put: they climb outward along radial glia, which act as ladders, stacking up the six layers of cortex — and they do it inside-out, with later-born neurons passing through the earlier ones to settle further out. Only once they've arrived do axons and dendrites grow, find their targets, and form synapses.

Overbuild first, then cut back by usage

Here's the counterintuitive stretch. Synapse density shoots past a peak well above adult levels in the first year or two, then is pruned steadily downward, not settling until around twenty. The prefrontal cortex is pruned last.

Which ones get deleted? The rule is the same one that governs adult plasticity: synapses that are used little, or whose activity doesn't line up with their partners, get tagged — and microglia then literally eat them. This isn't waste. Sketching an oversupply of connections and letting real experience decide which survive is cheaper in genes than specifying exact wiring, and it fits the environment the animal actually lands in.

Myelin: insulating the wires

The myelin wrapped around axons (made by oligodendrocytes in the central nervous system) speeds conduction up by tens of times and makes the timing between pathways precise. Its schedule runs long: sensory and motor pathways are insulated early, while the prefrontal cortex isn't finished until the mid-twenties — which lines up with the adolescent pattern of emotion arriving before the brakes do.

Critical periods and their brakes

Some circuits open a window during development in which they are unusually easy to change: experience during that stretch shapes the structure durably, and afterwards it becomes hard to shift. Ocular dominance in vision, speech-sound categories in hearing, accent in language — each has its own window.

So the accurate version of "plasticity declines in adulthood" is: the capacity isn't spent, the brain fitted brakes on purpose. That's the foundation for understanding where adult learning and rehabilitation can reach — and where they can't.