The adult brain is not fixed — it continues to form new synaptic connections, strengthen existing ones, and even generate new neurons in the hippocampus throughout life. Understanding how neuroplasticity works at the mechanistic level (LTP, BDNF, synaptic pruning, adult neurogenesis) changes how you approach cognitive enhancement: aerobic exercise is the strongest single stimulus, no supplement comes close to its effect on hippocampal structure, and chronic cortisol is the most destructive inhibitor. The complete four-layer protocol: exercise foundation, sleep consolidation, structural supplement stack, cortisol protection.