What you learned in school
Neuroscience textbooks taught that humans were born with all the brain cells they would ever have. After early childhood development, the brain was considered "fixed" and incapable of generating new neurons. Students learned that brain damage was permanent because dead neurons could never be replaced. This fundamental principle was taught as established fact in medical schools and shaped decades of neuroscience education.
What we know now
Neuroscience textbooks taught that humans are born with all the brain cells they'll ever have. After childhood brain development, no new neurons could be generated. Students learned that brain damage was permanent because dead neurons couldn't be replaced. This was considered an established fact about the adult brain. Research beginning in the 1990s discovered neurogenesis - the birth of new neurons - continues throughout adult life, particularly in the hippocampus. Exercise, learning, and environmental enrichment can promote new neuron growth. This discovery revolutionized understanding of brain plasticity and recovery.