True or false? The human body constantly replenishes old nerve cells in the brain with new cells.

a) True. A proper diet and mental stimulation are key to this process, which occurs over the course of a lifetime.
b) False. Nerve cells in the brain are not replaced but can live a long time.
c) Scientists aren't sure what happens to nerve cells.

Unlike most other body cells, nerve cells in the brain live a long time and have the capacity to survive more than 100 years. But when nerve cells die because of disease or injury, they are not replaced. Scientists still debate the degree of changes in the brain that occur with age.