In general, neurons do not undergo cell division—except perhaps for cells in the hippocampus and olfactory bulb. So you have what you have and they need to last your entire life. That’s one reason for making careful choices about how to take care of these vital cells. Although protected by your bony skull, neurons can be easily damaged. Blunt trauma, for example, can break off some of the axons, interfering with the transmission of information. That’s part of the controversy about some types of sports that can damage these delicate but vital cells. Pugilistic Parkinson’s is one example of problems that can arise from some types of sports. Wear a helmet when doing any type of sport or activity that might result in an injury to your head. A broken bone often heals ‘as good as new.’ A broken ‘head’ rarely does. If you smoke, stop. If you don’t, never start. Be careful about breathing in side smoke from tobacco products or from vehicle exhaust. Stay alert and avoid falls—a huge cause of head injuries and often of deteriorating brain function.