Brain damage

The brain is unique among the body's organs: it stores our memories and personality, so that it cannot simply be replaced if it starts to wear out. This poses a special problem for life extension: the information stored in the brain must be preserved over extended periods of time, safe from disease and accident.

Obviously it is good to prevent the premature death of neurons. Poisons such as alcohol, accidents such as stroke, and diseases such as Alzheimer's can all cause neurons to die. In each of these cases, neuron death can be greatly slowed if not prevented entirely by controlling the chemistry inside the cell. Injurious chemicals can be vacuumed up and converted into harmless ones. Damaged neurons, like other cells, sometimes go into suicide mode (called "apoptosis"); as mentioned above, this can be chemically prevented, and the neuron can be stabilized until the problem is fixed and the damage is repaired.
It is now known that brain cells do regenerate: the brain is adding new ones all the time. This implies that some neural death is normal. How do the new cells know how to behave? It seems that a new neuron can take its cues from the existing ones; this means that a person's mind may be intact even after the death and replacement of a large percentage of their neurons.

Finally, it may be possible to measure neural connections and/or activity in enough detail to simulate the firing pattern. This may make it possible to create an artificial neuron or even an artificial neural net that can be used to replace missing neurons and retain old memories. But even if this proves to be impossible, the worst-case scenario is one in which people can't remember much farther than a century back. We accept more memory loss than this as a natural consequence of aging.