Alzheimer’s and aging

At medical school we learnt very few certainties. Everything was a possibility, a probability, a risk, a trend, a likelihood. There were many rules of thumb, but very few rules. Yet amongst all the qualified circumspection, there was one iron-clad law: The human brain grows no new neurons. All the neurons we would ever have were present at birth, and from there it was all rapidly downhill for our neuron tally.