Aging of the brain is not enough to cause Alzheimer's or any of these other disorders. Over time the structure of the brain is known to change. It grows lighter, for example, and shrinks slightly. One of the cliches of neurology is that the human brain loses roughly a million neurons per year as it ages; this created a convenient rationale for seeing the onset of senility as the result of a decaying brain. However, the flaw in this explanation is that people who don't become senile presumably have lost the same number of neurons (this has to be left to conjecture, since reliable neuron counts can't be made in living people). At present it isn't known why one old brain stays lively and creative-one thinks of Michelangelo designing St. Peter's when he was nearly 90 or Picasso painting at the same age and Arthur Rubinstein playing the piano in Carnegie Hall- while another starts to deteriorate. One theory, based on animal research, is that our brains grow new connections as we age. As more neurons die, these new connections may compensate for the loss in some individuals.
No brain cells never actually touch physically. They reach toward each other across a gap, or synapse, using hundreds or thousands of hairlike filaments called dendrites. The effects is like that of two twiggy trees almost intertwining in the wind (the word dendrite is derived from the Greek word for "tree"). Just at the point where two filaments almost meet, a signal can be sent from one neuron to another. One of them is acetylcholine, the lack of which leads to Parkinson's disease for sending messages while others grow ten thousands. No one knows precisely why some neurons grow fifty dendrites for sending messages while others grow ten thousand. One encouraging finding, however, is that by remaining mentally active, older people may actually be growing new dendrites all the time. This much – publicized news was founded on brain research by Marian Diamond at Berkeley, who showed that the brains of rat grew or shrank according to the kind of experiences they were exposed to.
