Robustness?

[Bill Skaggs]

I wrote:

   >Neurons do die every so often, of course, but the rate 
   >has never been quantified (as far as I know) because 
   >it's so low, certainly much fewer than a thousand per 
   >day in ordinary, healthy people.

I was wrong.  There have been a number of studies of neuron
loss in aging.  It proceeds at different rates in different
parts of the brain, with some parts showing hardly any loss
at all.  Even in different areas of cortex the rates of loss
vary widely, but it looks like, overall, about 20% of the
neurons in cortex are lost by age 60.  

Using the standard estimate of ten billion neurons in the 
neocortex, this works out to about one hundred thousand
neurons lost per day of adult life.

Reference: "Neuron numbers and sizes in aging brain: Comparisons 
of human, monkey, and rodent data", DG Flood & PD Coleman,
*Neurobiology of Aging* 9:453-464 (1988).

Thanks to the people who wrote to me about this.

	-- Bill