|
|
Re: Holloway/Morgan ...the BAH....
Ralph L Holloway (rlh2@columbia.edu)
Sat, 29 Jul 1995 18:15:49 -0400
On 29 Jul 1995, Lloyd Jacobs wrote:
> As we became more and more dependent on our intelligence, our brain had
> to expand to make sure we had a good supply of brain cells to last ever
> longer lifespans. Culture came into play and having lucid old folk
> would help provide continuity in that culture. As well in case of head
> injury, extra cells might come in handy in preserving the all-important
> intellect.
An interesting idea here. The loss of brain cells with aging probably
starts immediately after adulthood is reached, i.e., age 20-25, and prior
to that there are phases of "selective cell death", genetically
programmed. The lost is gradual, but Brody once estimated that about 1/3
of the neurons were gone by age 65. After age 65, the rate of loss is
greater. Today, there is considerable controversy about how meaningful or
serious the loss is to intelligent social behavior. Most of the fossil
hominids had fairly short lives, with a few exceptions, and thus their
brain sizes were largest in their living days, before aging would have
wiped them out, so to speak. I think it is easier to look at the larger
size as something coming out of embryological development that allowed
for early learning to be efficient and adaptive, rather than as a buffer
against aging. It is not clear that the regulator genes responsible for
hypertrphy and hyperplasia of the nerve cells (neurons and neuroglia)
operate very differently between ourselves and chimpanzees. Noone has
idetified any such genes yet, that i know of. But the hyperplastic
aspect seems to commence early in humans, perhaps lasting somewhat longer
than in chimps, and then is followed by a longer period of hypertrophy,
during which as the cell size increases, and the perikarya enlarge, there
is more material available to increase the amount of dendritic branching
in the nerve cells, and thus synaptic connectivity, and thus too,
neuroglia ratios, as these are important for metabolic sustenance and
exchange.
The recent announcement, by the way, by Witelson et al, regarding an
increased neural density in female brains should mean LESS, not MORE,
synaptic connectivity, if the neural geometrics are the same between
males and females.
Ralph Holloway.
|