delanceyplace.com 2/12/10 - more on brains

In today s excerpt - human brains have almost
all of their 100 billion neurons in place at
birth, with some 250,000 being born every
minute during gestation, and these brains are
almost identical to all other human brains,
since even a slight variation can be lethal:

The trajectory by which a fusion of human
sperm and
ovum results, over nine months gestation, in
some 3-4 kilos of baby,
fully equipped with internal organs, limbs,
and a brain with most of its
100 billion neurons in place, is relatively
easy to describe, even
when it is hard to explain.

All humans are alike in very
many respects, all are different in some. (No
two individuals, not even
monozygotic twins, are entirely identical,
even at birth.) Yet chemically,
anatomically and physiologically there is
astonishingly little obvious variation to be
found between brains, even from people from
widely different
populations. Barring gross developmental
damage, the same structures
and substances repeat in every human brain,
from the chemistry of their
neurotransmitters to the wrinkles on the
surface of the cerebral cortex.
Humans differ substantially in size and
shape, and so do our brains, but
when a correction is made for body size, then
our brains are closely
matched in mass and structure, though men's
brains are slightly heavier
on average than are women's. So similar are
they though, that imagers
using PET (positron emission tomography) and
MRI (magnetic resonance imaging) have been
able to develop algorithms by which they can
transform and project the image derived from
any individual into a 'standard' brain.
Brains are so finely tuned to function, so
limited by
constraints, that anything more than
relatively minor variation is simply
lethal.

Of no body organ is the developmental
sequence more simultaneously dramatic and
enigmatic than the brain. How to explain the
complexity and apparent precision with which
individual neurons are
born, migrate to their appropriate final
sites, and make the connections
which ensure that the newborn on its arrival
into the outside world has
a nervous system so fully organized that the
baby can already see, hear,
feel, voice her needs, and move her limbs?
The fact that this is possible
implies that the baby at birth must have most
of her complement of
neurons already in place - if not the entire
100 billion, then getting on
for that number. If we assume a steady birth
of cells over the whole
nine months - although of course in reality
growth is much more
uneven, with periodic growth spurts and lags
- it would mean some
250,000 nerve cells being born every minute
of every day over the period.
As if this figure is not extraordinary
enough, such is the density of
connections between these neurons that we
must imagine up to 30,000
synapses a second being made over the period
for every square
centimeter of newborn cortical surface. And
to this rapid rate of production must be
added that of the glia, packing the white
matter below the
cortex and surrounding the neurons within it
- though admittedly they
do not reach their full complement by birth
but continue to be generated throughout
life.

Steven Rose, The Future of the Brain, Oxford,
Copyright 2005 by Steven Rose, pp. 57-63.