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."
|The Future of the Brain: The Promise and Perils of Tomorrow's Neuroscience
|Oxford University Press
|Copyright 2005 by Steven Rose