BUILDING THE BRAIN

From LeDoux Synaptic Self

3/18/2007

“…how do cells in the developing embryo become neurons, and how do they end up in just the right places.  How do the axons of all these cells find their way to their target areas?  And once having reached them, how do the terminals figure out exactly which neurons to make synapses with?  Because the various steps take time, and because different circuits go through these steps on different schedules, our behavioral and mental repertoire unfolds gradually, and unevenly, during childhood.  Somehow, though, it all comes together, and a person, a self with all its aspects, emerges.  (LeDoux/ss/65)”

Well, it is supposed to

“The job of genes is to make proteins, which regulate many aspects of brain development.  Some proteins are enzymes that trigger chemical reactions, others induce additional genes to make additional proteins, some form barriers that guide and restrict the many cell movements that take place, and still others provide adhesive surfaces on cells to which other cells cling while making their way to their final destination.  When we speak of genetic influences on brain development, we are essentially describing the effects of proteins and their chemical spin-offs.  (LeDoux/ss/66)”

“The embryo cannot make on its own the amino acids that are used to assemble the proteins that are required from brain and body development.  They have to be obtained from the mother….  (LeDoux/ss/66)”

“Although the major features of the brain are dictated by a genetic plan (which guarantees that all human brains look and work much the same), this plan requires certain conditions in the internal chemical environment in which neurons will grow.  If this gene-internal environment interaction is perturbed, so, too, will be the normal development of the brain.  Nature and nurture interact from the start.  (LeDoux/ss/67)”

Not normal in = not normal out

“Brain development begins in the ectoderm, which, together with the mesoderm and endoderm, make  up the three major parts of the embryo.  These give rise to the various regions and organs of the body.  A thickening of a part of the ectoderm forms the neural plate, which folds to form the neural tube, out of which the brain begins to be constructed.  (LeDoux/ss/67)”

“In humans, the vast majority of neurons are made in the months just prior to birth.  At the point, about 250,000 neurons are being generated per minute.  (LeDoux/ss/67)”

“It’ important to make a distinction here between neurogenesis – the birth of new neurons – and synaptogenesis – the creation of new synapses between existing neurons.  Synaptogenesis is a common phe- (LeDoux/ss/67) nomenon, one that probably occurs up until the moment of death….  Synapses are changed every time our brain records an experience.  (LeDoux/ss/68)”

“Somewhere between 50 and 70 percent of all genes in the human body are believed to be in the brain.  (LeDoux/ss/72)”

“Is the self sculpted from a preexisting set of synaptic choices, or does experience instruct and add to the synaptic basis of the self as we go through early life?  …since environmentally triggered neural activity is involved in both instructing and selecting connectivity, this is not so much a debate about genes vs. environmental experiences as one about the precise contribution of experience.  (LeDoux/ss/72)”

“Paraphrasing Socrates, Jerne noted that “learning consists of being reminded of what is already in the brain.  (LeDoux/ss/73)”

“…synapses in the brain, like animals in their environments, compete to stay alive.  Synapses that are used compete successfully and survive, while those that are not used perish.  According to Edelman, “The pattern of neural circuitry … is neither established nor rearranged instructively in response to external influences.”  External influences, instead, select synapses by initiating and reinforcing certain patterns of neural activity that involve them.  (LeDoux/ss/73)”

“Selectionists also assume that there’s a good deal of randomness involved – terminals and dendrites that happen to be in the same vicinity take the opportunity to form synaptic connections, independent of the overall guidance plan specified by genes.  As a result, in spite of a general genetically programmed plan, the preexisting connections upon which selection ultimately operates also have a unique, individualistic nature, from which experience then does the selecting.  (LeDoux/ss/73)”