Discovering the Brain (1992) / Chapter Skim
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6 The Development and Shaping of the Brain
6 The Development and Shaping of the Brain
Pages 86-103
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From page 86... ...
Does the genetic material of the fertilized egg already contain a full set of building specifications for the human brain, in which every cell is created as a minute increment in the overall design? And if the set of instructions is indeecT so closed and specific, how could chance or random mutations or the influence of the environment have played a role as 86
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From page 87... ...
:~ _~ L: ~ ~ To FIGURE 6.1. The development of the human brain during gestation is a highly complex project on a tight schedule.
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From page 88... ...
Less severe defects of the neural tube may give rise to varying degrees of spine bifida ("split spine") , with the spinal column missing the bony protection of some of its vertebrae.
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From page 89... ...
STUDYING THE BRAIN IN DEVELOPMENT Short of technology that would shrink a remote video camera down to the size of a cellular implant, how can neuroscientists study brain development as an ongoing process in the living animal? The multistage mode!
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From page 90... ...
The human brain develops from the tip of a 3-millimeter-long neural tube. At three to four weeks after conception, the neural groove closes into a tube, and three distinct regions a hindbrain, midbrain, and forebrain begin to take form.
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From page 91... ...
It comprises by far the largest portion of the human brain—about three-quarters, in the adult and is arguably the single anatomical structure that most sets us apart from other animals, even from other hominoids such as the chimpanzee (with whom we share well over 95 percent of our genetic makeup)
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From page 92... ...
it is through the neurotransmitters and their receptor sites that the brain translates its countless functions into chemical terms anct back again into function. in this regard, the research team of Pasko Rakic at the Yale University School of Medicine has worked extensively with areas 17 and 1S, which roughly correspond to the primary visual cortex the part of the brain that must receive sensory impulses from the eyes before the visual association cortex (located nearby)
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From page 93... ...
Even when quite subtle, these variations can yield evidence of the intermingled effects on development of genetic information random mutations, and environmental influences. MASS PRODUCTION OF BRAIN CELLS The assembly of a human brain, a complex undertaking on a non-negotiable schedule, calls for a vast number of cells of suitable design, available at a convenient location.
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From page 94... ...
An extra couple of cell divisions at this point would produce not more surface area, which is the essential property of a larger cortex, but only an extra layer of cells on top of the surface that is already taking shape. One of the reasons for this limitation, and a guiding principle in the construction of the brain, is that the proliferative ventricular zone apparently holds information about both the quantities of various cells needed and their eventual function or location.
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From page 95... ...
More specifically, the columns that make up the protomap at the ventricular surface could be seen as including a proliferative unit at the base and then a cellular pathway along which nerve cells travel when they have stopped dividing and begun to mature. As the neurons of each unit migrate along the pathway in a set order and settle into position in the cortex, they would reproduce faithfully the orderly arrangement of the units in which they originated a feature termed cytoarchitectonic, for "architecture of the cells." According to this model, the surface area (even if convoluted)
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From page 96... ...
The glial cells composing the pathways for most neurons are extremely elongated and stringy in form, making a dense radial pattern from the ventricular zone to the outer layers of the developing brain. Once the stage of migration is accomplished, some of these glial cells degenerate; others undergo cell division and join the mature network of supporting cells, the "white matter," in the brain.
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From page 97... ...
THE FORMATION OF SYNAPSES AND REGIONS After migration, the tendency of recently arrived neurons to cluster with similar cells into distinct regions determines the form and ultimately the function of each part of the brain. At the upper and outer surface, the cortical sheet becomes continuous at this stage and begins to compress into its characteristic folds and creases, as more cells from the proliferative units continue to adc!
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From page 98... ...
The target cell toward which an axon is growing can also help with synapse formation by providing some of the chemical compounds needed by the axon. The best-known of these compounds is nerve growth factor, which the axon takes up by means of specific receptors and transports back to its cell body.
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From page 99... ...
This uniqueness is a physical fact: the full universe of synaptic connections that takes form in any given human brain reflects the sum of the influences genetic, nutritional, toxic, environmental, social, psychological, educational, and even accidental that have all converged, unpredictably and irreproducibly, during the development of this particular brain. The elimination of great numbers of synapses, along with some neurons themselves, is a process widely observed among mammals (and among some other vertebrates as well)
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From page 100... ...
The contest goes on at all levels: a single neuron may first establish communication with its target cell by means of several axons, only one of which will ultimately survive; or a single dendrite may initially receive signals from a neighboring neuron on many dendritic spines, some of which will be eliminated with the onset of maturity. Axons may be retracted (as mentioned earlier)
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From page 101... ...
How are cells of, for example, the cortical plate (which will ultimately develop into the cerebral cortex) directed to differentiate into one of the six distinct cell types of the cortex and to migrate to the particular layer inhabited by that cell type?
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From page 102... ...
New cortical areas can be created by a mutation that controls cell proliferation when radial units are being formed. Such areas have the chance for specialization, and whole new sets of synapses transmitting information to and from these areas could prove advantageous now and again; and, if inheritable, they could spread through a population.
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From page 103... ...
in neuroscience, study of the formation and clevelopment of the human brain holds a special place; many lines of investigation converge here. New methods in molecular biology may now make it possible to uncover specialized genes, for example, that may control cell production in the ventricular zone or regulate the deployment of cell adhesion or cell recognition molecules along migratory pathways.
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