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LINKAGES Human Development and the Changing Brain

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LINKAGES Human Development and the Changing Brain
73
The Central Nervous System: Making Sense of the World
studied. In one case, nerve growth factor was put directly into the brain of a patient
with Alzheimer’s disease (Seiger et al., 1993). The early results seemed encouraging,
but the continuous infusion of the protein into the brain caused unacceptable side
effects (Nabeshima & Yamada, 2000). Another way to deliver growth factors is
through gene therapy, in which a gene for the desired growth factor is inserted into
a patient’s neurons (Bomze et al., 2001; Condic, 2001; Tuszynski, et al., 2002). Early
results from the use of this high-tech treatment are encouraging (Tuszynski et al.,
2005).
In the meantime, there are things that patients themselves can do to promote the
neural plasticity needed to restore lost central nervous system functions. Special mental and physical exercise programs appear useful in restructuring communication in the
brains of stroke victims and spinal cord injury patients, thus reversing some forms of
paralysis and improving some sensory and cognitive abilities (Blakeslee, 2001; Liepert
et al., 2000; Robertson & Murre, 1999; Taub, 2004). Christopher Reeve was an inspiring case in point. After his spinal cord injury, Reeve was told he would never again be
able to move or feel his body. He refused to accept this gloomy prediction, and after
years of devoted adherence to an exercise-oriented rehabilitation program, he regained
some movement and in the years before his death was able to feel sensations from much
of his body (Blakeslee, 2001).
LINKAGES
How do our brains change
over a lifetime? (a link to
Human Development)
F
LINKAGES
ortunately, most of the changes that take
place in the brain are not the kind assoHuman Development and
ciated with damage and disease. How
does the human brain change as we develop
the Changing Brain
throughout our lives? Researchers are using
PET and fMRI scans to begin to answer that
question. For example, they have found that association areas of the cerebral cortex
develop later than sensory and motor cortex (Casey, Galvan, & Hare, 2005). There are
also some interesting correlations between changes in neural activity and the behavior
of newborns and infants. Among newborns, scans show that activity is relatively high
in the thalamus but low in a portion of the forebrain related to smooth movement.
This finding may be related to the way newborns move. They make random, sweeping
movements of the arms and legs—much like patients with Huntington’s disease, who
have a hyperactive thalamus and a withering of the part of the forebrain that controls
smooth movement (Chugani & Phelps, 1986). During the second and third months
after birth, activity increases in many regions of the cortex. This change is correlated
with the loss of reflexes, such as the grasping reflex. At eight or nine months of age,
infants show increased frontal cortex activity, which correlates with the apparent
beginnings of cognitive activity (Chugani & Phelps, 1986). The brain continues to
mature even through adolescence, showing evidence of ever more efficient neural
communication in its major fiber tracts (Gogtay et al., 2004; Paus et al., 1999; Thompson et al., 2000).
Most of these changes reflect plasticity—changes in axons and synapses—not the
appearance of new cells. After birth, the number of dendrites and synapses increases.
Although different areas of the cortex sprout at different rates, the number of
synapses can increase tenfold in the first year after birth (Huttenlocher, 1990). In
fact, by the time children are six or seven years old, their brains have more dendrites
than those of adults, and they use twice as much energy. In early adolescence, the
number of dendrites and neural connections actually drops, so the adult level is
reached by about the age of fourteen. During childhood, the brain overproduces
neural connections and then “prunes” the extra connections (Sowell et al., 2001).
Figure 2.14 shows that as we grow, we develop more brainpower with less brain
(Sowell et al., 2003).
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