Plasticity in the Central Nervous System

The Central Nervous System: Making Sense of the World
FIGURE
71
2.13
Apparatus for Studying
Split-Brain Patients
When the person stares at the dot on the
screen, images briefly presented on one
side of the dot go to only one side of the
brain. For example, a picture of a spoon
presented on the left side of the screen
goes to the right side of the brain. The
right side of the brain can find the spoon
and direct the left hand to touch it. However, the language areas on the left side of
the brain did not see the spoon, so the
person is unable to say what it is.
their right hemispheres. In addition, these patients’ right hemispheres did better than
their left hemispheres at tasks involving spatial relationships (especially drawing threedimensional shapes) and recognizing human faces.
Having these two somewhat specialized hemispheres allows the normal brain to perform some tasks more efficiently, particularly difficult ones. But the differences between
the hemispheres should not be exaggerated. Remember, the corpus callosum usually
integrates the functions of the “two brains” (Rueckert et al., 1999). As a result, the hemispheres work closely together, each making up well for whatever lack of ability the other
may have (Banich & Heller, 1998; Staudt et al., 2001).
Plasticity in the Central Nervous System
The central nervous system has a remarkable property called plasticity, which is the ability to strengthen neural connections at synapses, as well as to establish new connections
(Cohen-Cory, 2002; Kolb, Gibb, & Robinson, 2003; Tailby et al., 2005). As a result, even
the simplest reflex in the spinal cord can be modified by experience (Chen, Chen, &
Wolpaw, 2003). In the brain, plasticity is the basis for our ability to form new memories
and learn new things. For example, more cells in the brain’s motor cortex become
involved in controlling hand movements in people who have learned to play a musical
instrument. The process can be seen in brain imaging studies; as nonmusicians get better at making rhythmic finger movements, the amount of motor cortex devoted to this
task increases (Munte, Altenmuller, & Jancke, 2002). Even more amazing is that merely
imagining practicing these movements causes changes in the motor cortex (PascualLeone, 2001). Athletes have long engaged in exercises in which they visualize skilled sports
movements; brain imaging research reveals that this “mental practice” can change the
brain.
plasticity A property of the central
nervous system that has the ability to
strengthen neural connections at
synapses, as well as to establish new
connections.
Repairing Brain Damage There are limits to plasticity, though, especially when it
comes to repairing damage in the brain and spinal cord. Unlike the skin or the liver,
the adult central nervous system does not automatically replace damaged cells. As a
result, most victims of severe stroke, Parkinson’s disease, Alzheimer’s disease, or spinal
cord injury are permanently disabled in some way. Nevertheless, scientists are searching
for ways to help a damaged central nervous system heal some of its own wounds.
72
HE WAS A SUPER MAN After a 1995
riding accident left actor/director
Christopher Reeve paralyzed below his
shoulders, he embarked on a long, intense rehabilitation regimen. He received
electrical stimulation to maintain muscle
tone and was strapped to a tilting table
to maintain bone density. He was suspended in a harness over a treadmill
while his legs were put through walking
movements. In 1999, he began using a
functional electrical stimulation bicycle,
which sent computer-controlled
electrical impulses to his legs, causing the
muscles to contract and move the bike’s
pedals. By the end of 2002, he could
move his fingers, right wrist, and upper
legs. In a swimming pool, he could move
his knees and upper arms. By the time of
his death in 2004, he had also regained
feeling in about 70 percent of his body.
Research is continuing on other exercise
programs to promote recovery from
spinal cord injury (Molteni et al., 2004;
Taub, 2004).
Chapter 2 Biology and Behavior
One approach has been to transplant, or graft, tissue from the still-developing brain
of a fetus into the brain of an adult animal. If the receiving animal does not reject it,
the graft sends axons out into the brain and makes some functional connections. This
treatment has reversed animals’ learning difficulties, movement disorders, and other
results of brain damage (Noble, 2000). The technique has also been used to treat a
small number of people with Parkinson’s disease—a disorder characterized by tremors,
rigidity of the arms and legs, and poor balance (Lindvall & Hagell, 2001). The initial
results have been encouraging (Mendez et al., 2005). Some patients showed improvement for several years, though improvement faded for others, and some patients suffered
serious side effects (Freed et al., 2001). Brain tissue transplants in humans are controversial because they require the use of tissue from aborted fetuses. As an alternative, some scientists have tried transplanting neural tissue from another species, such
as pigs, into humans (Drucker-Colin & Verdugo-Diaz, 2004). Russian physicians have
even tried transplanting neural tissue from fruit flies into the brains of Parkinson’s
patients. The results were beneficial, and there were no immediate side effects (Saveliev
et al., 1997), but the fruit fly neurons were eventually rejected by the patients’ bodies
(Korochkin, 2000).
The most promising source for new neurons now appears to be an individual’s own
tissues, because these cells would not be rejected. This is a revolutionary idea, because
it was long believed that once humans reached adulthood, the cells of the central nervous system stopped dividing, leaving each of us with a fixed set of neurons (Rakic,
2002). Then came research showing that cell division does take place in the adult central nervous systems of humans, nonhuman primates, and other animals (Blakeslee,
2000; Eriksson et al., 1998; Gould et al., 1999; Steindler & Pincus, 2002). It turns out
that the adult brain contains neural stem cells, a special kind of glial cells that are capable of dividing to form new tissue, including new neurons (Cheng, Tavazoie, & Doetsch,
2005; Sanai et al., 2004).
This discovery has created both excitement and controversy. There is excitement
because stem cells raise hope that damaged tissue may someday be replaced by cells
created from a person’s own body, but there is controversy because stem cells are linked
in many people’s minds with the cloning of whole individuals. If brain cells can indeed
be grown from cells found in bone marrow, the lining of the nose, or other sites, the
benefits in treating brain disorders would be substantial (Murrell et al., 2005). Patients
suffering from spinal cord injuries, as well as Parkinson’s disease and Alzheimer’s disease, might someday be cured by treatments that replace damaged or dying neurons
with new ones grown from the patients’ own stem cells (Chen, Magavi, & Macklis, 2004;
Cowan et al., 2005; Horner & Gage, 2002; Mezey et al., 2003; Teng et al., 2002; Zhao
et al., 2003).
Generating new neurons is only half the battle, however. The new cells’ axons and
dendrites would have to reestablish all the synaptic connections that had been lost to
damage or disease. Unfortunately, this process is hampered in the central nervous system by glial cells that actively suppress new connections between newly sprouted axons
and other neurons (Olson, 1997). Several central nervous system proteins, including
one aptly named Nogo, have the same suppressant effect.
Despite these challenges, scientists are reporting exciting results in their efforts to
promote healing in damaged brains and spinal cords. They have found, for example, that blocking the action of Nogo in mice and rats with spinal cord injuries
allows surviving neurons to make new axonal connections and repair the damage
(Cummings et al., 2005; Kastin & Pan, 2005). Other research with animals has shown
that both spontaneous recovery and the effectiveness of brain-tissue transplants can
be greatly enhanced by naturally occurring proteins called growth factors, which promote the survival of neurons (Hoglinger et al., 2001). One of these proteins is called
nerve growth factor. Another, called glial cell line–derived neurotrophic factor, or
GDNF, actually causes neurons to produce the neurotransmitter needed to reverse
the effects of Parkinson’s disease (Kordower et al., 2000; Theofilopoulos et al., 2001).
The best way to increase the amount of these growth factors in humans is still being