November 09, 1999
Exercise Improves Learning and Memory
Chalk up another benefit for regular exercise. Investigators from
the Howard Hughes Medical Institute (HHMI) have found that voluntary
running boosts the growth of new nerve cells and improves learning and
memory in adult mice.
"Until recently it was thought that the growth of new neurons, or
neurogenesis, did not occur in the adult mammalian brain," said Terrence Sejnowski,
an HHMI investigator at The Salk Institute for Biological Studies. "But
we now have evidence for it, and it appears that exercise helps this
happen."
“These observations support the idea that exercise enhances the formation and survival of new nerve cells as well as the connections between nerve cells, which in turn improves long-term memory.”
Terrence J. Sejnowski
Sejnowski, Salk colleague Fred Gage, and postdoctoral fellows
Henriette van Praag and Brian Christie, published their findings in the
November 9, 1999, issue of Proceedings of the National Academy of
Sciences.
The investigators began their study by comparing the memory skills
of a group of sedentary mice to those of a group of mice who exercised
freely on a running wheel for one month. Mice in the exercise group
logged an average daily distance of 4.87 kilometers, or 2.92 miles.
Both groups were trained to locate a submerged camouflaged platform
in a maze that was lying just below the surface in cloudy water. Mice
dislike swimming and instinctively seek the platform as a refuge from
the stressful activity.
"We can't ask a mouse if it remembered where the platform was
located, so we measure long-term memory by having them swim to the
platform," said Sejnowski.
After six days of training each group of mice for the swimming task,
the researchers began the study. The group of mice that had been
exercising made a beeline for the platform. In contrast, the sedentary
mice took significantly longer paths and times to find the dock. The
path chosen and time taken reflect long-term memory, or how well the
mice recall the platform's location. Based on the swimming test, mice
in the exercise group were better able to remember the platform's
location compared to mice in the sedentary group.
Next the researchers looked for changes in the number of nerve cells
between the two groups of animals. In comparison to sedentary mice, the
brains of mice that exercised had about 2.5 times more new nerve
cells.
New nerve cells were not distributed evenly throughout the brain,
but were concentrated in the dentate gyrus, a section of a larger area
of the brain called the hippocampus. The hippocampus plays a central
role in many memory formation processes, including spatial
learning—locating objects in the environment—and consciously
recalling facts, episodes, and unique events.
The investigators also examined brain slices from the two groups of
mice in order to measure a nerve-signaling process known as long-term
potentiation, or LTP. A large body of research supports the theory that
LTP—essentially a strengthening of the synaptic connections
between two neurons—is the primary mechanism involved in the
formation and storage of long-term memories by humans and animals.
"Presumably, LTP affects the flow of information through synapses,
the connections between two nerve cells," said Sejnowski. Neurons
communicate with each other by sending signals through synapses.
Examination of the tissue slices showed that the exercising mice
displayed twice as much long-term potentiation compared to their
sedentary counterparts. And, as was the case with neurogenesis,
increased LTP occurred only in the dentate gyrus.
"These observations support the idea that exercise enhances the
formation and survival of new nerve cells as well as the connections
between nerve cells, which in turn improves long-term memory,"
Sejnowski explained. He added that these data also confirm that the
structural and physiological changes that occurred in the dentate gyrus
correlate with a learning behavior associated with this region of the
brain.
Numerous human studies have shown that exercise increases alertness
and helps people to think more clearly. Recently, Gage demonstrated
that new nerve cells grow in the adult human dentate gyrus. If the same
correlation between exercise, nerve cell growth and memory observed in
mice plays out in the humans, "exercise could help you remember the
name of the person you met yesterday to or where you parked your car,"
said Sejnowski.
In future experiments, Sejnowski and his colleagues will follow
individual mice to see whether longer running times generate more new
nerve cells and stronger long-term potentiation. They will also explore
if other factors, such as hormones released by exercise, influence
memory and nerve cell growth. "We still don't have the causal link
between exercise and neurogenesis," said Sejnowski.
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