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Aging News & Information
A Senior Citizen’s Brain Has to Sometimes Yell for
the Muscles to Hear
Improving movement quality in older adults is
research goal
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UD
scientist Christopher Knight (left) and graduate student Dhiraj Poojari
are examining the nerves and muscles of the index finger to shed light
on how our motor-control system, which rules movement, changes as we
age. |
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March 9, 2007--Have your neurons been shouting at
your muscles again? It happens, you know, when you get older. As we grow
older, neurons--the nerve cells that deliver commands from our
brains--have to “speak” more loudly to get the attention of our muscles
to move, according to University of Delaware researcher Christopher
Knight, an assistant professor in UD's College of Health Sciences.
“As a result of age-related changes in muscle and
neurons, elderly people are often frustrated by poor control during
precision tasks, and slowed physical responses contribute to more falls
as people grow older,” Knight said.
Knight and co-author Gary Kamen, who directs the
Exercise Neuroscience Laboratory at the University of Massachusetts,
recently published the results of a study on motor-unit firing rates in
the Journal of Applied Physiology, and Knight is now beginning a new
project focusing on motor-control mechanisms in the elderly. Both
studies are sponsored by the National Institutes of Health.
The ultimate goal of the research, Knight said, is
to improve movement quality in older adults, as well as patients with
disorders such as cerebral palsy or multiple sclerosis, or who are
recovering from strokes.
Every move you make is made possible through a
miraculous communications network involving the brain at the command
center, the spinal cord, billions upon billions of nerve cells, and
thousands of muscle fibers.
“Muscles are the driving force behind our
movements,” Knight said. “Every time they get a command from the
neurons, the muscle fibers contract. In the generation of muscular
force, the smallest controllable unit consists of an individual neuron
and the muscle fibers it stimulates. We believe that our research is
very important to our understanding of motor-control mechanisms in
general and impaired control in patient populations.”
Shedding light on the communication between neurons
and muscles, and how it changes as we age, may lie right at our
fingertips, according to Knight's research.
Using an experimental apparatus he and his students
created in UD's Human Performance Lab, Knight has been examining
muscular force on a very small scale in the index finger, specifically,
the first dorsal interosseous muscle. Located between the index finger
and the thumb, this muscle contains 120 “motor units”--in other words,
120 individual neurons, or nerve cells, and the muscle fibers they
activate.
“It's a relatively simple muscle, so you get to see
more of a one-to-one relationship between the activity of the neurons
and the resulting muscular force,” Knight said.
Twenty-three subjects, ranging from 18 to 88 years
of age, participated in Knight's recent study.
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Inserted in the muscle of the index finger through a painless procedure,
this electrode, with its four tiny wires, records the electrical
impulses of individual neurons, or nerve cells. |
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In a virtually painless procedure, a small
needle-like electrode with four tiny wires was embedded in the muscle of
an index finger of each subject. The electrode was hooked up to a
computer to record the electrical impulses as they travel from neurons
to the muscle fibers.
As the index finger was held steady in a small
harness, each subject was asked to use the finger to follow the outline
of a sinusoidal curve, with its peaks and valleys, on a computer screen.
“More force--which is indicated by a corresponding
higher firing rate of neurons--is exerted just before you begin the
upturn toward one peak and then it eases off again in the downturn
toward a valley,” Knight noted.
Once recordings were completed at one site in the
muscle, the electrode was repositioned to sample from other motor units
within the muscle.
Knight and graduate student Dhiraj Poojari and
undergraduate researchers Maria Bellumori and Christopher Martens then
analyzed the firing-rate data for frequency and amplitude in a tedious
process that Knight hopes to automate in the future through the ongoing
development of a software program that will help sort out the
bang-bang-like “doublets,” the brief periods when the neurons fire
faster, from slower periods of activity.
The results showed lower firing rates among older
subjects versus younger subjects--a diminished ability of the muscle
fibers to “hear” and respond to the neurons' commands.
“The repeated contraction of muscles is essential
to movements such as walking,” Knight said. “However, our muscles have a
reduced capacity to contract or 'twitch' as we grow older. We lose
fast-twitch muscle fibers as we age.”
However, there are steps we can take to preserve
this critical motor capacity, according to Knight.
“After power training with weights, we see an
increase in firing rates,” Knight said. “For safety, we're commonly
advised to do things slowly when exercising, but it's important to also
do some fast exercises. You need a fast movement to prevent a fall. Even
in the frail elderly, it is possible to use exercise bands for manual
resistance to improve the speed of movement.”
Knight has always been interested in how the body
adapts to exercise. When he entered college years ago, his goal was to
become an elite track-and-field athlete. While he competed well, he
realized that his dreams lay elsewhere, and his attention focused full
force on academics.
At the University of Connecticut, a class on the
biology of the brain introduced him to the nervous system and movement,
and he was hooked. His interests were further piqued during a summer
research experience, where he had the opportunity to work with
wheelchair athletes.
“People with severe spinal cord injuries have
limited cooling because they can't perspire below the site of injury,”
Knight said, “so their core body temperature can reach dangerous
levels.”
In graduate school, he decided to pursue motor
control research, and he's never looked back.
“My early interests were based on sport, but my
career in this field now allows me to address a much larger population
that needs our knowledge,” Knight said. “Exercise is still the means for
improvement, and aging is a process that unites us all.”
Knight is now recruiting healthy, older subjects,
ages 70 and up, as well as individuals with Parkinson's disease or
multiple sclerosis for his next motor-control study. For more
information, please contact him at [caknight@udel.edu]
or (302) 831-6175.
>> Article by Tracey Bryant
>> Photos by Kathy F. Atkinson
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