When Your Brain Talks, Your Muscles Don't Always Listen

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When Your Brain Talks, Your Muscles Don't Always Listen

Have your neurons been shouting at your muscles again? It happens,

you know.

http://www.sciencedaily.com/releases/2007/03/070309103035.htm

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

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 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.

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 Bellumori and 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.