Page 24-25 - NeuRA 2013 in Review

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(top) PhD student David Kennedy performs a strong voluntary
contraction of the elbow flexor muscles to measure force output;
(bottom) Assoc Prof Janet Taylor and PhD student Jim Nuzzo use transcranial
magnetic stimulation to measure how well the brain drives the muscles.
The control of muscles by the nervous system
underlies all of our actions
To produce any voluntary movements, like reaching, grasping,
standing or walking, requires a chain of events. Signals from the
brain activate motor neurons in the spinal cord and these in turn
activate muscle fibres to make them contract.
Moreover, the neurons in the brain and spinal cord must
keep firing to keep the muscle working. As soon as they stop, the
muscle relaxes. Therefore, during exercise, not only the muscles
are working but so too is the nervous system.
Muscle fatigue with exercise is a common experience in
healthy people. It is also a prominent symptom in people with
many kinds of illnesses. Although processes in the muscle
cause some of the weakness of fatigue, processes in the nervous
system also contribute. Past research at NeuRA has shown that
when people make repeated or sustained muscle contractions,
the brain becomes less able to drive the motor neurons to drive
the muscles. In some kinds of exercise, the failure of the brain
is just as important as the failure of the muscles.
Recent NeuRA research shows that the behaviour of
neurons in the spinal cord may also play a part in fatigue.
When motor neurons have to fire repeatedly during deliberate,
voluntary activity, they become less responsive to input.
This may mean that the brain’s task during exercise is
made harder because extra drive from the brain is required to
keep the motor neurons firing and the muscles contracting.
For the person, the exercise will take more effort.
It is important to understand fatigue in healthy people
as it influences the performance of tasks in work, sport and
everyday life. Understanding its mechanisms will also allow
better management of this symptom in illness.
Your Nervous System | 23