Motor Unit Presentation
The Motor Unit
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Muscle fibers are innervated by neurons whose cell bodies are located in spinal
cord. The nerve fibers, or axons, of these motor neurons leave
the spinal cord and are distributed to the motor nerves. Each motor axon
branches several times and innervates many muscle fibers.
The combination of a single motor neuron and all the muscle fibers it
innervates is called a motor unit. Although the muscle fibers of a given
motor unit tend to be located near one another, motor units have
overlapping territories.
In response to an action potential from the neuron, a muscle fiber
depolarizes as the signal propagates along its surface and the fiber
twitches (contracts). This depolarization generates an electric field in
the vicinity of the muscle fibers which can be detected by a skin surface
electrode located near this field, or by a
quadrifilar
electrode inserted in the muscle. The resulting signal is called the
muscle fiber action potential. The combination of the muscle fiber action
potentials from all the muscle fibers of a single motor unit is the motor
unit action potential (MUAP). All of the muscle fibers in a motor unit
are fired each time a motor unit fires. The repetitive firing of a motor
unit creates a train of impulses known as the motor unit action potential
train (MUAPT). The summation of electrical activity created by each active
motor unit is the myoelectrical signal (ME) (4).
To sustain muscle contraction, the motor units must be repeatedly
activated (2). As the firing rates of motor units active in a contraction
increase, the twitches associated with each firing will eventually fuse to
yield large forces.
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Most of the experiments launched at the Motor Unit Lab have been done
on the first dorsal interosseous (FDI) and the tibialis anterior (TA).
Here is the procedure which is followed:
 The
hand (or the leg) of each subject is placed in a specially designed
device that constrained the muscle to contract isometrically. This
apparatus substantially isolates the force generated by the muscle. The
maximal voluntary contraction (MVC) level is measured for each subject.
Then the subject is instructed to contract his muscle so as to generate a
force-time course which tracked trajectory displayed on a monitor. The
trajectory consisted of a trapezoidal shape, with a sustained contraction
between 20 and 50% MVC during 30 seconds.
A special quadrifilar electrode is inserted
in the muscle. Myoelectric (ME) signals are obtained from three
differential combinations of the four wires (75 mm in diameter) exposed in
cross section at a side port on the cannula of the needle, as well as from
the cannula itself. The signals from the side port wires are amplified
with a bandwidth of 1 kHz - 10 kHz and are digitized at a rate of 50 kHz.
The force signal is amplified with a bandwidth of 10Hz to 1 kHz and is
digitized at a rate of 2000 Hz.
 The
ME signals are decomposed into their constituent MUAPT's by the Precision
Decomposition technique (Mambrito and De Luca , Le Fever and De Luca )
to obtain the map of MU firings. The Precision Decomposition technique is
a template matching technique which arrives at decisions for identifying
the shape of individuals MU's by a weighted combination of probability of
occurrence and the least-squared signal space distance between the MUAP
and an established template. The technique also continuously update the
templates if the shape of the MUAP's is modified slowly.
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A few tools
 The
output of the EMG Precision Decomposition algorithm is a set of impulse
trains. Each impulse corresponds to a firing of the appropriate motor
unit. Several patterns are useful to understand the control properties of
individual motor unit:
- The Bar Plot shows the location of all
the MUAPs of each motor unit. This plot may be used to determine the
recruitment and derecruitment threshold of a motor unit.
- The Dot Plot shows the duration of all the inter firing
interval of each motor unit. A dot represents the time between a
firing and the next one. This plot may be used to study the behavior
of the motor unit firings.
- The Instantaneous Firing Rate (IFR) is computed by inversing
the time between a firing and the next one.
 The
time varying Mean Firing Rate for each
Motor Unit is estimated by passing the impulse train showed in the IFI
plot through a Hanning filter, with symmetric unit area impulse
response. From empirical observation, it was found that a filter with
a window 400-800 ms long provides an acceptable compromise between
firing rate estimation bias and estimation stability. This type of
plots is very useful for studying relationships among different Motor
Units and the Force.
A few results
Number of studies have shown interesting results to understand the
neuromuscular system. The most important is the common drive of motor
units:
- Motor units have been found to modulate their firing rates in unison
and simultaneously.
- The firing rate of motor units is not constant, even during constant
force contractions; it fluctuates.
- The firing rates of earlier recruited motor units are greater than
those of later recruited motor units at any given force value. At a
force reversal, the firing rates of high threshold motor units reduce
their firing rates before the low threshold motor units.
- The fluctuations in a force output of a muscle during a
constant-force contraction are caused by the fluctuations in the
firing rates of the motor units.
- The firing rates of the motor units decrease during a constant-force
isometric contraction.
These phenomena suggests the following implications:
- The motor units have a net excitation which acts through a common
input. The most likely location of this common input is the anterior
horn cell.
- The control to the muscle is not designed to generate constant-force
contractions.
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