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The
motor unit
In anatomical terms, the motor unit consists of an anterior horn cell,
its axon, and all the muscle fibers innervated by that axon and its branches.
A motor unit may contain anywhere from a few muscle fibers (in the laryngeal
muscle) to several hundreds (in the gastrocnemius).
Muscle fibers belonging to one motor unit are not closely packed together.
They are scattered over a small area of muscle and intermingled with fibers
belonging to other motor units.
The motor unit action potential is the electrical field generated by
muscle fibers belonging to one motor unit as recorded by the tip of the
nearby needle electrode.
Normally muscle fibers belonging to one motor unit are all depolarized
and repolarized somewhat synchronously.
Amplitude, duration, number of phases, rise time, and firing rates characterize
a motor unit potential. Traditionally one measures the amplitude from peak
to peak; the duration from the first deflection of the baseline to the
last return to it; the number of phases by counting the number of times
the components of the motor unit potential cross the baseline plus one;
and the rise time as that elapsed between the peak of the initial positive
(down) deflection to the peak of the highest negative (up) deflection.
Note, however, that the number of fibers contained in a motor unit and
their degree of synchrony affect those characteristics.
The number of phases a motor unit contains depends largely on the synchrony
of depolarization of its muscle fibers and can be affected either by nerve
disease causing differential slowing in impulse conduction, or muscle disease
where the conduction characteristics of the muscle fibers themselves have
changed.
The rise time, strictly a function of the proximity of the needle tip
to the muscle fibers of the contracting unit, is usually between 200 and
300 µsec.
The firing rates of motor units depend on their type and size. Smaller
units are recruited early, with weak effort, and fire faster than large
units which are recruited later as effort is increased.
All the above characteristics vary with age, with the muscle under study,
and with muscle temperature. Minute changes in needle position can greatly
affect the shape of the motor unit potential. At a distance of 0.12 mm
of the depolarized fibers, the amplitude may be decreased by as much as
50 percent and at l mm by an astounding 90 percent.
In view of these variations, when single estimates of size and duration
from quick "eyeballing" of motor units is a problem, reading
should be done either by storing samples of the unit, by photographing
the unit or, better still, by having the unit trigger the sweep and using
a delay line to permit their study in detail.
Temperature Effect
At lower temperatures the motor unit duration and its amplitude are
increased.
Needle
Exam Description
There are four stages in the examination of a muscle by needle electrode:
when the muscle is at rest and during mild, moderate and full voluntary
effort.
The Muscle At Rest
Insertional activity: The
response of the muscle fibers to needle electrode insertion is called the
insertional activity. Normally it consists of brief, transient muscle action
potentials in the form of spikes, lasting only a few seconds and stopping
immediately when needle movements stop. Note that insertional activity
may be decreased, such as in fibrosis or fat tissue replacement; or prolonged,
such as in early denervation (the so-called irritability) and in myotonic
disorders.
 .
Spontaneous activity:
The persistence of any activity beyond insertion constitutes spontaneous
activity. This could be due to the normal end-plate noise, or to the presence
of fibrillations and positive waves, or other spontaneous activity (see
below).
A normal spontaneous activity is the end-plate noise. This can either
be monophasic (end-plate noise) or biphasic (end-plate spikes) potentials,
recorded when the needle is in the vicinity of a motor end-plate.
The monophasic potentials are of low amplitude and short duration and
cause a "thickened baseline" appearance. They give a typical
"sea shell" noise or "roar" on the loudspeaker.
The biphasic activity consists of irregular biphasic, 100-300 µv
spikes of short duration.
The muscle at rest must be examined in four or five different directions
once the needle is inserted to ensure adequate sampling. A pause of 0.5-1
second is required between each insertion to allow for the observation
of any spontaneous activity. When fasciculations are suspected, this time
is less than adequate and a 10 to 15 second pause is more appropriate.
For optimal observations of insertional activity set the oscilloscope
sweep speeds at 10 ms/division and amplification at 50 - 100 µv/division.
Filter settings chosen are 32 Hz for the low frequencies and 8000 or more
Hz for the high.
The Muscle During
Voluntary Effort
Assess voluntary activity during three stages of effort: mild, moderate,
and full. With mild and moderate voluntary effort, individual motor units
can be studied separately and their amplitude, duration, and number of
phases measured. Recruitment and firing rates are best assessed during
moderate effort, the interference pattern during full effort.
Mild effort: Only a few motor
unites are observed at this stage. These are the smaller motor units as
they are the ones to be recruited first. Ask the subject to maintain a
steady minimal contraction and sample the muscle in four or five different
areas. Sample at least 20 motor units and calculate an average amplitude,
duration and number of phases.
Moderate effort: The firing
rates and recruitment of motor units are best assessed during this stage.
As muscle effort increases, motor unit firing rates are increased and new
motor units are recruited. The units seen at this stage are larger than
those seen with mild effort.
Full efort: At maximum contraction,
the firing rates go even higher and more motor units are recruited into
the contraction making it difficult to distinguish them individually. When
all the motor units are recruited a complete interference pattern is observed.
Motor unit potentials are best studied with the same filter setting
used for insertional and spontaneous activity, i.e. 16-32 Hz low and 8000
Hz or more high. Motor unit potentials' duration is measured with an amplification
setting of 100-200 µv/division, and their amplitude at settings of
500 µv - 2000 µv/division, depending on the size of the motor
unit under study. The sweep speed setting is 5-10 ms/division. While these
settings are fairly widely accepted, different labs use different individual
settings. It is essential however to use the same settings consistently
to perform motor unit potential measurements.
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