Electrodes
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 effort:
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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