227
Electromyogram
of denervations. These spontaneous random depolarizations are called fibrillations,
which are short fluctuations with small amplitude. Mechanically these muscle fibrils
will show contraction twitches with minimal contractibility; however, the entire
muscle may not be noticeably affected.
In case of muscle denervation, generally the composition of the EMG recording
gets even more complicated. In the normal cases, one axon of a single neuron innervates only a single motor unit. However, when neurons leading to a motor unit die,
the affected muscle fibers can be innervated by branches from adjacent stimulating
neurons. The adjacent neurons form extensions on the abnormal motor unit that can
typically control several dozen muscle fibers. This process is referred to as collateral
enervation.
In collateral enervation, the measured EMG might be composed of the signals
from several neighboring motor units. Specifically, collateral enervation affects the
EMG recording in such as way that an increase in amplitude and duration of the
waveform is observed. In addition the configuration of the EMG becomes more complex because one neuron is innervating more muscle fibers and the motor unit is
spread out over a larger area.
11.4.2 PATHOLOGICAL MOTOR UNITS
Knowledge of the functions of individual motor units makes it possible to develop
an understanding of some of the major pathological conditions that affect predominantly the skeletal movement.
The relatively well-known condition of muscular dystrophy includes muscle fiber
degeneration and a direct interference with the contractile ability of the muscle
fibers. The EMG characteristics of muscular dystrophy are recurring sequences of
myotonic episodes.
One of the main disorders in peripheral motor neurons is anterior horn cell disease, or ALS. This is a disease in which the motor units are affected as a result
of nerve degeneration. This affliction is also known as Lou Gehrig’s disease. The
ALS is known to affect about 1 in every 100,000 people. The symptoms are severe
muscular atrophy associated with extreme weakness. The disease can result in death
often as a result of failure in the respiratory muscles. In this case, the EMG shows
spontaneous activity, aberrant discharges, abnormal recruitment, reduced interference pattern.
In a disease called radiculopathy, the nerve cells leading to the motor units
are ischemic and are subjected to an inflammatory process. In this disease, the
EMG reveals the signs of focal demyelination of the nerve cell (i.e., the damage
or lack of myelin cover around the axon). Radiculopathy is also characterized by
the excitation frequency that suddenly drops followed by an immediate increase
in the depolarization frequency. Under acute severe compression of the axon of
the nerve cell, the EMG shows fibrillations with 10–21 days following the injury
to the nerve cell.
Plexopathy, another disease associated with the damage on the stimulating
nerve, involves either the brachial or lumbosacral plexus. The visible results are
Electromyogram
of denervations. These spontaneous random depolarizations are called fibrillations,
which are short fluctuations with small amplitude. Mechanically these muscle fibrils
will show contraction twitches with minimal contractibility; however, the entire
muscle may not be noticeably affected.
In case of muscle denervation, generally the composition of the EMG recording
gets even more complicated. In the normal cases, one axon of a single neuron innervates only a single motor unit. However, when neurons leading to a motor unit die,
the affected muscle fibers can be innervated by branches from adjacent stimulating
neurons. The adjacent neurons form extensions on the abnormal motor unit that can
typically control several dozen muscle fibers. This process is referred to as collateral
enervation.
In collateral enervation, the measured EMG might be composed of the signals
from several neighboring motor units. Specifically, collateral enervation affects the
EMG recording in such as way that an increase in amplitude and duration of the
waveform is observed. In addition the configuration of the EMG becomes more complex because one neuron is innervating more muscle fibers and the motor unit is
spread out over a larger area.
11.4.2 PATHOLOGICAL MOTOR UNITS
Knowledge of the functions of individual motor units makes it possible to develop
an understanding of some of the major pathological conditions that affect predominantly the skeletal movement.
The relatively well-known condition of muscular dystrophy includes muscle fiber
degeneration and a direct interference with the contractile ability of the muscle
fibers. The EMG characteristics of muscular dystrophy are recurring sequences of
myotonic episodes.
One of the main disorders in peripheral motor neurons is anterior horn cell disease, or ALS. This is a disease in which the motor units are affected as a result
of nerve degeneration. This affliction is also known as Lou Gehrig’s disease. The
ALS is known to affect about 1 in every 100,000 people. The symptoms are severe
muscular atrophy associated with extreme weakness. The disease can result in death
often as a result of failure in the respiratory muscles. In this case, the EMG shows
spontaneous activity, aberrant discharges, abnormal recruitment, reduced interference pattern.
In a disease called radiculopathy, the nerve cells leading to the motor units
are ischemic and are subjected to an inflammatory process. In this disease, the
EMG reveals the signs of focal demyelination of the nerve cell (i.e., the damage
or lack of myelin cover around the axon). Radiculopathy is also characterized by
the excitation frequency that suddenly drops followed by an immediate increase
in the depolarization frequency. Under acute severe compression of the axon of
the nerve cell, the EMG shows fibrillations with 10–21 days following the injury
to the nerve cell.
Plexopathy, another disease associated with the damage on the stimulating
nerve, involves either the brachial or lumbosacral plexus. The visible results are