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ABSTRACT: When polio virus attacks the motor neuron H may be completely destroyed, damaged, or unaffected. Muscle fibers of a destroyed motor neuron are orphaned or reinnervated. Nearby functioning motor units will then send terminal axon sprouts to reinnervate the orphaned muscle fibers. If there are too many orphaned muscle fibers and not enough surviving motor units to reinnervate them, the orphaned muscle fibers will continue to fibrillate until they atrophy and the. The resultant effect of poliomyelitis upon the affected muscle is an overall loss of motor units with the remaining units innervating many more muscle fibers than they originally did.
There appears to be a late effect of polio upon these larger reinnervated motor units. After approximately 20 to 30 years, impulse transmission to the muscle fibers of the large reinnervated motor unit begins to fail. These transmission difficulties increase with age and time from recovery. These late onset transmission abnormalities may be factors in patient complaints of fatigue and progressive weakness.
The Normal Motor Unit
The motor unit is the functional component of the neuromuscular system.1 Motor units, which vary in size, are composed of a motor neuron, its axon, and all the muscle fibers innervated by that axon. Smaller motor neurons have smaller diameter axons and innervate fewer muscle fibers. They also have lower thresholds of excitation and are the first motor units recruited in a contraction. Muscle fibers of these first recruited or low threshold motor units are high in oxidative enzyme activity, have a slow twitch in rneir response to electrical stimulation and are referred to as Type I. Conversely, larger diameter motor neurons have larger axons and innervate larger numbers of muscle fibers. They are high in anaerobic enzyme activity, have a fast twitch response, and are classified as Type II. Type II motor units tend to be recruited at higher strengths of contraction. All muscle fibers belonging to one motor unit are of the same histochemical fiber type.
The Normal EMG
Electromyography (EMG) is the electrophysiological study of the motor unit performed by inserting a small wire or needle electrode into a muscle.2 While the muscle is at rest there is no electrical activity in the normal state. With very mild activation of a muscle,...