Clinical Disorders and the Motor System

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We would all like to thank Dr. Richard C. Schafer, DC, PhD, FICC for his lifetime commitment to the profession. In the future we will continue to add materials from RC’s copyrighted books for your use.

This is Chapter 9 from RC’s best-selling book:

“Basic Principles of Chiropractic Neuroscience”

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Chapter 9:   Clinical Disorders and the Motor System


Such clinical features as fatigue, weakness, nervousness, pain, tenderness, paralysis, sensory loss, paresthesia, and abnormalities of muscle mass or tone are the most common signs and symptoms noted in neural disorders. Fatigue, weakness, and nervousness are frequently presented together. This triune can usually be attributed to a functional disorder or appear as a complication in organic disease.

Abnormal striated muscle function has its origin in diseases of the brain, spinal cord, peripheral nerves, or muscle tissue itself. Dysfunction occurs in a variety of symptoms and signs such as:

(1) impaired movements,
(2) spontaneous movements,
(3) coordination defects,
(4) abnormal reflexes,
(5) distortions of muscle tone, and
(6) postural and movement distortions.

Weakness, wasting, and sometimes paralysis are represented in these conditions. Common types of motor lesions are shown in Table 9.1.

Basic Neuromuscular Activities

There are two fundamental types of neuromuscular activity. One type consists of reflex postural contractions, which are the basis of posture and physical attitudes and maintain muscle tone. The other type consists of phasic contractions, which produce movement. Phasic contractions may be either reflex or volitional in origin. While reflex actions are always purposeful, predictable, and involuntary, cortical activity is not.

Neurons carrying phasic and tonic impulses have distinctive characteristics. Phasic motor neurons are large, have a rapid conduction velocity, have a high threshold of physiologic excitability, present large impulses of short duration, and are electrically silent during rest. In contrast, tonic motor neurons are smaller, have a slower conduction velocity, have a lower threshold of physiologic excitability, present smaller impulses of longer duration, and are electrically active during rest.

Muscle and Joint Correlations

Functional and degenerative lesions of joints are principally the result not only of a pathologic process in a joint but of altered function of the motor system as a whole, according to the findings of Janda. Because of their physiologic properties, he believes that the muscles, which represent the most liable link of the motor system, respond early and distinctly in most clinical pictures of functional and degenerative joint disease. However, the muscular reaction is not of the same quality in all muscles: muscles with predominantly postural function tighten and muscles with predominantly phasic function weaken.

To understand the fine control of motion, the separate activity of individual muscles is not as important as their coordinated activity within different movement patterns. Understanding this, states Janda, seems to be the best basis for rational treatment and good long-term therapeutic results.


The strength of healthy muscle is generally proportional to its size (bulk). The term atrophy refers to the loss of muscle bulk as the result of disease. Atrophy is especially difficult to evaluate in the aged or malnourished individual.

The Determination of Atrophy

Palpation and mensuration are used to determine muscle volume. Upon palpation, there should be a mass that is symmetrical bilaterally. Measurements should be made with a flexible tape from a bony prominence to the belly of a suspected muscle and the point marked with a skin pencil. This distance should be recorded for future reference, then the circumference at that point is measured.

The same procedure is then conducted on the opposite side. The two sides should be approximately the same circumference unless there is a large degree of unilateral occupational activity. A decrease in size (eg, midcalf or thigh) indicates atrophy and is usually associated with some degree of hypotonicity and a decrease in strength.

Local Atrophy

General emaciation should not be confused with local atrophy. Local muscle atrophy occurs as a result of a peripheral nerve lesion, poliomyelitis, neuritis, or trauma to a spinal nerve. The affected muscle becomes shrunken, poor in tone, and weak in strength. Age, sex, occupation, and right- and left-handedness must be considered.

Weakness, flaccidity, and atrophy occur in the face, tongue, and pharyngeal muscles with disease of the lower motor neurons of the brain stem. The result is described by Daube/Sandok as a breathy, imprecise, nasal speech called flaccid dysarthria.

Review the complete Chapter (including sketches and Tables)
at the
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