Abnormally high muscle tone, called hypertonia, occurs in individuals with certain disease processes and is seen particularly clearly when a joint is moved passively at high speeds. The increased resistance is due to a greater-than-normal level of alpha motor neuron activity, which keeps a muscle contracted despite the individual's attempt to relax it. Hypertonia is usually found when there are disorders of the descending pathways that result in decreased inhibitory influence exerted by them on the motor neurons.
Clinically, the descending pathways—primarily the corticospinal pathways—and neurons of the motor cortex are often referred to as the "upper motor neurons" (a confusing misnomer because they are not really motor neurons at all). Abnormalities due to their dysfunction are classed, therefore, as upper motor neuron disorders. Thus, hypertonia indicates an upper motor neuron disorder. In this clinical classification, the alpha motor neurons—the true motor neurons—are termed lower motor neurons.
Spasticity is a form of hypertonia in which the muscles do not develop increased tone until they are stretched a bit, and after a brief increase in tone, the contraction subsides for a short time. The period of "give" occurring after a time of resistance is called the clasp-knife phenomenon. Spasticity may be accompanied by increased responses of motor reflexes such as
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Control of Body Movement CHAPTER TWELVE
the knee jerk, and by decreased coordination and strength of voluntary actions. Rigidity is a form of hypertonia in which the increased muscle contraction is continual and the resistance to passive stretch is constant. Two other forms of hypertonia that can occur suddenly in individual or multiple muscles are spasms, which are brief contractions, and cramps, which are prolonged and painful.
Hypotonia is a condition of abnormally low muscle tone, accompanied by weakness, atrophy (a decrease in muscle bulk), and decreased or absent reflex responses. Dexterity and coordination are generally preserved unless profound weakness is present. While hypotonia may develop after cerebellar disease, it more frequently accompanies disorders of the alpha motor neurons ("lower motor neurons"), neuromus-cular junctions, or the muscles themselves. The term flaccid, which means "weak" or "soft," is often used to describe hypotonic muscles.
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