Porth's Essentials of Pathophysiology, 4e

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Disorders of Neuromuscular Function

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somatosensory systems to coordinate sequential body and limb movements. The signs of cerebellar dysfunction include cerebel- lar ataxia and tremor. They result from defects in the smooth, continuously correcting functions of the cer- ebellum, and occur on the side of the cerebellar damage. Disorders of cerebellar function are typically caused by a congenital defect, cerebrovascular event, or growing tumor. Cerebellar gait ataxia is characterized by wide-based staggering, lurching, and uncontrolled gait. Visual monitoring of movement cannot compensate for cer- ebellar defects, and these abnormalities occur whether the eyes are open or closed. Because ethanol specifically affects cerebellar function, persons who are inebriated often walk with a staggering and unsteady gait. Rapid alternating movements such as supination–pronation– supination of the hands are jerky and performed slowly. Reaching to touch a target breaks down into small sequential components, each going too far, followed by overcorrection. The finger moves jerkily toward the tar- get, misses, corrects in the other direction, and misses again, until the target is finally reached. This is called over- and underreaching or dysmetria. Cerebellar tremor is a rhythmic back-and-forth movement of a finger or toe that worsens as the target is approached. The tremor results from the inability of the damaged cerebellar system to maintain ongoing fixation of a body part and to make smooth, continuous correc- tions in the trajectory of the movement; overcorrection occurs, first in one direction and then the other. Often, the tremor of an arm or leg can be detected during the beginning of an intended movement. The common term for cerebellar tremor is intention tremor. Cerebellar function, as it relates to tremor, can be assessed by ask- ing a person to touch one heel to the opposite knee, to gently move the toes along the back of the opposite shin, or to touch the nose with a finger. The ability to fix the eyes on a target also can be affected. Constant conjugate readjustment of eye posi- tion, called nystagmus, results and makes reading extremely difficult, especially when the eyes are deviated toward the side of cerebellar damage. Cerebellar func- tion also can affect the motor skills of chewing, swallow- ing, and speech. Normal speech requires smooth control of respiratory muscles and highly coordinated control of the laryngeal, lip, and tongue muscles. Cerebellar dysar- thria is characterized by slow, slurred speech of continu- ously varying loudness. Rehabilitative efforts directed by speech therapists include learning to slow the rate of speech and to compensate as much as possible through the use of less-affected muscles. Disorders ofThe Basal Ganglia The basal ganglia are a group of deep, interrelated sub- cortical nuclei that play an essential role in control of movement. They function in the organization of inher- ited and highly learned and rather automatic move- ment programs, especially those affecting the trunk and

Disorders of the Cerebellum and Basal Ganglia Aside from the areas in the cerebral cortex that stimu- late muscle contraction, two other brain structures, the cerebellum and basal ganglia, are also essential for normal motor function. Neither one can control muscle function by itself. Instead, they always func- tion in association with other components of motor control. Disorders of the Cerebellum The cerebellum has sometimes been referred to as the silent area of the brain because electrical stimulation does not produce any conscious sensation and rarely causes any motor movements. 3,31 However, removal or damage to the cerebellum causes movements to become highly abnormal. The cerebellum is especially vital dur- ing rapid muscular activities such as running, typing, and even talking. Loss of cerebellar function can result in total incoordination of these functions even though paralysis does not occur. The functions of the cerebellum are integrated into many connected afferent and efferent pathways throughout the brain. An extensive and important afferent pathway is the corticopontocerebellar path- way, which originates in the cerebral motor and pre- motor cortices as well as the somatosensory cortex. Other important afferent pathways link the cerebel- lum to input from the basal ganglia, muscle and joint information from the stretch receptors, visual input from the eyes, and balance and equilibrium sensa- tion from the vestibular system in the inner ear. There are three general efferent pathways leading out of the cerebellum: (1) the vestibulocerebellar pathway that functions in close association with the brain stem ves- tibular nuclei to maintain equilibrium and posture; (2) the spinocerebellar pathway that provides the circuitry for coordinating the movements of distal portions of the limbs, especially the hands and fingers; and (3) the cerebrocerebellar pathway that transmits output infor- mation in an upward direction to the cerebral cortex, functioning in a feedback manner with the motor and and the polyneuropathies, such as Guillain-Barré syndrome, which involve multiple peripheral nerves leading to symmetric sensory, motor, or mixed sensorimotor deficits. ■■ Back pain and pain related to spinal nerve root irritation can be caused by a number of disorders including muscle and ligament strains, age-related degenerative spine disorders, disk herniation, and narrowing of the central vertebral canal due to spinal stenosis.