Porth's Essentials of Pathophysiology, 4e

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Nervous System

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contractions of all the muscle fibers in a motor unit due to irritation and hyperexcitability of the cell body and its motor neuron, and suggest LMN disease. Spinal Reflex Activity Testing of deep tendon reflexes (see Understanding the Stretch Reflex and Muscle Tone) can provide important information about the status of the CNS in controlling muscle function. Hyperactive reflexes are suggestive of a UMN disorder. Clonus is the rhythmic contraction and alternate relaxation of a limb that is caused by suddenly stretching a muscle and gently maintaining it in the stretched position. It is seen in the hypertonia of spasticity associated with UMN lesions, such as spinal cord injury. Hyporeflexia or areflexia suggests the presence of an LMN lesion. The distribu- tion of abnormality in the reflexes is also helpful in determining the location of the lesion. For example, hyperreflexia in both lower extremities would suggest a lesion in the spinal cord, whereas hyperreflexia on one side of the body would suggest a lesion in the UMN along the motor pathway (e.g., in the motor cortex or internal capsule). Coordination of Movement Coordination of muscle movement requires that four areas of the nervous system function in an integrated manner—the motor system for muscle strength, the cerebellar system for rhythmic movement and steady posture, the vestibular system for posture and balance, and the sensory system for position sense. In cerebellar disease, one movement cannot be fol- lowed quickly by its opposite movement. Movements are slow, irregular, clumsy, unsteady, and inap- propriately varying in their speed, force, and direc- tion. Dysdiadochokinesia is the failure to accurately perform rapid alternating movements. Ataxia is a term used to describe a wide-based, unsteady gait. Dysmetria is a term used to describe inaccuracies of movements leading to a failure to reach a specified target. This can be tested by having the person touch the examiner’s finger and then alternately touch his or her own finger. These movements are normally smooth and accurate. Asking the person to touch the examiner’s finger with an outstretched arm and finger, first with the eyes open and then closed, provides a test for position sense. Repetitive and consistent devi- ation to one side (referred to as past pointing), which is worse with the eyes closed, suggests cerebellar or vestibular disease. Chorea (abnormal writhing movements), dystonia (abnormal simultaneous contractions of agonist and antagonist muscles, leading to abnormal postures), tremor (rhythmic movements of a particular body part), bradykinesia (slowness of movements), and myoclonus (involuntary jerking movement) indicate abnormalities in the basal ganglia, although the exact localization may be difficult to determine.

SUMMARY CONCEPTS

Disorders of the Motor Unit Most of the diseases of the motor unit cause weakness and wasting of skeletal muscles. The distinguishing fea- tures of these diseases vary depending on which of the components of the motor unit is primarily affected—the cell body of the motor neuron, its axon, the neuromus- cular junction, or the muscle fibers. 7–9 Disorders affect- ing the nerve cell body are often referred to as LMN disorders, those affecting the nerve axon as peripheral neuropathies, and primary disorders affecting the mus- cle fibers as myopathies. ■■ Motor function, whether it involves walking, running, or precise finger movements, requires functioning neural pathways consisting of upper motor neurons (UMNs) that project from the motor cortex to the brain stem or spinal cord, where they innervate the lower motor neurons (LMNs) of the contracting muscles. ■■ Proper control of muscle function requires not only excitation of the muscle by the LMNs located in the spinal cord, but also the function of reflex circuitry that monitors the functional status of the muscle fibers on a moment-by-moment basis.The muscle spindles of the stretch reflex function to monitor and correct for changes in muscle length when extrafusal fibers are either shortened (by contraction) or lengthened (by stretch). ■■ Assessments of muscle strength and muscle bulk, muscle tone and motor reflexes, and patterns of motor movement and posture provide the means for determining the location of disorders of motor function. Paresis (weakness) and paralysis (loss of muscle movement) reflect a loss of muscle strength. UMN lesions tend to produce spastic paralysis, and LMN lesions flaccid paralysis. Changes in muscle bulk are characterized by a loss of muscle mass (atrophy) or an increase in muscle mass (hypertrophy). Hypotonia is a condition of less-than-normal muscle tone, and hypertonia or spasticity is a condition of excessive tone. Abnormal and uncoordinated movements and postures are suggestive of a cerebellar or basal ganglia pathologic process.