Dyskinesias (dis-ki-ne-se-ahs) are a group of disorders often involving the basal ganglia in which unwanted, superfluous movements occur. Defects in the basal ganglia may result in brisk, jerky, purposeless movements that resemble fragments of voluntary movements. Sydenham's chorea (ko-re'ah; St. Vitus' dance) is a disease usually associated with a toxic or infectious disorder that apparently causes temporary dysfunction of the corpus striatum and usually affects children. Huntington's chorea is a dominant hereditary disorder that begins in middle life and causes mental deterioration and progressive degeneration of the corpus striatum in affected individuals.

Cerebral palsy is a general term referring to defects in motor functions or coordination resulting from several types of brain damage, which may be caused by abnormal brain development or birth-related injury. Some symptoms of cerebral palsy are related to basal ganglia dysfunction. Athetosis (ath-e-to'sis), often one of the features of cerebral palsy, is characterized by slow, sinuous, aimless movements. When the face, neck, and tongue muscles are involved, grimacing, protrusion and writhing of the tongue and difficulty in speaking and swallowing are characteristics.

Damage to the subthalamic nucleus can result in hemiballismus (hem'e-ba-liz'mus), an uncontrolled, purposeless, and forceful throwing or flailing of the arm. Forceful twitching of the face and neck may also result from subthalamic nuclear damage.

Parkinson's disease, characterized by muscular rigidity, loss of facial expression, tremor, a slow, shuffling gait, and general lack of movement, is caused by a dysfunction in the substantia nigra. The disease usually occurs after age 55 and is not contagious or inherited. A resting tremor called "pill-rolling" is characteristic of Parkinson's disease and consists of circular movement of the opposed thumb and index finger tips. The increased muscular rigidity in Parkinson' disease results from defective inhibitions of some of the basal ganglia by the substantia nigra. In this disease, dopamine, and inhibitory neurotransmitter substance, is deficient. The melanin-containing cells of the substantia nigra degenerate, resulting in a loss of pigment. Parkinson's disease can be treated with levodopa (L-dopa), a precursor to dopamine, or, more effectively, with Sinemet, a combination of L-dopa and carbidopa. Carbidopa prevents L-dopa from being absorbed by tissues other than the brain. A protein called glial cell line-derived neurotrophic factor (GDNF) has been discovered that selectively promotes the survival of dopamine-secreting neurons.

Cerebellar lesions result in a spectrum of characteristic functional disorders. Movements tens to be ataxic (jerky) and dysmetric (overshooting--for example, pointing past or deviating from a mark that one tries to touch with the finger). Alternating movements such as supination and pronation are performed in a clumsy manner. Nystagmus, which is a constant motion of the eyes, may also occur. A cerebellar tremor is an intention tremor (that is, the more carefully one tries to control a given movement, the greater the tremor becomes). For example, when a person with a cerebellar tremor attempts to drink a glass of water, the closer the glass comes to the mouth, the shakier the movement becomes. This type of tremor is in direct contrast to basal ganglia tremors described previously in which the resting tremor largely or completely disappears during purposeful movement.

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