Материал: Kaplan USMLE-1 (2013) - Anatomy

Section IV • Neuroscience

Figure IV-7-4. Coronal Section through Basal Ganglia and Other Subcortical Structures

(A)caudate nucleus (B) putamen (C) globus pallidus external segment

(D)globus pallidus internal segment (E) septal nuclei (F) fornix (G) lateral ventricle (H) anterior commissure (I) optic chiasm (J) basal nucleus of Meynert (K) preoptic hypothalamus (L} internal capsule, anterior limb

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Section IV • Neuroscience

Clinical Correlate

Lesions or Diseases ofthe Basal Ganglia

Lesions or diseases ofthe basal ganglia generally present with movement disorders, known as dyskinesias, and an involuntary tremor, ortremor at rest.

Most basal ganglia disorders seem to preferentially affect either the direct or the indirect pathways, altering the balance between the two.

Lesions ofthe direct pathway

Lesions of the direct pathway result in an underactive cortex and hypokinetic disturbances in which there is a slowing or absence of spontaneous movements. The best-known disorder of the direct pathway is caused by the degeneration of dopaminergic neurons of the substantia nigra in Parkinson disease. Because the cortex is underactive, Parkinson patients have problems initiating movements, combined with a reduction in the velocity and amplitude of the movements. The tremor at rest is the classic pill-rolling tremor seen in the fingers. Skeletal muscles in the upper limbs exhibit a cogwheel rigidity because of increased muscle tone. Patients also present with a stooped posture, an expressionless face, and a festinating or accelerating gait during which individuals seem to chase their center of gravity. One strategy for Parkinson patients is to give them L-dopa, a dopamine precursor that crosses the blood-brain barrier. Another strategy is to give anticholinergic drugs to inhibit the effects of acetylcholine on the indirect pathway.

Lesions ofthe indirect pathway

Other common disorders of the basal ganglia (chorea, athetosis, dystonia, tics) result from lesions to parts of the indirect pathway, which result in an overactive motor cortex. An overactive cortex produces hyperkinetic disturbances, expressed in numerous spontaneous movements. The involuntary tremors seen in these diseases range from being dancelike in chorea to ballistic with lesions to the subthalamic nucleus.

Chorea produces involuntary movements that are purposeless, quick jerks that may be superimposed on voluntary movements. Huntington chorea exhibits autosomal dominant inheritance (chromosome 4) and is characterized by severe degeneration of GABA neurons in the striatum. In addition to chorea, these patients frequently sufferfrom athetoid movements, progressive dementia, and behavioral disorders. Sydenham chorea is a transient complication in some children with rheumatic fever.

Athetosis refers to slow, worm like, involuntary movements that are most noticeable in the fingers and hands but may involve any muscle group. It is present in Huntington disease and may be observed in many diseases that involve the basal ganglia.

Dystonia refers to a slow, prolonged movement involving predominantly the truncal musculature. Dystonia often occurs with athetosis. Blepharospasm (contraction ofthe orbicularis oculi causing the eyelids to close), spasmodic torticollis (in which the head is pulled toward the shoulder), and writer's cramp (contraction ofarm and hand muscles on attempting to write) are all examples

of dystonic movements.

(Continued)

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Chapter 7 • Basal Ganglia

Clinical Correlate (Continued)

Hemiballismus results from a lesion of the subthalamic nucleus usually seen in hypertensive patients. Hemiballismus refers to a violent projectile movement of a limb and is typically observed in the upper limb contralateral to the involved subthalamic nucleus.

Tourette syndrome involves facial and vocaltics that progress to jerking movements ofthe limbs. It is frequently associated with explosive, vulgarspeech.

Wilson disease results from an abnormality of copper metabolism, causing the accumulation of copper in the liver and basal ganglia. Personality changes, tremor, dystonia, and athetoid movements develop. Untreated patients usually succumb because of hepatic cirrhosis. A thin brown ring around the outer cornea, the Kayser-Fleischer ring, may be present and aid in the diagnosis.

ChapterSummary

•The basal ganglia play important motor functions in starting and stopping

voluntary motor functions and inhibiting unwanted movements. The basal ganglia consists of 3 nuclei masses deep in the cerebrum (caudate nucleus, putamen, and globus pallidus), one nucleus in the midbrain (substantia nigra), and the subthalamic nucleus ofthe diencephalon. The striatum combines the caudate nucleus and the putamen while the corpus striatum

consists ofthese 2 nuclei plus the globus pallidus.

•There are 2 parallel circuits (direct and indirect) through the basal ganglia. These circuits receive extensive input from the cerebral cortex that project back to the motor cortex after a relay in the ventrolateral (VL) nucleus ofthe thalamus. Both ofthese pathways demonstrate disinhibition. The direct pathway increases the level of cortical excitation and promotes movement. The indirect pathway decreases the level of cortical excitation and suppresses unwanted movement.

•The striatum is the major input center and the globus pallidus is the major output centerforthe pathways through the basal ganglia. Critical to proper function ofthe striatum is dopamine production by the substantia nigra.

Dopamine excites the direct pathway and inhibits the indirect pathway.

•Lesions of the direct pathway result in an underactive cortex, which produces hypokinetic motor disturbances. The classic disorder caused by degeneration of dopaminergic neurons of the substantia nigra is Parkinson disease. These patients are characterized by tremor at rest (pill-rolling), increased muscle tone, mask face, and hypokinetic movement.

•Hyperkinetic disorders result from lesions ofthe indirect pathway and cause an overactive motor cortex. These movements occur spontaneously at rest and cannot be controlled by the patient. Examples ofthese disorders include chorea (multiple quick movements), athetosis (slow serpentine movements), and hemiballismus (violent flinging movements). Hemiballismus results from hemorrhagic destruction ofthe contralateral subthalamic nucleus.

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