Материал: Kaplan USMLE-1 (2013) - Anatomy

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Chapter 6 • The Cerebellum

ChapterSummary

•The cerebellum controls posture, muscle tone, and learning of repeated motor functions, and coordinates voluntary motor activity. Diseases ofthe cerebellum result in disturbances of gait, balance, and coordinated motor actions, but there is no paralysis or inability to start or stop movement.

•The cerebellum is functionally divided into (1) the vermis and intermediate

zone, (2) the hemisphere, and (3) the flocculonodular lobe. Each ofthese 3 areas receives afferent inputs mainly from the spinal cord, cortex and inferior olivary nucleus, and vestibular nuclei, respectively. These afferent fibers (mossy and climbing) reach the cerebellum via the inferior and middle cerebellar peduncles, which connect the cerebellum with the brain stem. The afferent fibers are excitatory and project directly or indirectlyvia granule cells to the Purkinje cells ofthe cerebellar cortex. The axons of the Purkinje cells are inhibitory and are the only outflow from the cerebellar cortex. They project to and inhibit the deep cerebellar nuclei (dentate, interposed, and fastigial nuclei) in the medulla. From the deep nuclei, efferents project mainly through the superior cerebellar peduncle and drive the upper motor neurons ofthe motor cortex. The efferents from the hemisphere project through the dentate nucleus, to the contralateral ventral lateral/ventral anterior nuclei of the thalamus, to reach the contralateral precentral gyrus.

These influence contralateral lower motor neurons via the corticospinal tract.

•Symptoms associated with cerebellar lesions are expressed ipsilaterally. Unilateral lesions ofthe cerebellum will result in a patient falling toward the side of the lesion. Hallmarks of cerebellar dysfunction include ataxia, intention tremor, dysmetria, and dysdiadochokinesia.

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Basal Ganglia

GENERAL FEATURES

The basal ganglia initiate and provide gross control over skeletal muscle move ments. The major components of the basal ganglia include:

•Striatum, which consists of the caudate nucleus and the putamen (telen- cephalon)

•External and internal segments of the globus pallidus (telencephalon)

•Substantia nigra (in midbrain)

•Subthalamic nucleus (in diencephalon)

Together with the cerebral cortex and the ventrolateral (VL) nucleus of the thal amus, these structures are interconnected to form 2 parallel but antagonistic circuits known as the direct and indirect basal ganglia pathways (Figures IV-7- 1 and IV-7-2). Both pathways are driven by extensive inputs from large areas of cerebral cortex, and both project back to the motor cortex after a relay in the VL nucleus of the thalamus. Both pathways use a process known as "disinhibition'' to mediate their effects, whereby one population of inhibitory neurons inhibits a second population ofinhibitory neurons.

Direct Basal Ganglia Pathway

In the direct pathway, excitatory input from the cerebral cortex projects to striatal neurons in the caudate nucleus and putamen. Through disinhibition, activated inhibitory neurons in the striatum, which use y-aminobutyric acid (GABA) as their neurotransmitter, project to and inhibit additional GABA neurons in the internal segment ofthe globus pallidus.

The GABA axons ofthe internal segment ofthe globus pallidus project to the thala mus (VL). Because their input to the thalamus is disinhibited, the thalamic input excites the motor cortex. The net effect of the disinhibition in the direct pathway results in an increasedlevel ofcortical excitation and the promotion ofmovement.

Indirect Basal Ganglia Pathway

In the indirect pathway, excitatory input from the cerebral cortex also projects to striatal neurons in the caudate nucleus and putamen. These inhibitory neurons in the striatum, which also use GABA as their neurotransmitter, project to and inhibit additional GABA neurons in the external segment ofthe globus pallidus.

The GABA axons ofthe external segment ofthe globus pallidus project to the sub thalamic nucleus. Through disinhibition, the subthalamic nucleus excites inhibi tory GABA neurons in the internal segment ofthe globus pallidus, which inhibits the thalamus. This decreases the level of cortical excitation, inhibiting movement. The net effect of the disinhibition in the indirect pathway results in a decreased level ofcortical excitation, and a suppression ofunwanted movement.

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Dopamine and cholinergic effects

In addition to the GABA neurons, 2 other sources of chemically significant neurons enhance the effects of the direct or indirect pathways.

Dopaminergic neurons in the substantia nigra in the midbrain project to the striatum. The effect of dopamine excites or drives the direct pathway, increasing cortical excitation. Dopamine excites the direct pathway through D1 receptors and inhibits the indirect pathway through D2 receptors.

Cholinergic neurons found within the striatum have the opposite effect. Acetyl choline (Ach)drives the indirect pathway, decreasing cortical excitation.

Figure IV-7-3. MRI of Horizontal Section through

Diencephalon, Basal Ganglia, and Cortex.

(a) Thalamus {b) Head of Caudate Nucleus (c) Genu of Internal Capsule Containing CorticobulbarAxons {d) Posterior Limb of Internal Capsule (e) Primary Visual Cortex (f) Splenium of Corpus Callosum (g) Putamen {h) Broca's Motor Speech Area

(i) Wernicke's Oral Comprehension Area

Chapter 7 • Basal Ganglia

Note

All basal ganglia connections are with ipsilateral cortex.

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Источник: https://studfile.net/preview/14638320/

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