Thalamus motor circuits

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The specific relay nuclei of the thalamus that are linked with the motor systems, the basal ganglia and the cerebellum, are the ventral lateral (VL) and the ventral anterior (VA) nuclei (see Figure 12 and Figure 63). These project to the different cortical areas involved in motor control, the motor strip, the premotor area, and the supplementary motor area (as shown in the upper insets). These thalamic nuclei also receive input from these cortical areas, in line with the reciprocal connections of the thalamus and cortex. One of the intralaminar nuclei, the centromedian nucleus, is also linked with the circuitry of the basal ganglia (described in the previous illustration).

Basal Ganglia: The neostriatum receives input from wide areas of the cerebral cortex, as well as from the dopamergic neurons of the substantia nigra. Fibers are then sent to the globus pallidus. The major outflow from the basal ganglia, from the internal (medial) segment of the globus pallidus, follows two slightly different pathways to the thalamus, as pallido-thalamic fibers. One group of fibers passes around, and the other passes through the fibers of the internal capsule (represented on the diagram by large stippled arrows). These merge and end in the ventral anterior (VA) and ventral lateral (VL) nuclei of the thalamus (see Figure 63). (The ventral anterior nucleus is not seen on this section through the thalamus.) The other outflow from the basal ganglia via the pars reticulata of the substantia nigra generally follows the same projection to these thalamic nuclei (not shown). The projection from these thalamic nuclei to the cerebral cortex goes to the premotor and supplementary motor areas, as shown in the small insets (in the upper figures; see Figure 14A and Figure 17; also Figure 60), cortical areas concerned with motor regulation and planning.

The pathway from thalamus to cortex is excitatory. The basal ganglia influence is to modulate the level of excitation of the thalamic nuclei. Too much inhibition leads to a situation that the motor cortex has insufficient activation, and the prototypical syndrome for this is Parkinson's (discussed with Figure 24 and Figure 52). Too little inhibition leads to a situation that the motor cortex receives too much stimulation and the prototypical syndrome for this is Huntington's chorea (discussed with Figure 24). The analogy that has been used to understand these diseases is to a motor vehicle, in which a balance is needed between the brake and the gas pedal for controlled forward motion in traffic.

The MOTOR areas of the cerebral cortex that receive input from these two subsystems of the motor system are shown diagrammatically in the small insets, both on the dorsolateral surface and on the medial surface of the hemispheres (see Figure 14 and Figure 17).

Cerebellum (to be reviewed after study of the cerebellum): The other part of the motor regulatory systems, the cerebellum, also projects (via the superior cerebellar peduncles) to the thalamus. The major projection is to the VL nucleus, but to a different portion of it than the part that receives the input from the basal ganglia. From here, the fibers project to the motor areas of the cerebral cortex, predominantly the precentral gyrus as well as the premotor area, areas 4 and 6, respectively (see Figure 57).

Clinical Aspect

Many years ago it was commonplace to refer to the basal ganglia as part of the extrapyramidal motor system (in contrast to the pyramidal motor system — discussed with Figure 45, the cortico-spinal tract). It is now known that the basal ganglia exert their influence through the appropriate parts of the cerebral cortex, which then acts either directly, i.e., using the cortico-spinal (pyramidal) tract, or indirectly via certain brainstem nuclei (cortico-bulbar pathways, see Figure 46) to alter motor activity. The term extrapyramidal should probably be abandoned, but it is still frequently encountered in a clinical setting.

Tourette's syndrome is a motor disorder manifested by tics, uncontrolled sudden movements; occasionally, these individuals have bursts of uncontrolled language, which rarely contains vulgar expletives. This disorder starts in childhood and usually has other associated behavioral problems, including problems with attention. There is growing evidence that this disorder is centered in the basal ganglia. The condition may persist into adulthood.

Supplementary motor area

Precentral gyrus (area 4) Premotor area (area 6)

Supplementary motor area

Precentral gyrus (area 4) Premotor area (area 6)

Thalamo-cortical fiber

Cerebral cortex

C=Fibers of principal striatal circuit Fibers forming internal loop Fibers from dentate n. of cerebellum

Cerebello-thalamic fibers

Internal Capsule

C=Fibers of principal striatal circuit Fibers forming internal loop Fibers from dentate n. of cerebellum

Internal capsule

Nigro-striatal and Striato-nigral fibers

Putamen Striato-pallidal fibers Globus pallidus Pallido-thalamic fibers

Internal capsule

Nigro-striatal and Striato-nigral fibers

Ventral lateral n. Intralaminar n. Centromedian n.

Cerebello-thalamic fibers

Decussation of superior cerebellar peduncles

Substantia nigra

FIGURE 53: Thalamus — Motor Circuits

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