Basal ganglia circuitry

The Parkinson's-Reversing Breakthrough

Medication for Parkinson Disease

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Upper Illustration

This is the same view of the basal ganglia as shown previously (see Figure 24), with the head of the caudate nucleus removed. The illustration includes the two other parts of the basal ganglia as a functional "system" — the subthalamic nucleus and the substantia nigra.

• The subthalamic nucleus (S) is situated in a small region below the level of the thalamus.

• The substantia nigra (SN) is a flattened nucleus located in the midbrain region. It is composed of two parts (see Figure 65A).

• The pars compacta has the pigment-containing cells (see Figure 15B and Figure 65). These neurons project their fibers to the caudate and putamen (the striatum or neostria-tum). This is called the nigro-striatal "pathway," although the fibers do not form a compact bundle; the neurotransmitter involved is dopamine.

• The pars reticulata is situated more ven-trally. It receives fibers from the striatum and is also an output nucleus from the basal ganglia to the thalamus, like the internal segment of the globus pallidus (see below).

Lower Illustration: Basal Ganglia Circuitry

Information flows into the caudate (C) and putamen (P) from all areas of the cerebral cortex (in a topographic manner, see next illustration), from the substantia nigra (dopaminergic from the pars compacta), and from the centromedian nucleus of the thalamus (see below). This information is processed and passed through to the globus pallidus, internal segment (GPi), and the pars reticulata of the substantia nigra; these are the output nuclei of the basal ganglia.

Most of this information is relayed to the specific relay nuclei of the thalamus, the ventral anterior (VA) and ventral lateral (VL) nuclei (see Figure 12 and Figure 63). These project to the premotor and supplementary motor cortical areas (see Figure 14A, Figure 17, and Figure 60).

(This is to be contrasted with the projection of the cerebellum to the cortex, discussed with Figure 57.) The circuitry involving the basal ganglia, the thalamus, and the motor cortical areas will be described in detail with the next illustration.

In addition, there is a subcircuit involving the subtha-lamic nucleus (S): the external segment of the globus pallidus sends fibers to the subthalamic nucleus, and this nucleus sends fibers to the internal segment of the globus pallidus, the output portion.

Another subloop of the basal ganglia involves the centromedian nucleus of the thalamus, a nonspecific nucleus (see Figure 12). The loop starts in the striatum (only the caudate nucleus is shown here), to both segments of the globus pallidus; then fibers from the globus pallidus internal segment are sent to the centromedian nucleus, which then sends its fibers back to the striatum (see Figure 63).

Clinical Aspect (see also Figure 24)

Parkinson's disease: The degeneration of the dopamine-containing neurons of the pars compacta of the substantia nigra, with the consequent loss of their dopamine input to the basal ganglia (the striatum) leads to this clinical entity. Those afflicted with this disease have slowness of movement (bradykinesia), reduced facial expressiveness ("mask-like" face), and a tremor at rest, typically a "pill-rolling" type of tremor. On examination, there is rigidity, manifested as an increased resistance to passive movement of both flexors and extensors, which is not velocity-dependent. (This is to be contrasted with spasticity, discussed with Figure 49B.) In addition, there is no change in reflexes.

The medical treatment of Parkinson's disease has limitations, although various medications and combinations (as well as newer drugs) can be used for many years. For these patients, as well as in other select clinical cases, a surgical approach for the alleviation of the symptoms of the Parkinson's disease has been advocated, including placing lesions in the circuitry or using stimulating electrodes (with external control devices). To date, the theory has been that these surgical approaches are attempting to restore the balance of excitation and inhibition to the thalamus, thereby restoring the appropriate influence to the cortical areas involved in motor control.

The motor abnormality associated with a lesion of the subthalamic nucleus is called hemiballismus. The person is seen to have sudden flinging movements of a limb, on the side of the body opposite to the lesion. The likely cause for this is usually a vascular lesion.

Putamen (P)

Globus pallidus (external segment; GPe)

Globus pallidus (internal segment; GPi)

Substantia nigra (SN)

Subthalamic n. (S)

Amygdala

Putamen (P)

Globus pallidus (external segment; GPe)

Globus pallidus (internal segment; GPi)

Substantia nigra (SN)

Subthalamic n. (S)

Amygdala

Centromedian n.

Globus Pallidus

Fibers forming internal loop

Pallido-subthalamic and subthalamo-pallidal fibers Striato-nigral and nigro-striatal fibers

Centromedian n.

Fibers forming internal loop

Pallido-subthalamic and subthalamo-pallidal fibers Striato-nigral and nigro-striatal fibers

Th = Thalamus Md = Midbrain

FIGURE 52: Basal Ganglia Circuitry

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