The Basal Ganglia Include Five Major Nuclei

The meaning of the term “basal ganglia” has changed over the years, but most folks would now agree that there are five major structures on the list: the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus (Fig. 19-1). The caudate nucleus and putamen have similar but parallel connections and are referred to in combination as the striatum. The putamen and globus pallidus have very different connections but are physically stuck together; in combination, they are referred to as the lenticular nucleus (from the Latin word for “lentil”).

Figure 19-1 Major components of the basal ganglia, and terminology used to refer to them. Nucleus accumbens, the third component of the striatum, lies anterior to this plane of section. GPe, External segment of the globus pallidus; GPi, internal segment of the globus pallidus; STN, subthalamic nucleus.

The terms strio– and –striate are used to refer to fibers coming from or going to the striatum; for example, corticostriate fibers start in the cerebral cortex and end in the caudate nucleus or putamen. Similarly, the terms pallido– and –pallidal, nigro– and –nigral, and subthalamo– and –subthalamic are used to refer to fibers coming from or going to the globus pallidus, the substantia nigra, or the subthalamic nucleus.

The caudate nucleus parallels the lateral ventricle, having an enlarged head in the wall of the anterior horn, a smaller body adjacent to the body of the ventricle, and a still smaller tail adjacent to the inferior horn.

The caudate nucleus and putamen merge with each other anteriorly at the base of the septum pellucidum; the area of fusion is nucleus accumbens, now recognized as a third division of the striatum.

The lenticular nucleus (putamen + globus pallidus) underlies the insula and is shaped like a wedge cut from a sphere (THB6 Figures 19-1 and 19-2, pp. 475 and 476). The globus pallidus is the more medial, tapering part of the wedge, extending toward the interventricular foramen and thalamus; it has two parts with distinctive connections, an external segment (GPe) adjacent to the putamen and an internal segment (GPi) closer to the thalamus.

The substantia nigra is mostly located in the rostral midbrain, between the cerebral peduncle and the red nucleus; part of it extends rostrally just into the diencephalon. The substantia nigra is another two-part structure. The compact part (SNc), closer to the red nucleus, contains the pigmented, dopaminergic neurons for which the substantia nigra was named; the reticular part (SNr), adjacent to the cerebral peduncle, is in effect a displaced piece of GPi.

The subthalamic nucleus, as its name implies, is located inferior to the thalamus, just above the most rostral part of the substantia nigra.

Basal Ganglia Circuitry Involves Multiple Parallel Loops That Modulate Cortical Output

How does damage to the basal ganglia cause movement (and other) disorders? For the most part, we know only the broad outlines of an answer, but there is one basic fact to keep in mind: The basal ganglia have no major outputs to lower motor neurons. Instead, they work primarily by influencing what comes out of the cerebral cortex.

The striatum is, in a sense, the major input part of the basal ganglia, collecting excitatory (glutamate) inputs from large cortical areas (different areas for different parts of the striatum). GPi and SNr are the principal output structures, sending inhibitory (GABA) projections to the thalamus, which in turn projects back to a restricted portion of this large cortical area (Fig. 19-2