Basal Nuclei

The Basal Nuclei Include Five Major Groups of Cells

The term basal nuclei is slowly replacing the more traditional terminology— basal ganglia; both terms are appropriate and both are commonly used. The meaning of the term basal nuclei has evolved over time, but it is now used to refer to those structures whose damage causes the distinctive kinds of movement disorders (and other disorders) described in this chapter. These structures include some large subcortical nuclei of each cerebral hemisphere—the putamen, caudate, nucleus accumbens, and globus pallidus —together with the diencephalic subthalamic nucleus and the substantia nigra of the rostral midbrain ( Figs. 19.1 and 19.2 ).

Various names are applied to different combinations of members of the basal nuclei ( Fig. 19.3 ). The putamen, caudate nucleus, and nucleus accumbens have a common embryological origin, identical histological appearances, and similar connections. One indication of this common origin is their physical continuity just above the orbital surface of the frontal lobe, where the head of the caudate merges with nucleus accumbens, ^a

a Named historically for its physical location, its original name was nucleus accumbens septi —“nucleus leaning against the septum”—because the region of apparent fusion of the putamen and caudate nucleus seems to lean up against the base of the septum pellucidum (see Fig. 19.2A ).

which in turn merges with the anterior part of the putamen. Another indication is the bridges of gray matter that cross the internal capsule between the putamen and the caudate nucleus, giving this region a striped appearance in many planes of section ( Fig. 19.4 ). Because of this appearance, the putamen, caudate nucleus, and nucleus accumbens together are referred to as the striatum. The putamen and globus pallidus have different connections but are physically apposed, and together they are referred to as the lenticular or lentiform nucleus (from the Latin word for “lentil”).

These assorted names give rise to prefixes and suffixes that are used to describe fibers coming from or going to different parts of the basal nuclei. Strio - and - striate are used for the striatum; thus striopallidal fibers go from, for example, the putamen to the globus pallidus, and corticostriate fibers go from the cerebral cortex to the putamen, caudate nucleus, or nucleus accumbens. The globus pallidus is also called the pallidum, so pallidothalamic fibers go from the globus pallidus to the thalamus. Nigroreticular fibers go from the substantia nigra to the reticular formation.

The Striatum and Globus Pallidus (Internal and External Segments) Are the Major Forebrain Components of the Basal Nuclei

The lenticular nucleus is shaped somewhat like a wedge cut from a sphere (see Figs. 19.1 and 19.2 ). The putamen (from the Latin for “husk”), which is approximately coextensive with the insula, forms the outermost portion of this wedge. It is separated from the more medial globus pallidus by a thin lateral medullary lamina of myelinated fibers. The globus pallidus is divided into internal (medial) and external (lateral) portions by a medial medullary lamina. In unstained sections through the lenticular nucleus, the globus pallidus has a distinctively pale appearance as a result of the large number of myelinated fibers that originate from it or traverse it. (In myelin-stained sections such as those shown in Figs. 19.1 and 19.2 , it is therefore relatively dark.)

The putamen is located where the diencephalon and telencephalon fuse during development (see Fig. 2.11 ). The caudate nucleus is found in the wall of the lateral ventricle and grows with it in a C -shaped course during development. The result is that the caudate nucleus (Latin for “nucleus with a tail”) winds up with an enlarged head that bulges into the anterior horn, a body in the lateral wall of the body of the ventricle, and a slender tail that borders on the inferior horn ( Fig. 19.5 and see Figs. 19.1 and 19.2 ). In the temporal lobe, the tail of the caudate nucleus is continuous with the amygdala, which in turn is continuous with the putamen (see Fig. 25.5 ).

The Subthalamic Nucleus and Substantia Nigra Are Interconnected With the Striatum and Globus Pallidus

The subthalamic nucleus, shaped like a biconvex lens enveloped in white matter, is located between the medial part of the cerebral peduncle and the thalamus (see Fig. 19.2C ). Although relatively small, it has major interconnections with other parts of the basal nuclei (see Fig. 19.17 ).

The region referred to as the substantia nigra actually has two parts ( Fig. 19.6 )—a dorsal compact part (SNc) that contains closely packed, pigmented neurons, and a reticular part (SNr) nearer the cerebral peduncle that contains more loosely packed neurons, most of which are nonpigmented. These correspond to the two distinctly different ways in which the substantia nigra participates in the circuitry of the basal nuclei: the compact part provides widespread, modulatory, dopaminergic projections to other parts of the basal nuclei, whereas the reticular part is one of the basal nuclei output nuclei ( Fig. 19.7 and see Fig. 19.9 ).

Basal Nuclei Circuitry Involves Multiple Parallel Loops That Modulate Cortical Output

Although the basal nuclei are best known for their role in motor control, mediated by their interactions with motor cortex and subcortical structures, it is now clear that they are more generally involved in many disparate cortical functions. The basis of this involvement is a series of loops, each starting with projections from an area of cerebral cortex to the basal nuclei and then returning, by way of the thalamus, to part of this cortical area. The cortical starting and ending points of each loop determine its function, with some related to movement, others to cognition, and still others to emotion and motivation. Interconnections within the basal nuclei determine the pattern of activity in each loop on a moment-to-moment basis.

Afferents From the Cortex Reach the Striatum and Subthalamic Nucleus; Efferents Leave From the Globus Pallidus and Substantia Nigra

Projections from the cerebral cortex reach the striatum and subthalamic nucleus (see Fig. 19.7 ), which in this sense are the principal input elements of the basal nuclei. Outputs leave from the internal segment of the globus pallidus (GPi) and the SNr. The links between these input and output structures are discussed a little later in this chapter. The corticostriate inputs and the projections back to the cortex from the thalamus make excitatory (glutamate) connections, whereas the GPi-SNr outputs are inhibitory (γ-aminobutyric acid [GABA]). There are multiple versions of this loop, all similar in principle but each using different cortical areas and a distinctive portion of the striatum, globus pallidus, and other nuclei. Some loops begin and end in parietal or temporal association cortex, so interactions with the basal nuclei may characterize most cortical areas.

The major circuit through which the basal nuclei participate in the control of movement provides one example of such a loop. The striatum and globus pallidus form by far the largest part of the basal nuclei, yet they have no way to affect motor neurons directly (see Fig. 18.7 ). Thus the only way the basal nuclei can play a role in the control of movement is by influencing one or more of the descending pathways mentioned in the previous chapter. They do so primarily by affecting the activity of motor areas of the cerebral cortex (and, to a degree, through projections to the reticular formation). Somatosensory and motor areas project to a portion of the striatum (mostly putamen), which in turn projects by way of GPi to the ventral anterior and ventral lateral (VA/VL) nuclei; the circuit is completed by projections from VA/VL back to motor areas of the cortex. Other basal nuclei loops use their own distinctive portions of the striatum, subthalamic nucleus, globus pallidus, substantia nigra, thalamus, and cerebral cortex. Collectively the loops roughly correspond to the putamen, caudate nucleus, and nucleus accumbens, which receive inputs from motor and somatosensory cortex, association cortex, and limbic areas, respectively ( Fig. 19.8 ). The partitioning of connections among striatal subdivisions is not absolute. For example, limbic inputs reach not only nucleus accumbens but also adjoining parts of the putamen and caudate nucleus. For this reason, all these limbic-recipient regions are recognized as a separate striatal division called the ventral striatum.

Interconnections of the Basal Nuclei Determine the Pattern of Their Outputs

Most of the remaining connections of the basal nuclei fall into four categories: widespread dopaminergic projections from the compact part of the substantia nigra to other parts of the basal nuclei, especially the striatum; multiple inhibitory interconnections between different parts of the basal nuclei (see Fig. 19.13 ); excitatory projections from the subthalamic nucleus to the globus pallidus (see Fig. 19.17 ); and interconnections of thalamic intralaminar nuclei with the striatum and globus pallidus ( Fig. 19.9 and see Fig. 19.14 ). A striking feature of these connections is the large proportion of inhibitory synapses. All the outputs from the striatum, globus pallidus, and reticular part of the substantia nigra are inhibitory, using GABA as a transmitter. The only prominent source of excitatory (glutamate) projections is the subthalamic nucleus.

In the account that follows, only the best documented connections of the basal nuclei are described. There are still many mysteries about the basal nuclei, and the precise function of most of these connections is unknown. However, there has been enough progress that some of the consequences of damage are beginning to make sense.