Basal Ganglia

Historically, the basal ganglia have been considered as major components of the motor system. In fact, they have a much broader role than that and are probably involved to some extent in most forebrain functions. However, their relationship to movement is their best understood aspect, and that is what shows up clinically in disorders like Parkinson disease and Huntington disease. The interrelationships of the basal ganglia and motor areas of the cerebral cortex are emphasized in this chapter, but you should keep in mind that the basal ganglia have extensive connections, similar in principle and parallel in detail, with most other areas of the cerebral cortex.

The Basal Ganglia Include Five Major Nuclei

Key Concepts

• The striatum and globus pallidus are the major forebrain components of the basal ganglia.

• The substantia nigra and subthalamic nucleus are interconnected with the striatum and globus pallidus.

The meaning of the term “basal ganglia” has changed over the years, but most folks would now agree that there are six major structures on the list: the caudate nucleus , putamen , globus pallidus , nucleus accumbens , substantia nigra , and subthalamic nucleus ( Figs. 19.1 and 19.2 ). 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”). The nucleus accumbens is only found at the more anterior part of the basal ganglia and is identified as congruent between the putamen and caudate.

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 , 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. 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

Key Concept

• Afferents from the cortex reach the striatum and subthalamic nucleus; efferents leave from the globus pallidus and substantia nigra.

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. Hence the basal ganglia play a role in the initiation of things like movements, thoughts, and motivations.

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 (gamma-aminobutyric acid [GABA]) projections to the thalamus, which in turn projects back to a restricted portion of this large cortical area ( Fig. 19.3 ). Because thalamocortical projections are excitatory, the globus pallidus is in a position to suppress or facilitate cortical activity by way of varying patterns of inhibition in the thalamus; the balance of excitatory and inhibitory connections interposed between the striatum and GPi/SNr helps determine the pattern. For example, inhibiting an inhibitory GPi neuron could have the same net effect on the thalamus as increasing excitatory inputs to the same part of the thalamus ( Fig. 19.4 ).

All these structures, and most of their interconnections, are in one cerebral hemisphere, so damage to any of them results in contralateral deficits.

Although the cortex-striatum-globus pallidus-thalamus-cortex pathway is commonly drawn as a single loop, it is actually a system of parallel, overlapping loops, each wired up according to the same principles ( Fig. 19.5 ). So the caudate nucleus, for example, receives its inputs from a different widespread area of the cortex than does the putamen or nucleus accumbens (ventral striatum) and influences its own part of the globus pallidus, which in turn projects to its own part of the thalamus and its own restricted cortical area.

The putamen subsystem is the part most directly involved in movement disorders. Its inputs come from the motor and somatosensory areas flanking the central sulcus. Its outputs reach motor areas of cortex (particularly the supplementary motor area ) by way of the ventral lateral (VL) and ventral anterior (VA) nuclei of the thalamus. This set of connections is consistent with the notion that the putamen and supplementary motor area are somehow involved in planning or initiating voluntary movements.

The caudate nucleus, in contrast, is more involved in cognitive functions, although less is known about how this involvement shows up in our day-to-day activities. Caudate inputs come from widespread association areas of the cerebral cortex; caudate outputs, by way of VA and the dorsomedial nucleus (DM), reach prefrontal cortex. So the anatomical connections are appropriate for an involvement of the caudate in cognitive functions.

Finally, nucleus accumbens and adjacent striatal areas (collectively called the ventral striatum ) receive inputs from limbic structures and project, by way of part of the globus pallidus and thalamus, back to limbic cortex (described in Chapter 23 ).