The basal ganglia are a part of the motor system. The principal nuclei of the basal ganglia are the caudate nucleus, the putamen, and the globus pallidus, all of which lie in the subcortical white matter of the telencephalon. These nuclei are connected to each other, and to the motor cortex, in complex regulatory circuits. They exert both excitatory and inhibitory effects on the motor cortex. They play an important role in the initiation and modulation of movement and in the control of muscle tone. Lesions of the basal ganglia, and of other, functionally related nuclei, such as the substantia nigra and the subthalamic nucleus, can produce either an excess or a deficiency of movement-related impulses, and/or pathological alterations of muscle tone. The most common disease of the basal ganglia is Parkinson’s disease, which is characterized by the clinical triad of rigidity, akinesia, and tremor.
The hierarchically uppermost center for the control of movement is the cerebral cortex, whose signals are transmitted by the pyramidal pathway to the motor cranial nerve nuclei and to the anterior horn cells of the spinal cord (pyramidal system). A number of other structures in the central nervous system participate in the initiation and modulation of movement. The most important of these “accessory motor centers” are the basal ganglia, a set of subcortical nuclei located within the deep white matter of the telencephalon. The pyramidal system was long regarded as the “major” system for the control of movement, as it provides the most direct and most rapid connection between the cortex and the motor neurons of the brainstem and spinal cord. All other structures playing a role in movement were relegated to the so-called extrapyramidal system. This term is misleading, however, because the pyramidal and extrapyramidal systems do not, in fact, operate separately. Rather, they are subunits of a single, integrated motor system and, as such, are closely linked to each other, both structurally and functionally. Thus, there are extensive connections, for example, between the motor cortex and the striatum, an important nucleus within the basal ganglia. The term extrapyramidal system is now obsolete and will be avoided in this book. Instead, we will speak of normal and abnormal function of the basal ganglia.
The corpus striatum is an important control center for the motor system. We will briefly consider its phylogenetic development in this section in order to make its function and anatomical connections easier to understand.
The phylogenetically oldest motor centers in the central nervous system are the spinal cord and the primitive apparatus of the reticular formation in the midbrain tectum. Over the course of phylogeny, the paleostriatum (globus pallidus) developed next, and then the neostriatum (caudate nucleus and putamen), which enlarged in parallel with the cerebral cortex. The neostriatum is particularly well developed in higher mammals, including humans. As the phylogenetically more recent structures grew larger, the older structures came under their influence to an increasing extent. In phylogenetically older species, the older neural centers are primarily responsible for the maintenance of normal muscle tone and for the more or less automatic control of locomotion.
As the cerebral cortex developed, the phylogenetically older motor centers (paleostriatum and neostriatum) came increasingly under the control of the new motor system, i.e., the pyramidal system. While most mammals, including the cat, can still walk without much difficulty after the cerebral cortex is removed, humans are entirely dependent on an intact pyramidal system. Human phylogenetic development has reached the point that the older neural centers can no longer compensate for the functional loss of the new ones. Yet, even in humans, a spastically paralyzed limb can still be seen to make certain involuntary movements, called associated movements, which are generated by the older motor centers.
The basal ganglia include all of the functionally interrelated nuclei within the deep white matter of the telencephalon that are embryologically derived from the ganglionic eminence (anterior portion of the telencephalic vesicle). The major nuclei of the basal ganglia are the caudate nucleus, the putamen, and part of the globus pallidus (▶Fig. 8.1 and ▶Fig. 8.2); other nuclei that are considered part of the basal ganglia on embryological grounds are the claustrum (▶Fig. 8.5 and ▶Fig. 8.6) and the amygdala (▶Fig. 8.1 and ▶Fig. 8.2). The amygdala has already been discussed in connection with the limbic system (see ▶Functions of the Limbic System, Chapter 7). Like the claustrum, whose function is not precisely known, the amygdala has no direct functional connection to the remainder of the basal ganglia. These two structures will not be discussed any further in this chapter.
Caudate Nucleus. The caudate nucleus forms part of the wall of the lateral ventricle and, like it, has an arched shape, due to the rotation of the telencephalon during embryonic development (▶Fig. 9.15). The head of the caudate nucleus forms the lateral wall of the lateral ventricle; its tail forms the roof of the inferior horn of the lateral ventricle in the temporal lobe, extending as far forward as the amygdala, which lies at the anterior end of the inferior horn (▶Fig. 8.2). The caudate nucleus can therefore be seen in two separate locations on some coronal sections (cf. ▶Fig. 8.3, ▶Fig. 8.4, ▶Fig. 8.5, ▶Fig. 8.6, ▶Fig. 8.7, ▶Fig. 8.8, especially ▶Fig. 8.7), in the lateral wall of the body of the lateral ventricle as well as in the roof of the inferior horn. The rostral portion (head) of the caudate nucleus is continuous with the putamen.
Fig. 8.4 Two horizontal sections through the basal ganglia (for planes of section, see ▶Fig. 8.3).
Putamen. The putamen lies lateral to the globus pallidus (or pallidum, so called because of its relatively pale coloration), covering it like a shell and extending somewhat beyond it both rostrally and caudally. The putamen and globus pallidus are separated by a thin layer of white matter called the medial medullary lamina.
The caudate nucleus and putamen are connected by numerous small bridges of gray matter, which are seen as stripes in anatomical sections. These two nuclei together have, therefore, been given the alternative name corpus striatum (striped body), or striatum for short (▶Fig. 8.2). The striation arises during development, when the fibers of the internal capsule grow through the originally uniform basal ganglion. Ventral segments of the striatum are also called the nucleus accumbens.
Globus pallidus. The third major nucleus of the basal ganglia is made up of an internal and an external segment (pars interna and pars externa). Because the globus pallidus is phylogenetically older than the other nuclei, it is also called the paleostriatum. Part of it is, embryologically speaking, a component of the diencephalon. The putamen and globus pallidus are collectively termed the lentiform or lenticular nucleus (lens-shaped nucleus).
Associated nuclei. Further nuclei that are closely functionally related to the basal ganglia include two midbrain nuclei—the substantia nigra (reciprocally connected to the striatum) and the red nucleus—and one diencephalic nucleus, the subthalamic nucleus (reciprocally connected to the globus pallidus). The globus pallidus caudally borders the rostral portion (red zone) of the substantia nigra. The pallidum, substantia nigra, and red nucleus contain large amounts of iron. The dark pigmentation of the substantia nigra (“black substance”) is due to its high melanin content.
The neural connections of the basal ganglia with one another and with other regions of the brain are not yet completely understood. The major afferent and efferent pathways will be described in this section.
Afferent pathways to the corpus striatum. The corpus striatum receives afferent input from extensive areas of the cerebral cortex, particularly the motor areas of the frontal lobe, i.e., Brodmann areas 4, 6aα, and 6aβ. These cortical afferents are derived from projection neurons of the cerebral cortex (pyramidal cells of the fifth layer of the cortex), are glutamatergic, run ipsilaterally, and are topically organized. There are probably no reciprocal fibers running from the corpus striatum back to the cortex. A further point-to-point afferent input to the corpus striatum is derived from the centromedian nucleus of the thalamus, and is probably excitatory. This afferent pathway transmits impulses from the cerebellum and the midbrain reticular formation to the striatum. The substantia nigra sends dopaminergic afferent fibers to the striatum, whose loss is the cause of Parkinson’s disease. Finally, the striatum also receives a serotonergic input from the raphe nuclei.
Other afferent pathways. The globus pallidus derives its major afferent input from the corpus striatum and receives no direct afferent fibers from the cerebral cortex. Cortically derived afferent fibers do, however, travel to the substantia nigra, red nucleus, and subthalamic nucleus.