329 8

8 Basal Ganglia

Preliminary Remarks

on Terminology . . . . . . . . . . . . . . . 330

The Role of the Basal Ganglia

in the Motor System:

Phylogenetic Aspects . . . . . . . . . . 331

Components of the Basal Ganglia

and Their Connections . . . . . . . . . 332

Function and Dysfunction

of the Basal Ganglia . . . . . . . . . . . 340

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8330

8Basal Ganglia

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 disease, which is characterized by the clinical triad of rigidity, akinesia, and tremor.

Preliminary Remarks on Terminology

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

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The Role of the Basal Ganglia in the Motor System: Phylogenetic Aspects · 331 8

and will be used only rarely in this book. Instead, we will speak of normal and abnormal function of the basal ganglia.

The Role of the Basal Ganglia in the Motor System: Phylogenetic Aspects

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.

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8332 · 8 Basal Ganglia

Components of the Basal Ganglia and Their

Connections

Nuclei

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 (Figs. 8.1 and 8.2); other nuclei that are considered part of the basal ganglia on embryological grounds are the claustrum (Figs. 8.5 and 8.6) and the amygdala (Figs. 8.1 and 8.2). The amygdala has already been discussed in connection with the limbic system (p. 319). 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.

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 (cf. p. 352). 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. Figs. 8.3­8.8, 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.

Theputamen lieslateraltotheglobuspallidus(orpallidum,socalledbecauseof its relatively pale coloration), covering it like a shell and extending somewhat beyonditbothrostrallyandcaudally.Theputamenandglobuspallidusareseparated 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.

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

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Components of the Basal Ganglia and Their Connections · 333 8

Thalamus

Globus pallidus

Putamen

Caudate nucleus

Amygdala

Fig. 8.1 Topographical relationships of the basal ganglia (in red)

Head of the caudate nucleus

Putamen
Subthalamic nucleus	Lateral
Amygdala	ventricle
Tail of the caudate nucleus	Thalamus
Fig. 8.2 Lateral view of the basal ganglia and ventricular system

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8334 · 8 Basal Ganglia

Putamen

Caudate nucleus

X

XX

Amygdala

Thalamus

Fig. 8.3 Lateral view of the basal ganglia. X, XX: horizontal planes of section for Fig. 8.4. 1−4: coronal planes of section for Figs. 8.5−8.8.

		Thalamus
Head of the caudate nucleus		Tail of the caudate nucleus
Putamen		Choroid plexus of
Lateral		the lateral ventricle
		Splenium of the
ventricle
Globus		corpus callosum
pallidus
Genu of the
corpus	X
callosum
	XX	Vermis
		Superior
		and inferior
		colliculi
		Pineal body
		Inferior horn of
		the lateral ventricle

Fig. 8.4 Two horizontal sections through the basal ganglia (for planes of section, see. Fig. 8.3)

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		Components of the Basal Ganglia and Their Connections · 335		8
Corpus	Lateral	Head of the	Fig. 8.5 Coronal section 1 through the
callosum	ventricle	caudate nucleus	basal ganglia (for planes of section,
			see. Figs. 8.3 and 8.4)

Internal

capsule

Putamen

Insula

Claustrum

Septum pellucidum

		Corpus	Lateral
		callosum	ventricle	Hypothalamus
				Body of the
				caudate nucleus
				Internal
				capsule
				Putamen
				Claustrum
				Insula
				Globus
				pallidus
		Septum		Anterior
		pellucidum
				commissure
	Fig. 8.6 Coronal section 2 through the	Optic recess of
		the third ventricle
	basal ganglia (for planes of section,			Olfactory area
	see Figs. 8.3 and 8.4)		Optic chiasm

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8336 · 8 Basal Ganglia

Corpus	Fornix	Choroid plexus of the	Fig. 8.7 Coronal section 3 through the
callosum		lateral ventricle	basal ganglia (for planes of section, see Figs.
		Thalamostriate v.	8.3 and 8.4)
		Body of the
		caudate nucleus
		Thalamus

Internal

capsule

Claustrum

Putamen

Globus pallidus

Tail of the caudate nucleus

Mamillary

body

Hippocampus;

inferior horn of Mamillo- the lateral ventricle thalamic tract

Optic tract

Subthalamic nucleus

Splenium of the

corpus callosum

Great cerebral v. (of Galen)

Tapetum

Posterior horn of the lateral ventricle

Nucleus of the inferior colliculus

Superior cerebellar peduncle

Cerebellum

		Cerebral aqueduct
		Middle cerebellar
Fig. 8.8 Coronal section 4 through the		peduncle
basal ganglia (for planes of section, see Figs.	Periaqueductal	Medial
8.3 and 8.4)	gray matter	lemniscus

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