Introduction
The previous three chapters have described the anatomy and function of the four levels of the motor system hierarchy: the spinal cord, the brainstem, the motor cortex, and the association cortex. Two other brain structures can be considered as “side loops” in the motor hierarchy. They influence the processing of motor control and modulate the output of the descending pathways without directly causing motor output. Both of these structures—the basal ganglia and the cerebellum—are now known to have other functions in addition to modulating motor control. Because the most obvious clinical signs of damage to these areas are a wide variety of motor impairments, they are still generally considered to be motor structures. Basal ganglia dysfunction causes a set of symptoms that are quite different from damage to descending motor pathways, and thus the basal ganglia were at one time considered to form an “extrapyramidal motor system” that was distinct from the pyramidal tract pathways. It is now known that the basal ganglia do not originate a separate motor pathway. Instead, they influence and modulate the activity of motor cortex and the descending motor pathways in ways that cause distinct symptoms when different basal ganglia structures are damaged.
Gross Anatomy of the Basal Ganglia
The basal ganglia comprise a distributed set of brain structures in the telencephalon, diencephalon, and mesencephalon (Figure 4.1 and Table 1). The forebrain structures include the caudate nucleus, the putamen, the nucleus accumbens (or ventral striatum) and the globus pallidus. Together, these structures are named the corpus striatum. The caudate nucleus is a C-shaped structure that is closely associated with the lateral wall of the lateral ventricle. It is largest at its anterior pole (the head), and its size diminishes posteriorly as it follows the course of the lateral ventricle (the body) all the way to the temporal lobe (the tail), where it terminates at the amygdaloid nuclei. The putamen is also a large structure that is separated from the caudate nucleus by the anterior limb of the internal capsule. The putamen is connected to the caudate head by bridges of cells that cut across the internal capsule. Because of the striated appearance of these cell bridges (Figure 4.1B), the caudate and putamen are collectively referred to as the striatum or neostriatum, and the nucleus accumbens is often called the ventral striatum. Functionally, the caudate nucleus and the putamen are considered equivalent to each other; indeed, most mammals have only a single nucleus called the striatum. It is unclear whether there is any functional significance of the separation of the striatum into the caudate and putamen in primates. The putamen and the globus pallidus are collectively called the lenticular nucleus, or lentiform nucleus. The globus pallidus is divided into two segments: the internal (or medial) segment and the external (or lateral) segment.
Figure 4.1 |
Table I |
The subthalamic nucleus is part of the diencephalon; as its name implies, it is located just below the thalamus. The substantia nigra is a midbrain structure, composed of two distinct parts: the pars compacta and the pars reticulata. The substantia nigra is located between the red nucleus and the crus cerebri (cerebral peduncle) on the ventral part of the midbrain. The pars compacta is the source of a clinically important dopaminergic pathway to the striatum; loss of neurons in this area is the cause of Parkinson’s disease (see below). An area that is functionally analogous to the substantia nigra pars compacta is the ventral tegmental area, which is located nearby and makes a dopaminergic projection to the nucleus accumbens.
Historically, the amygdaloid complex and the claustrum were considered parts of the basal ganglia. However, modern usage usually restricts the term to those structures that cause the motor impairments characteristic of the extrapyramidal syndrome (caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra).
Basal Ganglia Afferents
The striatum is the main recipient of afferents to the basal ganglia (Figure 4.2). These excitatory afferents arise from the entire cerebral cortex and from the intralaminar nuclei of the thalamus (primarily the centromedian nucleus and parafascicularis nucleus). The projections from different cortical areas are segregated, such that the frontal lobe projects predominantly to the caudate head and the putamen; the parietal and occipital lobes project to the caudate body; and the temporal lobe projects to the caudate tail. The primary motor cortex and the primary somatosensory cortex project mainly to the putamen, whereas the premotor cortex and supplementary motor areas project to the caudate head. Other cortical areas project primarily to the caudate. Thus, along the C-shaped extent of the caudate nucleus, the caudate cells receive their input from the cortical regions that are close by. The enlarged head of the caudate reflects the large projection from the frontal cortex to the caudate. In addition, the nucleus accumbens (ventral striatum) receives a large input from limbic cortex.
In the motor regions of the basal ganglia, there is a motor homunculus similar to that seen in the primary motor cortex. Thus, the projections from the medial wall of the anterior paracentral lobule (the part of M1 that contains a representation of the legs and torso) innervate regions of the striatum that are next to the recipient zones from the dorsal surface of the precentral gyrus (the part of M1 that contains a representation of the arms and hands). Similarly, the projections from the lateral surface of the precentral gyrus (the part of M1 that contains a representation of the face) innervate regions that are next to the arm and hand representation. This topography of projections is maintained in the intrinsic circuitry of the basal ganglia.
Basal Ganglia Efferents
The major output structures of the basal ganglia are the globus pallidus internal segment (GPint) and the substantia nigra pars reticulata (SNr) (Figure 4.3). Both of these structures make GABAergic, inhibitory connections on their targets. The GPint projects to a number of thalamic structures by way of two fiber tracts: the ansa lenticularis and the lenticular fasciculus. The loop that processes sensorimotor information from the motor cortex and the somatosensory cortex projects to the ventral anterior (VA) and ventral lateral (VL) nuclei. The loop that processes other neocortical information projects to the dorsomedial nucleus (DM), intralaminar nuclei, and parts of the VA nucleus. The SNr projects to the superior colliculus, which is involved in eye movements, as well as to the VA/VL thalamic nuclei.
Figure 4.3 |
