43: The Basal Ganglia

Introduction

• The Basal Ganglia Consist of Several Interconnected Nuclei

• A Family of Cortico–Basal Ganglia–Thalamocortical Circuits Subserves Skeletomotor, Oculomotor, Associative, and Limbic Functions

• The Cortico–Basal Ganglia–Thalamocortical Motor Circuit Originates and Terminates in Cortical Areas Related to Movement

  • The Motor Circuit Plays a Role in Multiple Aspects of Movement

  • Dopaminergic and Cholinergic Inputs to the Striatum Are Implicated in Reinforcement Motor Learning

• Other Basal Ganglia Circuits Are Involved in the Regulation of Eye Movements, Mood, Reward, and Executive Functions

• Diseases of the Basal Ganglia Are Associated with Disturbances of Movement, Executive Function, Behavior, and Mood

  • Abnormalities in the Basal Ganglia Motor Circuit Result in a Wide Spectrum of Movement Disorders

  • A Deficiency of Dopamine in the Basal Ganglia Leads to Parkinsonism

  • Reduced and Abnormally Patterned Basal Ganglia Output Results in Hyperkinetic Disorders

  • Abnormal Neuronal Activity in Nonmotor Circuits Is Associated with Several Neuropsychiatric Disorders

• An Overall View

The traditional view that the basal ganglia play a role in movement stems largely from the fact that diseases of the basal ganglia, such as Parkinson and Huntington disease, are associated with prominent disturbances of movement and from the earlier belief that basal ganglia neurons send their output exclusively to the motor cortex by way of the thalamus. However, we now know that the basal ganglia also project to nonmotor areas of the cerebral cortex, providing a mechanism whereby they may participate in a wide variety of nonmotor functions, and that diseases of the basal ganglia are associated with complex behavioral and neuropsychiatric disturbances.

In this chapter we first describe the individual nuclei of the basal ganglia anatomically and then discuss their function in the context of the larger networks in which they participate. The delineation of brain circuits into which the basal ganglia are incorporated has enabled researchers to better understand the pathophysiology of some of the major diseases affecting basal ganglia functions. These disease states are described at the end of the chapter.

The Basal Ganglia Consist of Several Interconnected Nuclei

The basal ganglia comprise four principal structures: the striatum, globus pallidus, substantia nigra, and subthalamic nucleus (Figure 43–1).

The striatum is separated by the internal capsule into the caudate nucleus and the putamen. The striatum is the major input structure of the basal ganglia, receiving prominent projections from the cerebral cortex, brain stem, and thalamus. The globus pallidus consists of two separate nuclei, the external and internal segments, each with different connectivity and functions. The internal segment is one of the major output structures of the basal ganglia, whereas the external segment is part of their intrinsic circuitry.