Although the basal ganglia nuclei have first been recognized more than 300 years ago by the English anatomist Thomas Willis, our modern view of the basal ganglia circuitry dates back from the mid-1950s after the introduction of tract-tracing methods that allowed investigators to dissect out at the macroscopic and microscopic levels the main constituents and neural connections involved in the transmission and processing of information through these brain regions. In the early 1990s, these efforts led to the development of working models of basal ganglia function and dysfunction (the so-called “direct and indirect basal ganglia pathways”), which, since then, have been the basis for our current view of the basal ganglia circuitry and the framework for the development of modern surgical approaches aimed at lesioning or stimulating the subthalamic nucleus (STN) or the internal globus pallidus (GPi) for basal ganglia diseases. Based on their close anatomical and functional relationships with the basal ganglia, new deep brain stimulation (DBS) targets will be discussed in this chapter.
In light of their clear involvement in neurodegenerative diseases characterized symptomatically by movement disorders, such as Parkinson's disease (PD) and Huntington's chorea, the basal ganglia are commonly seen as integral constituents of the extrapyramidal motor system. Although they, indeed, play significant roles in regulating motor behaviors, basal ganglia functions extend far beyond the motor systems and also include high-order processing of cognitive- and limbic-related information. The functional anatomy of the neural circuits and synaptic networks that underlie motor and non-motor functions of the basal ganglia will be presented and discussed in this chapter.
Because of space constraints, this chapter does not aim at covering the whole literature on basal ganglia anatomy. The readers are referred to other comprehensive reviews and compendiums for a survey of the early literature and a more general overview of this field.1–41
In primates, the basal ganglia are made up of the caudate nucleus and putamen, which form the dorsal striatum and the nucleus accumbens and olfactory tubercle commonly referred to as the ventral striatum. In rats and mice, a single mass of gray matter, called the caudate–putamen complex, forms the dorsal striatum. The other basal ganglia structures include the GP, which is made up of the internal and external segments commonly named GPi and GPe (external GP), in primates. In rodents, the entopeduncular nucleus is the homologue of GPi, while the GP is the homologue of the GPe. Another key basal ganglia structure, often considered as the “pacemaker” of the basal ganglia is the STN, a small almond-shaped nucleus ventral to the thalamus between the diencephalon and midbrain.10,20,21,30,31 The importance of the STN in the normal and pathological circuitry of the basal ganglia is highlighted by the fact that it is a prime surgical target in PD.42–52 Finally, the substantia nigra, located at the basis of the midbrain, is another key component of ...