Basal ganglia: functional anatomy and neuropharmacology

Introduction
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By Philip A Hanna MD, Olukayode Onasanya MD, and Jonathan W Mink MD PhD

The basal ganglia comprise several interconnected nuclei in the forebrain, midbrain, and diencephalon that play important roles in various processes including motor, associative, cognitive, and mnemonic functions. In primates, the basal ganglia are divided into dorsal and ventral parts. The dorsal division consists of the striatum (caudate and putamen), the globus pallidus (external segment, Gpe, and internal segment, Gpi, which is known as the entopeduncular nucleus in rodents), the subthalamic nucleus (nucleus of Luys), and substantia nigra (the dorsal pars compacta, SNpc, and the ventral pars reticulate, SNpr). These are associated with motor and associative functions. In addition, a ventral division of the basal ganglia, ventral striatum or nucleus accumbens, ventral pallidum, and the ventral tegmental area is associated with limbic functions. The putamen and the globus pallidus together make up the lentiform or lenticular nucleus. The volume of the basal ganglia seems significantly smaller on the left compared to the right side, irrespective of gender and handedness (Afifi 2003). The striatum and subthalamic nucleus are the primary input structures of the basal ganglia, receiving excitatory input from cerebral cortex. Globus pallidus internal segment and substantia nigra pars reticulata are the primary output nuclei, sending inhibitory output to thalamus and brainstem targets. Acting through the thalamus, the basal ganglia output influences frontal lobe cortical neurons. By virtue of the inhibitory output from the basal ganglia, conditions associated with destruction of the output nuclei are associated with unwanted and nonspecific overactivity of thalamocortical and brainstem targets; conditions associated with excessive activity of the output neurons are associated with underactivity of thalamocortical and brainstem targets.