The basal ganglia

The basal ganglia, said Kinnear-Wilson in the 1920s, have all the clarity of a dark basement. And so it has remained for the better part of a century. There are now major new ideas about what these deeplying structures in the forebrain may be doing. Lesions of the basal ganglia lead to devastating motor disorders, including Parkinson’s disease and Huntington’s disease. In addition, the basal ganglia have been implicated in a range of neuropsychiatric disorders, and basal ganglia function is disrupted in addictive states. The basal ganglia are also thought to have a major role in learning and memory. How can these structures have so many functions? Or is it possible that there is some common theme to these apparently disparate functions? The position of the basal ganglia in the circuitry of the brain gives a clue to their function (Figure 1). They are the largest subcortical structures in the human forebrain. The basal ganglia receive inputs from the neocortex and, by way of their output nuclei, the basal ganglia nuclei project massively to thalamic nuclei, which in turn project to the frontal cortex. This anatomy means the basal ganglia are in a prime position to influence the executive functions of the forebrain, such as planning for movement and even cognitive behaviors. In addition to this tight linkage with the frontal cortex, the basal ganglia send outputs to brainstem nuclei involved in motor control, including the superior colliculus, which controls axial orientation and saccadic eye movements. The basal ganglia consist of several different nuclei (Figure 1), and each of these is profoundly important clinically. The striatum (made up of the caudate nucleus and the putamen) receives most of the cortical input to the basal ganglia. Degeneration of neurons in the striatum leads to Huntington’s disease and related hyperkinetic disorders. The pallidum (consisting of external and internal segments) receives most of the output of the striatum. The pallidum is the site of therapeutic lesion (pallidotomy) and deep-brain stimulation procedures used to relieve Parkinson’s disease. The subthalamic nucleus is a key structure controlling pallidal function, and is an increasingly favored site for deep-brain stimulation in the treatment of Parkinson’s disease. The substantia nigra (with a dopamine-containing region, the pars compacta, and a second region, the pars reticulata) is critical to basal ganglia function. Dopamine-synthesizing neurons in the substantia nigra pars compacta degenerate in Parkinson’s disease and related parkinsonian disorders. The pedunculopontine nucleus of the brainstem is also associated with the basal ganglia and affects function in several nuclei of basal ganglia circuits, most notably the substantia nigra pars compacta.