Chapter 31: Basal Ganglia Structure & Function

The functional complex is defined as including the corpus striatum—composed of the caudate nucleus, putamen, and globus pallidus—along with the subthalamic nucleus and substantia nigra. The putamen and caudate nucleus are jointly termed the striatum or neostriatum, which serves as the primary 'input' structure, receiving the majority of afferents from the cerebral cortex (glutamatergic), intralaminar thalamus, and aminergic inputs from the substantia nigra pars compacta (dopamine). The globus pallidus, particularly its medial segment, and the substantia nigra pars reticulata form the 'output' structures of the basal ganglia, projecting mainly to the thalamus. Within the striatum, neurochemical organization is layered into a mosaic of striosomes (patches) and a surrounding matrix. Striatal output utilizes GABAergic medium spiny neurons and follows two critical, contrasting pathways: the direct pathway projects to the medial globus pallidus, while the indirect pathway projects via the lateral globus pallidus and the excitatory, glutamatergic subthalamic nucleus. Clinically, disorders of the basal ganglia are characterized by severe abnormalities in movement, tone, and posture. The chapter emphasizes the pathophysiology of Parkinson’s disease, caused by the degeneration of dopaminergic neurons, leading to the overactivity of the indirect pathway and underactivity of the direct pathway, resulting in excessive inhibition of the motor thalamus. Conversely, involuntary movements like dyskinesias are associated with underactivity of the basal ganglia output structures. The deep understanding of these circuits has spurred advanced neurosurgical treatments, notably deep brain stimulation (DBS) and lesioning of key structures like the medial globus pallidus and the subthalamic nucleus, as well as the pedunculopontine nucleus for gait and postural disturbances. Additionally, basal ganglia dysfunction is implicated in other behavioral disorders like obsessive-compulsive disorder, as the basal ganglia participate in parallel, functionally segregated cortico-striato-thalamic loops subserving locomotor, cognitive, and affective behaviors.