A part of the basal ganglia, the cluster of neurons deep within the brain that regulates voluntary movement. The striatum consists of the putamen and the caudate, two components of the basal ganglia, and controls the smoothness and coordination of movement. The name means “striped,” a reference to the striatum’s cordlike appearance. The striatum is primarily a bidirectional conduit linking the basal ganglia and other structures of the brain that participate in movement. Striatal neurons in the putamen are the main targets in the basal ganglia for dopamine from the substantia nigra, the key neurotransmitter affected by Parkinson’s disease. It provides inhibitory (mainly GABA) outputs to the globus pallidus, thalamus, and brainstem.
The substantia nigra produces most of the dopamine that these structures require. The pig-mented dopamine-producing neurons of the substantia nigra have long projections, called the nigrostriatal fibers, that carry dopamine to the putamen. These fibers are lost as Parkinson’s progresses. The striatonigral fibers conduct signals from the striatum and the thalamus to the substantia nigra; this pathway seems to remain intact as Parkinson’s progresses, although the diminished supply of dopamine alters neuron function just as it does elsewhere in the basal ganglia.
The striatum also produces and circulates small amounts of acetylcholine, a neurotransmitter that facilitates communication between neurons and muscle cells. Although acetylcholine levels do not change in Parkinson’s disease, researchers believe the imbalance between dopamine and acetylcholine that results when dopamine becomes depleted allows acetylcholine to become hyperactive, overstimulating neurons to the extent that nerve signals overwhelm muscle cells. Such changes in the striatum may cause the tremors characteristic of Parkinson’s disease. Surgical treatment that targets the globus pallidus or especially the thalamus, structures that receive and send nerve signals through the striatum, is sometimes successful in controlling tremors for an extended time.