Four masses of gray matter located deep in the cerebral hemispheres: caudate, lentiform, and the claustrum. Parkinsonism and Huntington's chorea are diseases of the basal ganglia, which are key components in the formation of habits and unconscious motor programs. The caudate and lentiform nuclei and the fibers of the internal capsule that separate them constitute the corpus striatum. The function of the basal ganglia is complex. They contribute to some of the subconscious aspects of voluntary movement such as accessory movements and inhibiting tremor. They do not initiate movement but rather provide coordination of complex motor circuits. Neurotransmitters that affect the basal ganglia are acetylcholine, dopamine, gamma-aminobutyric acid (GABA), and serotonin.