<P> The importance of these subcortical nuclei for normal brain function and behavior is emphasized by the numerous and diverse neurological conditions associated with basal ganglia dysfunction, which include: disorders of behavior control such as Tourette syndrome, hemiballismus, and obsessive--compulsive disorder; dystonia; addiction; and movement disorders, the most notable of which are Parkinson's disease, which involves degeneration of the dopamine - producing cells in the substantia nigra pars compacta, and Huntington's disease, which primarily involves damage to the striatum . The basal ganglia have a limbic sector whose components are assigned distinct names: the nucleus accumbens, ventral pallidum, and ventral tegmental area (VTA). There is considerable evidence that this limbic part plays a central role in reward learning, particularly a pathway (mesolimbic pathway) from the VTA to the nucleus accumbens that uses the neurotransmitter dopamine . A number of highly addictive drugs, including cocaine, amphetamine, and nicotine, are thought to work by increasing the efficacy of this dopamine signal . There is also evidence implicating overactivity of the VTA dopaminergic projection in schizophrenia . </P> <P> In terms of development, the human central nervous system is often classified based on the original three primitive vesicles from which it develops: These primary vesicles form in the normal development of the neural tube of the embryo and initially include the prosencephalon, mesencephalon, and rhombencephalon, in rostral to caudal (from head to tail) orientation . Later in development of the nervous system each section itself turns into smaller components . During development, the cells that migrate tangentially to form the basal ganglia are directed by the lateral and medial ganglionic eminences . The following table demonstrates this developmental classification and traces it to the anatomic structures found in the basal ganglia . The structures relevant to the basal ganglia are shown in bold . </P> <Table> <Tr> <Th> Primary division of the neural tube </Th> <Th> Secondary subdivision </Th> <Th> Final segments in a human adult </Th> </Tr> <Tr> <Th> Prosencephalon </Th> <Td> <Ol> <Li> Telencephalon </Li> <Li> Diencephalon </Li> </Ol> </Td> <Td> <Ol> <Li> On each side of the brain: the cerebral cortices, caudate, putamen </Li> <Li> Globus pallidus, ventral pallidum, thalamus, hypothalamus, subthalamus, epithalamus, subthalamic nucleus </Li> </Ol> </Td> </Tr> <Tr> <Th> Mesencephalon </Th> <Td> <Ol> <Li> Mesencephalon </Li> </Ol> </Td> <Td> <Ol> <Li> Mesencephalon (midbrain): substantia nigra pars compacta (SNc), substantia nigra pars reticulata (SNr) </Li> </Ol> </Td> </Tr> <Tr> <Th> Rhombencephalon </Th> <Td> <Ol> <Li> Metencephalon </Li> <Li> Myelencephalon </Li> </Ol> </Td> <Td> <Ol> <Li> Pons and cerebellum </Li> <Li> Medulla </Li> </Ol> </Td> </Tr> </Table> <Tr> <Th> Primary division of the neural tube </Th> <Th> Secondary subdivision </Th> <Th> Final segments in a human adult </Th> </Tr>

The putamen and globus pallidus are considered to be subdivisions of the