Human Basal Ganglia

Cross references: 

Basal ganglia (Wiki)   
    "The basal ganglia (or basal nuclei) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus and other brain areas. The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or "habits" such as bruxism, eye movements, and cognitive,[1] emotional functions.[2] "  

Brain: Basal ganglia
Basal Ganglia and Related Structures.svg
Basal ganglia labeled at top right.
Click on the image to enlarge.

Currently popular theories implicate the basal ganglia primarily in action selection, that is, the decision of which of several possible behaviors to execute at a given time.[1][3] Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The "behavior switching" that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.[2][4]
The main components of the basal ganglia are the striatum, or neostriatum (composed of the caudate and putamen), the globus pallidus, or pallidum (composed of globus pallidus externa (GPe) and globus pallidus interna (GPi)), the substantia nigra (composed of both substantia nigra pars compacta (SNc) and substantia nigra pars reticulata (SNr)), and the subthalamic nucleus (STN).[5]
    The largest component, the striatum, receives input from many brain areas but sends output only to other components of the basal ganglia. The pallidum receives its most important input from the striatum (either directly or indirectly), and sends inhibitory output to a number of motor-related areas, including the part of the thalamus that projects to the motor-related areas of the cortex.
    One part of substantia nigra, the reticulata (SNr), functions similarly to the pallidum, and another part (compacta or SNc) provides the source of the neurotransmitter dopamine's input to the striatum. The subthalamic nucleus (STN) receives input mainly from the striatum and cortex, and projects to a portion of the pallidum (interna portion or GPi). Each of these areas has a complex internal anatomical and neurochemical organization.
The basal ganglia have a limbic sector whose components are assigned distinct names: the
Nucleus Accumbens Septi (NA), ventral pallidum, and ventral tegmental area (VTA). VTA efferents provide dopamine to the nucleus accumbens (ventral striatum) in the same way that the substantia nigra provides dopamine to the dorsal striatum. Because there is much evidence that it plays a central role in reward learning, the VTA→NA dopaminergic projection has attracted a great deal of attention. For example, a number of highly addictive drugs, including cocaine, amphetamines, and nicotine, are thought to work by increasing the efficacy of the VTA→NA dopamine signal. There is also evidence implicating overactivity of the VTA dopaminergic projection in schizophrenia.[6]"