REGIONS are the second level down from the topmost level of our conceptual
hierarchy, and are often divided into Sub-Regions.  Within Regions are Groups of
Nuclei. Each of those Nuclei are clusters of specific neurons (Cell Types). 
Circuits link Cells in Nuclei to Cells in the same or other Nuclei.  In some
brain regions    (like Cortex) Cell Types form Layers (sheets of cells) not
Nuclei. The cell populations in these layers define Areas of those Regions.