The basic functional unit of the nervous system is the neuron or nerve cell. In addition to neurons, the brain contains large numbers of supporting cells termed glial cells and a rich vascular supply. The limbic lobe of the forebrain lies on the medial surface of the cerebral hemispheres where it forms an anatomical border around the brainstem. Collectively, the nuclei of the corpus striatum and their interconnecting neural regions comprise a complex circuitry involved in the initiation and programming of movements. Although usually considered the seat of higher cognition, the neocortex is also a complex sensorimotor processing center. Classically, anatomists have divided the cortex into primary sensory and motor regions, sensory association and premotor areas, and association areas. Brodmann's system assigns each cytoarchitectonic area a unique number. Another cytoarchitectonic system, that of von Economo and Koskinas, uses a lettering system with the first letter of each region referring to a particular cortical lobe.