<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (September 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> In neuroanatomy, a nucleus (plural form: nuclei) is a cluster of neurons in the central nervous system, located deep within the cerebral hemispheres and brainstem . The neurons in one nucleus usually have roughly similar connections and functions . Nuclei are connected to other nuclei by tracts, the bundles (fascicles) of axons (nerve fibers) extending from the cell bodies . A nucleus is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex . In anatomical sections, a nucleus shows up as a region of gray matter, often bordered by white matter . The vertebrate brain contains hundreds of distinguishable nuclei, varying widely in shape and size . A nucleus may itself have a complex internal structure, with multiple types of neurons arranged in clumps (subnuclei) or layers . </P> <P> The term "nucleus" is in some cases used rather loosely, to mean simply an identifiably distinct group of neurons, even if they are spread over an extended area . The reticular nucleus of the thalamus, for example, is a thin layer of inhibitory neurons that surrounds the thalamus . </P>

Bundles of neurons in the cns are called