In
neuroanatomy, a
nucleus (plural form: nuclei) is a cluster of densely packed
cell bodies 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.