Inneuroanatomy, anucleus (pl.:nuclei) is a cluster ofneurons in thecentral nervous system,[1] located deep within thecerebral hemispheres andbrainstem.[2] The neurons in one nucleus usually have roughly similar connections and functions.[3] Nuclei are connected to other nuclei bytracts, the bundles (fascicles) ofaxons (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 thecerebral cortex orcerebellar cortex. In anatomical sections, a nucleus shows up as a region ofgray matter, often bordered bywhite matter. Thevertebrate 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.
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. Thereticular nucleus of the thalamus, for example, is a thin layer of inhibitory neurons that surrounds the thalamus.
Some of the major anatomical components of the brain are organized as clusters of interconnected nuclei. Notable among these are thethalamus andhypothalamus, each of which contains several dozen distinguishable substructures. Themedulla andpons also contain numerous small nuclei with a wide variety of sensory, motor, and regulatory functions.
In theperipheral nervous system (PNS), a cluster of cell bodies of neurons (homologous to a CNS nucleus) is called aganglion. The fascicles of nerve fibers in the PNS (homologous to CNS tracts) are callednerves.
A collection of nerve cell bodies in the CNS is called a nucleus.
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