In
neuroanatomyNeuroanatomy is the study of the anatomy of nervous tissue and neural structures of the nervous system. In vertebrate animals, the routes that the myriad nerves take from the brain to the rest of the body , and the internal structure of the brain in particular, are both extremely elaborate...
, a
nucleus is a
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
structure consisting of a relatively compact cluster of
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the
cerebral cortexThe cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It constitutes the outermost layer of the cerebrum. In preserved brains, it has a grey color, hence the name "grey matter"...
or
cerebellar cortexThe cerebellum is a region of the brain that plays an important role in the integration of sensory perception, coordination and motor control...
. In anatomical sections, a nucleus shows up as a region of
gray matter"Gray Matter" is a short story by Stephen King, first published in the October 1973 issue of Cavalier magazine, and later collected in King's collection Night Shift...
, often bordered by
white matterWhite matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...
. The
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
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. The
reticular nucleus of the thalamusThe thalamic reticular nucleus is part of the ventral thalamus that forms a capsule around the thalamus laterally. It is separated from the thalamus by the external medullary lamina...
, 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 the
thalamusThe thalamus is a midline paired symmetrical structure within the brain of vertebrate animals, including humans. It is between the cerebral cortex and the midbrain, both in terms of its location and its neurological connections...
and
hypothalamusThe hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions of the hypothalamus is to link the nervous system to the endocrine system via the pituitary gland ....
, each of which contains several dozen distinguishable substructures. The
medullaThe medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
and
ponsThe pons is a structure located on the brain stem. It is cranial to the medulla oblongata, caudal to the midbrain, and ventral to the cerebellum...
also contain numerous small nuclei with a wide variety of sensory, motor, and regulatory functions.
In the
spinal cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain. The brain and spinal cord together make up the central nervous system. It is around 45 cm long in men and around 43 cm long in women. The length of the spinal cord is much shorter than...
and
peripheral nervous systemThe peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain...
, a cluster of neurons is usually called a
ganglionIn anatomy, a ganglion is a biological tissue mass. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
instead of a nucleus.