In neuroanatomy, a nucleus is a central nervous system structure that is composed mainly of gray matter, and which acts as a hub or transit point for electrical signals in a single neural subsystem. For example, the lateral geniculate nucleus mediates signals in the vertebrate visual system. The vestibular nucleus stores head motion information and guides eye movements via the vestibulo-ocular reflex.

Other examples include the Raphe nucleus, which is involved in sleep, and the suprachiasmatic nucleus, which controls circadian rhythm. All the nerve cell axons terminating on a nucleus tend to employ the same neurotransmitter at their synapses. As a result, the effects of certain psychoactive drugs are concentrated in particular nuclei. Morphine is believed to act via synapses of the arcuate nucleus, for example.