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.
Other examples include the Raphe nucleus, which is involved in sleep, and the suprachiasmic 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.