Body-Brain Integration: The Lower Brainstem in Sleep-Wake Regulation.

The lower brainstem is a key hub linking visceral physiology with the regulation of brain states. This review synthesizes recent findings demonstrating how several nuclei within this region-including the nucleus of the solitary tract (NST), parabrachial nucleus (PBN), and other medullary circuits-function as an integrated network that couples sleep-wake regulation to the body's homeostatic demands. The NST serves as a central gateway, translating cardiovascular, immune, and digestive signals into a sleep drive, whereas the PBN plays a pivotal role in processing threat-related inputs to promote arousal. Several populations of GABAergic neurons in the medulla induce both motor suppression and sleep. In addition, cholinergic neurons in the nucleus ambiguus and catecholaminergic cells in the ventrolateral medulla and locus coeruleus regulate sleep-wake states together with somatic and autonomic motor activity. Collectively, these findings establish the brainstem not merely as a collection of reflex centers but as a master coordinator aligning global brain states with peripheral bodily functions.