Nuclei arrayed around the midline of the brain stem (‘raphe’ referring to a ridge separating two symmetrical parts of a body). Forming part of the reticular system, the nuclei are evident at most levels of the brain stem (from the midbrain to the spinal cord and forebrain). Their main role involves the manufacture and synthesis of the neurotransmitter serotonin (but also some non-serotonin neurotransmitters). The rostral part of the raphe nuclei contains around 85% of all serotonin neurons. Caudally, and nearer the brain stem, is home to a markedly fewer of such neurons. Projections from the raphe nuclei have widespread connections throughout the central nervous system. Functionally, they play an important role in the regulation of sleep-wake cycles and especially during wakefulness. Also, they are implicated in the inhibition of pain. They develop from mesopontine and medullary primordia, with the distinction between rostral and caudal clusters being maintained well into adulthood, and they support the modulation brain development. Consequently, alterations in the functioning of the raphe nuclei (i.e., in the transmission of serontonin) early in development can give rise to a range of neuropsychiatric and neurological disorders.
See Brain stem, Entorhinal cortex, Inhibitory control, Locus coeruleus (or ceruleus), Mesencephalic reticular activating system, Pons, Serotonin (5-hydroxytryptamine, or 5HT)