Raphe nuclei

The raphe nuclei ("raffe", Greek: ραφή = seam) are a moderate-size cluster of nuclei found in the brain stem. Their main function is to release serotonin to the rest of the brain. Selective serotonin reuptake inhibitor (SSRI) antidepressants are believed to act in these nuclei, as well as at their targets.

Anatomy
The raphe nuclei are traditionally considered to be the medial portion of the reticular formation, and they appear as a ridge of cells in the center and most medial portion of the brain stem.

In order from caudal to rostral, the raphe nuclei are known as the nucleus raphe obscurus, the raphe magnus, the raphe pontis, the raphe pallidus, the nucleus centralis superior, nucleus raphe dorsalis, nuclei linearis intermedius and linearis rostralis. Some scientists chose to group the linearis nuclei into one nucleus, shrinking the number of raphe to seven, e.g., NeuroNames makes the following ordering:
 * Raphe nuclei of medulla
 * Nucleus raphe obscurus (nucleus raphe obscurus)
 * Nucleus raphe magnus (raphe magnus)
 * Nucleus pallidus (raphe pallidus)


 * Raphe nuclei of the pontine reticular formation
 * Pontine raphe nucleus (raphe pontis)
 * Inferior central nucleus


 * Raphe nuclei of the midbrain reticular formation
 * Superior central nucleus (nucleus centralis superior)
 * Dorsal raphe nucleus (nucleus raphe dorsalis)

Projections
These nuclei interact with almost every pertinent portion of the brain, but only a few of them have specifically independent interaction. These select nuclei are discussed as follows.

Overall, the caudal raphe nuclei, including the raphe magnus, pallidus and raphe obscurus, all project towards the spinal cord and brain stem. The more-rostral nuclei, including the raphe pontis, centralis (also called median), dorsal, tend to project towards the brain areas of higher function



Function
The raphe nuclei have a vast impact upon the central nervous system. Many of the neurons in the nuclei (but not the majority) are serotonergic; i.e., contain serotonin, a type of monoamine neurotransmitter. It is important to note that pharmacology traditionally affects global serotonin levels, while the actions of the raphe nuclei are dependent on the complex interplay between nuclei.

Projections from the raphe nuclei also terminate in the dorsal horn of spinal gray matter where they regulate the release of enkephalins, which inhibit pain sensation.

The raphe nuclei provides feedback to the suprachiasmatic nuclei (SCN), thus contributing in circadian rhythms in animals. The SCN transmits to the raphe nuclei via the dorsomedial hypothalamus nucleus altering serotonin levels for sleep/wake states. The raphe nuclei will then transmit feedback to the SCN about the animal's vigilance and levels of alertness. This reciprocal feedback between the two structures provides an adaptable yet stable basis of circadian rhythms.