The raphe nuclei are a cluster of neurons in the brain stem. They are responsible for releasing serotonin to other parts of the brain, and that serotonin is involved in a range of physiological and pathological functions in the human body. Several different nuclei exist in the brain stem and regulate the release of serotonin that controls sleep patterns, mood, pain response, and motor functions.
"Raphe" refers to a seam or ridge of the body where the left and right structures join together. This part of the brain is labeled with the term because it is at the junction of both sides of the brain stem. It also is where other parts of the brain, such as the medulla oblongata, come together.
All of these components, each just millimeters in length, are contained in parts of the brain below the lobes of the cerebrum, where the higher functions take place. The cerebrum is the largest part of the human brain. It includes the frontal lobe, which controls problem solving and other cognitive functions; the occipital lobe; and the parietal and temporal lobes.
Within raphe nuclei, cell density varies significantly between regions, meaning the number of neurons in a given space differs from one spot to another. These structures include the dorsal raphe nucleus, which includes more serotonin-producing, or serotonergic, neurons than anywhere else in the brain stem of humans. It is also the largest of the raphe nuclei. Second in size is the raphe obscurus.
The caudal linear nucleus also has many serotonergic brain cells. Other components of the raphe nuclei include the median raphe nucleus; the raphe magnus nucleus, at the edge of the medulla; and the raphe pallidus nucleus. The latter is the smallest raphe nucleus of the bunch.
If this all seems complex, it really is. Even scientists who specialize in the human brain don’t fully understand what each of the raphe nuclei do. The nuclei have projections — some that lead toward the central brain and others that lead to the spinal cord — that serve as pathways for serotonin. For example, the dorsal raphe projects into the cerebral cortex, amygdala, and substantia nigra. Projections are common traits of brainstem structures, including the locus coeruleus, a section that has been correlated with depression and sleep.
Serotonin is also known to be produced in the medulla, making the interaction of each of the raphe nuclei and other parts of the brain harder to analyze. Although the role of serotonin in brain development and in basic functions is known, its production is still not fully understood. The raphe nuclei structures have been painstakingly analyzed in human neuroanatomy, but their complexity still leaves a few mysteries to the world of science.