The Raphe nuclei are constellations of neurons lying along the median plane of the brainstem (medulla oblongata, pons, and midbrain). They are parts of the reticular formation. The nerve cells that make up these nuclei are characterized by the serotonin they contain. They have extensive distribution of efferent fibers, which have largely different targets. Thus, the caudal raphe nuclei send their efferent axons to the spinal cord, while the rostral nuclei send fibers upwards, to the thalamus and hypothalamus.
Aside from serotonin, most raphe nuclei also contain neuropeptides and thyrotropin-releasing hormone, which contribute to the synaptic effects of efferent fibers. Serotonin influences the cerebral circulation. A peculiarity of the raphe nuclei is that their neurons send axons to the ependymal cells, which cover the interior walls of the brain ventricles. Presumably, such fibers contribute to the regulation of transport processes through the ependyma, which is the membrane lining the cerebral ventricles and the central canal of the spinal cord.
Function
The raphe nuclei regulate sympathetic activity, also playing a role in the regulation of the circadian rhymthms as they receive afferent fibers from the suprachiasmatic nuclei located in the anterior hypothalamic area. Various kinds of behavioral changes have been reported after damage to raphe nuclei, such as aggression and increased motor activity.
Below, a photograph of a transverse section of the medulla shows the raphe nuclei situated in the two median portions on each side of the midline. They can be seen as narrow light grey stripes.
An schematic picture of side view of encephalon, showing the location of the raphe nuclei and their fiber projections in the brain.
