Nucleus Raphe Magnus

Nucleus Raphe Magnus

The nucleus raphes magnus, located directly rostral to the nucleus raphes obscurus, is afferently stimulated from axons in the spinal cord and cerebellum.

This makes the nucleus raphes magnus a likely candidate for part of the motor system; however, it seems to participate in the endogenous analgesia system. Mounting evidence suggests that the nucleus raphes magnus plays an important role in homeostatic regulation.

The nucleus raphes magnus receives descending afferents from the periaqueductal grey, the paraventricular hypothalamic nucleus, central nucleus of the amygdala, lateral hypothalamic area, parvocellular reticular nucleus and the prelimbic, infralimbic, medial and lateral precentral cortices.

All of these brain areas influence the main function of the nucleus raphes magnus.

The main function of the nucleus raphes magnus is mostly pain mediation; in fact it sends projections to the dorsal horn of the spinal cord to directly inhibit pain. The nucleus raphes magnus releases serotonin when stimulated. Raphespinal neurons project to enkephalin releasing interneurons in the posterior horn of the spinal cord.

The periaqueductal grey, the center of analgesia, sends efferent connections to the nucleus raphes magnus when it is stimulated by opiates (endogenous or otherwise).

Electrical stimulation of the PAG produces analgesia, as well as administration of morphine to the PAG or nucleus raphes magnus. The antinociceptic effects of electrical stimulation of the PAG can be blocked by administering naloxone, an opiate antagonist, to the nucleus raphes magnus.

Similarly, afferent fibres from the spinothalamic tract synapse at the periaqueductal grey matter. This in turn is linked to the nucleus raphes magnus, which when stimulated directly inhibits pain fibers in the dorsal horn of the spinal cord, thus alleviating pain.

All of this seems to indicate that the nucleus raphes magnus is part of the endogenous opiate system, and acts to inhibit pain in the spinal cord.