Medullary serotonergic system

Medullary neurons synthesizing serotonin (5-HT) encompass several groups projecting to the spinal cord and brainstem. The medullary serotonergic groups include the nucleus raphe magnus (RMg), which projects primarily to the spinal and trigeminal dorsal horn, and the nucleus raphe obscurus (ROb) and raphe pallidus (RPa), which project to motor and autonomic nuclei of the spinal cord and medulla. Other serotonergic groups in the ventrolateral medullary reticular formation contribute bulbospinal and propriospinal projections to the spinal cord and brainstem. Serotonin exerts complex effects on its targets via several subtypes of 5-HT receptors. Medullary serotonergic neurons exert a major modulatory influence on nociceptive processing, motoneuron excitability, respiratory control, and autonomic function, particularly thermoregulatory sympathetic responses to cold. Not surprisingly, these neurons have been implicated in the pathophysiology of pain, spasticity, and respiratory failure potentially related to sudden death.