This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Physiology of Pain, Subject name: Physiology
Neurons located in which area release serotonin as their neurotransmitter? 
Select and explain. 
A. Periaqueductal gray area, B. Interneurons of the spinal cord, C. Periventricular area, D. Nucleus raphe magnus
D: Nucleus raphe magnus
Answer: D: Nucleus raphe magnus 
Explanation: Neurons of the nucleus raphe magnus release serotonin at their nerve endings. These terminate in the spinal cord on interneurons that in turn release enkephalin and block the incoming signals from the pain fibers. The analgesia system consists of three major components: Periaqueductal gray and periventricular areas of the mesencephalon and upper pons. Raphe magnus nucleus, a midline nucleus located in the lower pons and upper medulla. Dorsal pain inhibitory complex (interneurons) located in the spinal cord.