Neuroscience Online

Section I: Cellular and Molecular Neurobiology 12. Biogenic Amine Neurotransmitters Part 5 of 11 Jack C. Waymire, Ph.D.

Figure 12.5 5-HT neuronal pathways in the rat CNS are in two clusters of cells, one in the caudal brain stem (B1-B4) and the other in the rostral brainstem (B5-B9) Serotonin - Anatomy As shown in Figure 12.5, serotonin cells are located in two clusters: a caudal system in the medulla (B1-B4, green in Figure 12.5) a rostral system in the midbrain (B5-B9, purple and blue in Figure 12.5). Both project widely throughout the CNS. Caudal System: The caudal cluster of 5-HT cells (B1-B4) is located close to the midline and project caudally to the spinal cord dorsal and ventral horns as well as the intermediolateral cell column. These pathways are believed to mediate the role of 5-HT in sensory, motor and autonomic functions, respectively. Rostral System: The rostral midbrain cluster of cells (B5-B9), (raphe nuclei) are distributed throughout the midbrain. A cluster of cells located medially and another located dorsally provide over 80% of the 5-HT innervation of the forebrain. These cells project to the diencephalon, basal ganglia, limbic system, cortex, mesencephalic gray and inferior and superior colliculi. Some evidence supports the conclusion that the innervation of forebrain structures by seratonergic processes is complementary to that of NE. Another important aspect of 5-HT microanatomy is that two distinct patterns of innervation exist for these medial and dorsal systems. The dorsal system is similar in its anatomy to that of catecholamine neurons with thin diffusely branching axons lacking classic synaptic contacts (volume neurotransmission). The medial system, in contrast, appears to have classical synapses and is characterized by the presence of thick axons with large round nerve endings that make extensive synaptic contacts. These differences imply a marked difference in the physiological function of these two 5-HT systems.

Other Systems: In addition to the above two pathways, another 5-HT pathway projects partially from one of the rostral nuclei (B5) and partially from two caudal nuclei (B2 & B3, dark green in Figure 12.5) to innervate the cerebellar cortex and deep cerebellar nuclei. There is also a widespread 5-HT projection to structures within the brainstem, including the locus coeruleus, several cranial nuclei, inferior olivary nucleus, and nucleus solitarius.

Cross Reference: Links for Serotonin