Serotonin neurons on the ventral brain surface.

Serotonin neurons and fibers on the subpial surface of the ventral medulla oblongata in the rat are described by immunohistochemistry and autoradiography. The neurons are concentrated in the area encompassed by the origins of the abducens, hypoglossal, glossopharyngeal, and vagus nerves. The highest number of serotonin surface neurons appears along the median medullary fissure or basilar sulcus, where they may represent the most ventral extensions of the raphe pallidus group. As these cells lie on the surface of the brain, they could be directly affected by alterations in the chemical composition of the cerebrospinal fluid and, depending on their connections, could influence important medullary functions.