Cytoarchitectural subdivisions of the periaqueductal gray matter in the cat

Abstract

Abstract The neuronal population of the PAG consists of relatively small cells, ranging from approximately 8 μ to 30 μ in diameter. These neurons are of three different types and are classified as I, II, and III for the sake of convenience. Each class aggregates in certain areas of the central gray to form subdivisions distinguishable by differing neuronal distributions as well as by cellular characteristics. Class I neurons are small, spindle shaped cells that stain darkly with cresyl violet. They aggregate loosely around the cerebral aqueduct, thus forming the comparatively acellular inner ring of the PAG, the nucleus medialis. The class II cells are also small and darkly staining, but they are fusiform to spherical in shape. These cells cluster more closely and are surrounded by many glial cells. They congregate in the area dorsal to the cerebral aqueduct to form the nucleus dorsalis. The largest cells of the PAG, the class III neurons are spherical or tringular in shape and stain lightly due to their sparse Nissl substance. Frequently glial cells abut the perikaryon of these neurons. They aggregate very closely to form the dense cellular outer portion of the PAG. This mantle of cells is called the nucleus lateralis.