JOHNSON, J. I.; SWITZER III, R. C.; SUDHEIMER, K. D.; FERREIRA, F. M.; FOBBS JR., A. J.; MARINO, L. ; Michigan State Univ.; NeuroScience Associates, Inc.; Michigan State Univ.; Michigan State Univ.; National Museum of Health and Medicine, Armed Forces Institute of Pathology; Emory Univ.: Lateralized Cerebral Peduncles, Extensive Midbrain Pallidum, and Other Distinctive Features of the Midbrains of Whales and Dolphins.

Examination of magnetic resonance images through the brains of bottlenose dolphins Tursiops truncatus, common dolphin Delphinus delphis, striped dolphin Stenella longirostris, harbor porpoise Phocoena phocoena, dwarf pygmy whale Kogia simus, and white whale Delphinapteras leucas, along with stained sections from four bottlenose dolphins, showed distinctive features of the midbrain common to all of these toothed whales and dolphins. The cerebral peduncle is on the lateral aspect of the midbrain, rather than the latero-ventral aspect as in other mammals. The peduncles enter the pons at its lateral rather than its rostral aspect. On the ventral surface of the midbrain is a cellular mass continuous with the forebrain pallidum. This midbrain pallidum probably represents the reticular substantia nigra. Large cells in the caudal midbrain, extending into the pons, may correspond to those of the dopaminergic compact substantia nigra and ventral tegmental area of other mammals. These distinctive arrangements of midbrain components may be related to: folding of the cetacean forebrain around the mesencephalic flexure, a greater number of corticopontine relative to corticospinal fibers, and a greater number of midbrain dopaminergic cells projecting to an enhanced corpus striatum. Supported by: NSF grants IBN 98114911, 98114912, 9812712, 0131028, 0131267, 0131826