JOHNSON, J. I.*; SWITZER, R. D. III; SUDHEIMER, K. D.; FERREIRA, F. M.; MARINO, L.; FOBBS, A. J. Jr.; Michigan State Univ.; Neuroscience Assoc., Inc.; Univ. of Michigan; Michigan State Univ.; Emory Univ.; Nat’l. Mus. Health & Med., A. F. I. P.; ; : Distinctive organization of the basal ganglia in the brains of cetaceans.
The brains of cetaceans, compared with those of the sister group artiodactyls, show a distinctive organization of the basal forebrain. In and around the cerebral peduncle, artiodactyls, like most other mammals, have several separate nuclear formations of basal ganglia, , which in recent years have been shown to have common functions and interconnections. These include the lateral and medial globus pallidus (entopeduncular nucleus), and the reticular substantia nigra, which can be considered an �extended pallidum�. In cetaceans, in contrast, the cerebral peduncle lies more laterally, and medial to the peduncle is a continuous cell mass from the corpus striatum to the pons. T2-weighted magnetic resonance images of brains from six genera of cetaceans, along with stained sections from bottlenose dolphins < Tursiops truncatus/i> show that this cell mass is segmented into 3 subdivisions, which we term P1, P2 and P3. We propose that these subdivisions correspond to the separated cell groups of artiodactyls and other mammals: P1 to the lateral globus pallidus (external segment), P2 to the medial globus pallidus (internal segment or entopeduncular nucleus), and P3 to the substantia nigra. Nissl-stained sections, from < Tursiops truncatus /i>, reveal parvocellular rostral and magnocellular caudal divisions of P3, which could correspond to the reticular and compact parts of the substantia nigra of other mammals. The neotenic flexed posture of the midbrain and forebrain in cetaceans may be responsible for this distinctive spatial organization of these components of the basal ganglia and cerebral peduncle. In other mammals this part of the brain is stretched out, which could draw the cerebral peduncle into a course that interrupts the pallidal cell column. Supported by NSF grants IBN 0131267, 0131028, 0131826.
