Organization of the efferent projections of the pedunculopontine tegmental nucleus of the midbrain of the dog pallidum.

Studies of the pedunculopontinopallidal projections of the dog brain based on the retrograde axonal transport of horseradish peroxidase demonstrated that the compact zone (PPNc) and the lateral area of the diffuse zone (PPNd) of the pedunculopontine tegmental nucleus (PPN) of the midbrain project to the globus pallidus, entopeduncular nucleus, and ventral pallidum. The medial area of the PPNd, adjacent to the chiasm of the upper cerebellar peduncles and seen in other animals as the mesencephalic extrapyramidal area (MEA), projects only to the globus pallidus. In dogs, this area of the tegmentum is not a major source of projections to the striopallidum, such that it is inappropriate to regard it as a separate structure, comment being restricted to the topical organization of PPNd projections to the pallidum. Projection fibers to pallidal structures arise from both cholinergic and non-cholinergic PPN neurons.