Distribution of Pontomesencephalic Neurons Projecting to the Medullary Reticular Areas and Spinal Cord in Relation to the Pedunculopontine Nucleus in the Monkey

The pedunculopontine tegmental nucleus (PPN) is located in the mesopontine tegmentum lateral to the decussation of the superior cerebellar peduncle, at the levels from the caudal pole of the red nucleus to the ponto-mesencephalic junction, and innervated by descending projections from the motor outflow of the basal ganglia. It is interconnected with the substantia nigra (SN), the internal segment of pallidum (GPi) and the subthalamic nucleus (STN), and is an important interface between the basal ganglia and the brainstem motor system (Jackson and Crossman, 1983; Garcia-Rilli et al., 1991; Rye et al., 1996; Winn et al., 1997). It also receives cortical afferents mainly from the primary motor cortex, and afferents from the nucleus accumbens directly or indirectly via its output stations, such as the ventral pallidum, lateral hypothalamic area, and the ventral tegmental area (VTA) (Garcia-Rill et al., 1983c). The PPN transfers limbic information concerned with motivation reinforcement from the ventral striatum to the basal ganglia thalamo-cortical loops and to the ponto-medullary systems (limbic-motor integration). The PPN could be involved in locomotion and a variety of behavioral functions, such as the control of sleep and wake, learning and reinforcement processes, and autonomic functions. It is also involved in response choice to interrupt ongoing behavior; either to switch to a new response or simply to cease the current response (Winn et al., 1997).