Microcircuits of the Pedunculopontine Nucleus

The notion of the pedunculopontine nucleus (PPN) is being updated in light of increasing evidence that identifies it as a key structure in the processing of exteroceptive information and its transmission to the forebrain, influencing the activity of PPN's multiple targets (from basal ganglia to the cerebral cortex). Here we review the anatomical evidence supporting the existence of a local network that forms the basis for such processing functions and propose a more complex relationship with its efferent systems. We also identify some of the critical issues that remain to be answered in order to have a comprehensive understanding of the roles of the PPN. A better insight into this connectivity will also help to understand the contribution of the PPN to the diverse systems it influences.