The Pedunculopontine Nucleus

The dense interconnections of the PPN and basal ganglia enable the two structures to maintain a close functional relationship that has significant bearing on a wide range of behaviours, including sleep/arousal, locomotion, and posture. Our synthesis of the literature suggests that it is no longer sufficient to view PPN neurons as playing roles in only one of two distinct effector systems, namely the RAS and MLR, but that individual PPN neurons are ideally suited to play central roles in both systems. The impact of this hypothesis on our understanding of basal ganglia function is difficult to predict at this time. What is certain, however, is that an appreciation of the functional integration of the PPN and basal ganglia is critical for a better understanding of the roles played by each circuit in behaviour. The cellular heterogeneity present in the PPN indicates that the development of a realistic scheme for functional integration is a major challenge for the future. In meeting this challenge, it is imperative that correlative analyses of the physiology, neurochemistry and morphology of the non-cholinergic neurons, as well as the cholinergic neurons, of the PPN are undertaken. Furthermore, it is important that we consider the different functional aspects of the PPN at the level of single identified neurons. Using this and other approaches, it will be possible to dissect the influences of the basal ganglia and PPN as they work together as a single ‘system’, rather than two separate entities.