On the relationships between the striatum and the pedunculopontine tegmental nucleus.

In this essay we consider the role of the pedunculopontine tegmental nucleus as a striatal output station. We review the relevant anatomical, electrophysiological, behavioral, and pathological studies and conclude that the pedunculopontine tegmental nucleus occupies an important position in striatal outflow, receiving motor output from the dorsal striatum and information from the ventral striatum relating to limbic processes of motivation and reinforcement. The hypothesis we present is that the pedunculopontine tegmental nucleus is at the very least an integral component of the limbic-motor interface, although in discussing this concept we also assess the likelihood that the limbic-motor interface is in fact a distributed system-that is, that limbic-motor interfacing is not all done by a single structure in the central nervous system but that different aspects of it are served by different systems. We present the hypothesis that the pedunculopontine tegmental nucleus is one critical site through which limbic information concerned with motivation, reinforcement, and the construction of novel associations can gain access to a stream of motor outflow coming from the caudate-putamen and directed toward pontomedullary systems without reference back to the cerebral cortex. This hypothesis is important because it highlights striatal outflow, which is not processed through the cortical re-entry systems, and also emphasizes the importance of pontine systems in cognitive processing.