THE PEDUNCULOPONTINE TEGMENTAL NUCLEUS - WHERE THE STRIATUM MEETS THERETICULAR-FORMATION

The pedunculopontine tegmental nucleus (PPTg) contains a population of cholinergic neurons (the Ch5 group) and non-cholinergic neurons. Ther e appears to be functional interdigitation betwen these two groups, wh ich both have extensive projections. The principal ascending connectio ns are with thalamic nuclei and structures associated with the striatu m, including the substantial nigra pars compacta. The descending conne ctions are with a variety of nuclei in the pens, medulla and spinal co rd, concerned with autonomic and motor functions. In the past, emphasi s has been laid on the role of the PPTg in locomotion and behavioural state control. In this review, we emphasise the role of the PPTg in pr ocessing outputs from the striatum. The non-cholinergic neurons receiv e outflow from both dorsal and vental striatum, and lesions of the PPT g disrupt behaviour associated with each of these. Our review indicate s that the PPTg is less concerned with the induction of locomotion and more concerned with relating reinforcement (information about which c omes from the Ventral striatum) with motor output from the dorsal stri atum. The conclusions we draw are: (1) the PPTg is an outflow system f or the striatum, but also forms a 'subsidiary circuit', returning info rmation to striatal circuitry; in this, the PPTg has an anatomical org anisation that resembles that of the substantia nigra. (2) As well as a role in the mediation of REM sleep, cholinergic PPTg neurons have an important role in the waking state, providing feedback into the thala mus and striatum. (3) The precise function of the computations perform ed on striatal outflow by the PPTg is uncertain. We discuss whether th is function is complementary (parallel to other routes of striatal out flow), integrative (modifying other forms of striatal outflow) or both .