Participation of the pedunculopontine tegmental nucleus in arousal-demanding functions

Abstract

The pedunculopontine tegmental nucleus (PPTg) is part of the locomotor mesencephalic region and the reticular activating system (RAS). As such, it is involved in regulating some aspects of the motor control and the wake/sleep cycle, but the way in which it operates is unclear. PPTg neurons respond to a variety of sensory stimuli and to cues that trigger the execution of goal-directed motor acts. In this paper, we discuss data that suggest that in addition to operating via thalamic nuclei, the PPTg may also operate by activating cerebellar nuclei, especially the dentate nucleus. In such a way, an intense and diffuse activation of the cerebral cortex may be achieved. In arousal-demanding reward-related tasks, separate populations of PPTg neurons modulate their discharge pattern to encode, in particular, expectancy and magnitude of reward. These neuronal responses, which should reach thalamic, basal ganglia, and cerebellar nuclei through ascending PPTg fibers are essential for the formation of stimuli-reward associations and action selection. In parallel, PPTg neurons, via descending fibers directed to lower brainstem and spinal motor structures, may automatically influence motor mechanisms and muscle tone in order to cause movements that are executed in a smooth and timely manner. In patients affected by Parkinson's disease, PPTg deep brain stimulation may increase arousal, thus making patients more attentive to behavioral stimuli that trigger motor act, while simultaneously facilitating movement.