Putative cholinergic neurons of the pedunculopontine tegmental nucleus projecting to the superior colliculus consist of sensory responsive and unresponsive populations which are functionally distinct from other mesopontine neurons.

We examined the sensory properties of putative cholinergic neurons of the pedunculopontine tegmental nucleus projecting to the superior colliculus. Projection neurons were identified by antidromic activation from the contralateral posterior superior colliculus; stimulation of the anterior half was essentially ineffective. Identified neurons fell into two groups, one with a somatosensory input (39%) and one without a sensory input. Somatosensory responsive projection neurons were low threshold and rapidly adapting. Receptive fields were contralateral (94%) and predominantly orofacial (57%). Sensory responsive and unresponsive projection neurons were intermingled within the pedunculopontine tegmental nucleus as identified histologically by reduced nicotinamide adenine dinucleotide phosphate diaphorase or acetylcholinesterase. The properties of neurons outside the nucleus differed significantly. They could not be activated antidromically from the superior colliculus; many had ipsi- or bilateral receptive fields (75%) and wide dynamic range or nociceptive response patterns (52%). The presence of two functionally distinct groups of projection neurons implies a dual or more complex modulation of tectal neurons by the pedunculopontine tegmental nucleus. The pedunculopontine tegmental nucleus has been implicated in a multiplicity of behaviors and, in particular, in rapid eye movement sleep and alerting or arousal functions. By virtue of its many connections with the basal ganglia, limbic system and reticular structures, the projection to the superior colliculus of two distinct groups may provide an important differentiating element of the tectal organization of orienting and spatial cognitive behavior.