Our understanding of the functional organization of central pathways for pain and temperature sensations depends on anatomical elucidation of the substrates involved. We recently identified a thalamic nucleus in monkeys and humans (VMpo) that we believe is a sensory relay nucleus for specific pain and temperature activity. We will test this hypothesis by examining the synaptic organization of the lamina I trigemino- and spino-thalamic (TSTT) terminations in VMpo in order to determine whether they provide the basis for secure synaptic relay of sensory information. We will build volume-rendered three-dimensional models of single anterogradely-labeled TSTT terminals from serial ultrathin sections, using a methodology that we have applied successfully in the cat thalamus. Though technically difficult and time consuming, this is the only means of fully characterizing the relationships of these terminals with adjacent ultrastructural elements. In reconstructing their synaptology, we will seek to identify triadic arrangements with presynaptic dendrites, which are characteristic of thalamic sensory relay nuclei but which others say are not associated with TSTT terminals in the primate. Further, we will examine the possibilities that the terminations of nociceptive and thermoreceptive lamina I TSTT fibers may be different, as suggested by prior work in the cat, and that lamina I terminations in the ventral posterior n. (VP) may differ from those in VMpo. These studies will improve our understanding of the thalamic substrates underlying the processing of pain and temperature information, and they will offer insight into the synaptic basis for the sensations of pain and cold that can be produced by stimulation of this region in the human thalamus.