SQUIRREL-MONKEY LATERAL THALAMUS .1. SOMATIC NOCIRESPONSIVE NEURONS AND THEIR RELATION TO SPINOTHALAMIC TERMINALS

The incidence and response properties of nociresponsive neurons, their locations relative to spinothalamic terminals, and their relations to cytoarchitectonic borders were studied in the lateral thalamus of the squirrel monkey. Nociceptive neurons were found in ventral posterior inferior nucleus (VPI), in the lateral and medial nuclei (VPL and VPM) of the ventral posterior complex (VP = VPL + VPM), as well as the pos terior complex (PO). The overall incidence of nociresponsive cells was 19% (50 of 270 cells). The proportion of nociresponsive neurons withi n VPI was 50% (23 of 46), 38% in PO (8 of 21), and 10% in VP (19 of 20 3). Most nociresponsive cells (90%) in VP were of wide-dynamic-range t ype, while within VPI 43% of nociresponsive cells were nociceptive-spe cific type. Most of these nociresponsive cells had thermal and mechani cal responses, and a small number also responded to cooling. The recep tive fields of nociresponsive cells in VPL were in continuity, in both size and body location, with surrounding low-threshold units. The rec eptive fields of VPI and PO nociresponsive cells were larger than thos e in VPL. The probability of encountering nociresponsive cells located within 100 mu m of spinothalamic terminations was high in VPI (73%) a nd low in VPL (33%). On the other hand, the probability of encounterin g non-nociceptive cells located within 100 mu m of spinothalamic termi nals was low in both VPI (20%) and VPL (26%). The results indicate seg regation of nociresponsive cell types across VP, VPI, and PO and sugge st that VPI, and perhaps PO, is an important region for discriminative processing and perception of painful stimuli.