SQUIRREL-MONKEY LATERAL THALAMUS .2. VISCEROSOMATIC CONVERGENT REPRESENTATION OF URINARY-BLADDER, COLON, AND ESOPHAGUS

The response properties of 106 visceroceptive lateral thalamic neurons were investigated in anesthetized squirrel monkeys. Most neurons were located in the ventral posterior lateral nucleus (VPL), and a smaller number of cells was also found in a variety of thalamic nuclei around VPL. Ninety (85%) of these cells responded to distension of the urina ry bladder, the distal colon, and/or the lower esophagus. The majority of the visceral-responsive cells also had convergent somatic and mult ivisceral responses (71% of the 85%). A small population (6%) was visc eral specific; that is, these neurons were not activated with somatic stimuli. Visceral responses were excitatory, inhibitory, or mixed, and most were either visceral nociceptive specific (65%) or visceral wide -dynamic-range type (34%). Very few visceral responses (1%) could be c lassified as low threshold. The incidence of these response types were highly dependent on the specific viscus stimulated. Most visceral res ponses were able to code stimulus duration and intensity. The majority (69%) of the visceroceptive neurons had somatic low-threshold converg ent input mainly from the surface of the lower body. The somatic recep tive field locations and the somatic response properties seem unrelate d to the convergent visceral input and the visceral response propertie s, although there were some exceptions. No obvious viscero-topical org anization was found in VPL. The results lead us to propose two differe nt modes of representation for processing of and distinguishing betwee n visceral and somatic inputs: a distributed population code for visce ral inputs, and a local code for somatic inputs. Based on these codes, we discuss a new hypothesis for referred pain.