Thalamic mechanism of pain: shell theory of thalamic nociception.

In both cats and Japanese macaques, there are nociceptive specific (NS) and wide dynamic range (WDR) neurons in the shell region of the caudal ventrobasal (VB) complex of the thalamus. This comprises both the nucleus ventralis posteromedialis proper (VPM proper), and the nucleus ventralis posterolateralis (VPL). These two classes of nociceptive neurons are spatially segregated, WDR neurons being located both more anteriorly and in a narrow belt around the VB complex. Both NS and WDR neurons are somatotopically organized. These two classes of nociceptive neurons may constitute a thalamic link in the pain pathway from both the dorsal horn of the spinal cord and its trigeminal homologue, to the primary somatosensory cortex. Visceral sympathetic afferent inputs also project to the VPL part of the shell region of the caudal VB complex, there being viscerosomatic convergence onto both NS and WDR neurons. Visceral and cutaneous pain pathways thus appear to share a common projection locus in the shell region of the caudal VB complex.