Regions of the thalamus that have been implicated in the processing of nociceptive information
Two major regions of thalamus have been implicated in the processing of nociceptive information: (1) the lateral thalamus, including the ventral posterolateral (VPL) and ventral posteromedial nuclei (VPM), and (2) the intralaminar nuclei of the medial thalamus. The VPL receives input via the spinothalamic tract, as well as a major input from non-nociceptive lemniscal pathways originating in the dorsal column nuclei. The VPM receives input via the nucleus caudalis and the principal trigeminal nucleus. Stimulation of the lateral thalamus in patients who are not experiencing pain does not produce significant pain. By contrast, in patients who have ongoing pain, electrical stimulation can reproduce pain, suggesting a reorganization of the nociceptive input to the thalamus under conditions of persistent injury.
The output of the lateral thalamus is largely to the somatosensory cortex. Neurons in this circuit code for the sensory-discriminative features of pain, namely intensity and location. In contrast, connections with the anterior cingulate and insular cortex process information that underlies the affective components of pain. As noted earlier, “pain” inputs engage the limbic system via connections from the spinal cord to the amygdala via the spinoparabrachio-amygdala pathway. The medial thalamus, including the intralaminar nuclei, receives direct spinothalamic and spinoreticular thalamic projections. Cells in this region have larger receptive fields and are thought to contribute to the diffuse character of pain perception. The cortical connections of the more medial regions of the thalamus, notably the anterior cingulate gyrus, are involved in the affective component of the pain perception.