Does the spinothalamic tract to ventroposterior lateral thalamus and somatosensory cortex have roles in both pain sensation and pain-related emotions?

Willis and colleagues review evidence that spinothalamic tract (STT) projections from both lamina I and deep layers of the dorsal horn to the ventroposterior lateral (VPL) nucleus of the thalamus and from there to S1 and S2 somatosensory cortical areas (STT-VPL-S1–2) are critical for sensory processing of pain. They have restored balance to current views of ascending spinal pathways for pain by reviewing evidence, including their own recent work, in favor of this major pathway. We agree almost entirely with their characterization and wish to provide our own functional interpretation of this component of STT-VPL-S1–2. In so doing, we expand on 2 points not discussed in detail by them. The first is that different ascending spinal pathways for pain contain very different proportions of wide dynamic range (WDR) neurons and nociceptive-specific (NS) neurons, and these different proportions may have functional significance for processing the different dimensions and stages of pain. The second is that we agree with Willis et al that STT-VPL-S1–2 is critical for sensory processing of pain; however, we explain our view that STT-VPL-S1–2 is critical for both pain sensation and pain affect. This view is in strong disagreement with the parallel 2-pathway concept of Melzack and Casey.

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