Pain and emotion

Chapter 10 endnote 19, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

It’s even possible that nociception is a form of interoception.

Interoceptive and nociceptive signals from your body meet in the brainstem and travel to the rest of your brain along the same ascending pathways (C, Aδ, Lamina I and vagus pathways). As your body speaks to your brain, interoceptive sensations from various causes (exercise, stress, etc.) are indistinguishable, and nociception is strongly related. Furthermore, there is crosstalk among the neurons, meaning that the signals influence one another along these paths.The neurons are unmyelinated, or lightly myelinated, meaning that the signal will move slowly along each axon, but signals from different neurons can influence one another as their signals move along the axons themselves and not just at the synapses. Instances of pain and emotion can be distinguished, but nociceptive and interoceptive sensations might contribute to both.

Furthermore, the brain regions that are important for processing nociception, known as the “pain connectome” or the “pain matrix,” are in the interoceptive and control networks.

So what does it mean when a scientist reports that emotion can enhance or diminish pain? The answer: “emotion induction” is changing inner body predictions, which changes the dial on nociception and pain. When the brain constructs an emotion concept, this contains contain interoceptive predictions that can change affect, and negative affect changes physical discomfort into suffering.

Predictions can cause opioid release, which turns down nociception in the brainstem, but also there are opioid receptors in other parts of the interoceptive network and even in spinal cord. So opioids dial down not only nociception but also inner body changes that have implications for interoception and affect. Opioids, for example, which are released during placebo, not only reduce experienced pain, but also reduce autonomic nervous system reactivity, as well as brainstem and spinal cord responses to painful stimulation. Placebo effects even influence the release of another neurotransmitter, dopamine, which used to be considered a “reward” neurotransmitter, but actually is better understood as the juice for effortful action (that's usually needed to obtain a reward). It juices the system to allow a person to seek the reward, but the rewarding properties themselves appear to derive from opioids (and possibly another system, called the cannabinoid system — internal marijuana).