A Rose By Any Other Name….
Pain researchers use words in very deliberate ways. The most infamous faux pas of the pain lexicon is to use the word “pain” when you mean “nociception.” Let’s briefly refer to the IASP Taxonomy to understand the difference:
Nociception is defined as the “neural process of encoding noxious stimuli.” Encoding noxious stimuli is a surprisingly complex process and can include any or all of the following:
- Detection of noxious stimuli at the skin’s surface,
- Generation of neural discharges in nociceptors,
- Modulation of neuron circuits in the dorsal horn of the spinal cord,
- Descending inhibition of these circuits by the brainstem, and
- Sensory and affective processing of this information by the brain.
Pain is defined as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” The word “experience” is the key to differentiating nociception from pain, because experience requires consciousness.
Therefore the difference between nociception and pain is consciousness.
As a pain researcher, I notice that even I occasionally use the word pain instead of nociception. The underlying reason is that it is a shorter word, requires less expenditure of energy, and if I’m talking to another pain researcher they implicitly understand whether I’m referring to nociception or pain. This is a bad habit I am trying to correct, so feel free to call me on it!
Somatic versus Visceral
The words “somatic” and “visceral” are also important terms with very specific meanings in pain research. Somatic has historically referred to the body in general. Terms like psychosomatic and somatoform disorders were historically used to describe physical symptoms caused by psychological factors. In the past 20 years, the medical community has made major strides to validate chronic idiopathic pain because doctors and researchers have increasingly acknowledged that our inability to identify pain mechanisms does not mean such mechanisms do not exist. This sense of humility continues to gain traction.
In the pain world, “somatic” refers to specific tissues: skin, muscle, and bone. Nociceptors in somatic tissues share the similar properties. They activate in proportion to the intensity of a noxious stimulus (intensity encoding nociceptors), which means that somatic pain:
- begins with the application of a noxious stimulus and ends with its removal,
- is well-localized, and
- has a very distinct quality (sharp, cutting, etc).
Nociceptors located in visceral tissues such as hollow organs and fascia behave differently. Visceral nociceptors not activate until noxious stimulation has passed a high threshold of intensity. This means that visceral pain:
- may begin seconds to minutes after the application of a noxious stimulus (usually distension),
- is diffusely localized, and
- has a vague quality that is difficult to put into words.
Potential Evolutionary Significance
Why would different tissue types have nociceptors with different functional profiles? Consider the last time you had a stomach ache from the flu or with food poisoning. You likely curled up in a fetal position, stayed very still, and desperately wished you were still a child so someone would take care of you and rub your belly. (Perhaps that’s just me…) I can wager with almost perfect certainty that you did NOT decide to go to your 8:00 pm tango lesson, train for your half marathon, or have your girlfriends over for a tupperware party (unless you are a red-headed trooper!). Visceral pain promotes behaviors that preserve energy and maximize safety. Evolutionarily, visceral pain would have meant that you have sustained a serious injury to organs that you need to survive. Notice that visceral pain does this by changing your motivation to move, seek out pleasure, or focus on anything other than your misery. These natural reactions to visceral pain are important to remember in the case of chronic visceral pain patients. Consider how you might feel if you had an almost constant stomach ache for 5 years!
Somatic pain is a different story.
Somatic pain can be serious, of course, but we can all agree that you can survive without an arm or leg. Evolutionarily, injuries to the superficial tissues of the body (skin, muscle, bone) will not kill you. As a result, it is somehow easier to distract oneself from somatic pain. There is a comfort in somatic pain because you know that under normal conditions, it will stop eventually. You can learn what events or stimuli trigger the pain and avoid them to prevent future pain. In no way am I minimizing the seriousness of chronic somatic pain, but I am arguing that there are more coping strategies available that can diminish its impact.
Blurring the Somatic/Visceral Divide
I have discussed somatic and visceral pain as if they are distinct phenomena. In reality, the nociceptive signals from somatic and visceral tissue overlap at multiple points before even reaching the brain. By “overlap,” I mean that a neurons transmitting somatic information will at some point synapse onto a neuron that also receives visceral information. This first place this happens is at dorsal horn interneurons and is referred to as viscero-somatic convergence. About 90% of sensory neurons that terminate in the spinal cord are from somatic tissue, and less than 10% are from visceral tissue. Visceral nociceptors always synapse onto interneurons that also receive input from somatic nociceptors (at least no one has observed an exception to this pattern). This means that somatic and visceral nociceptive signals can interact in the spinal cord, before they even reach the brain. These interactions are referred to as “crosstalk.”
Spinal crosstalk blurs the line between pure somatic and pure visceral pain. It remains a mystery why–despite this overlap–somatic information retains its spatial topography whereas visceral information does not.
[CONTENT UNDER CONSTRUCTION. PLEASE BE PATIENT!]