The nociceptive system has evolved a range of intriguing characteristics. Spatial summation is one such characteristic, whereby increasing the area of a stimulus, or the distance between multiple stimuli, results in more intense pain—not only a greater area of pain. This befits pains’ protective function, because larger/multi-site injuries are likely to be more dangerous.
Defining terms. The effects of increased stimulus area, and inter-stimulus distance, have been labelled area- and distance-based summation respectively. In the lab, area-based spatial summation can be examined by contrasting the pain evoked by different sized noxious stimulations. Distance-based spatial summation can be examined by contrasting the pain evoked by pairs of noxious stimuli at increasing separations, relative to a pair adjacent. So far, studies show summation at separations up to at least 20cm.
One of the many questions that get me up in the morning is: Why is chronic spinal pain so prevalent compared to, say, chronic hand pain? It seems to me that pathological tissue models have so far failed to fully explain this discrepancy. One idea is that anatomical differences in the characteristics of the nociceptive system, such as the degree to which inputs summate, might help explain the vulnerability of one region over another. In a recent study we hypothesised that the effectiveness of the spatial summation effect would be greater at the spine than it is at the arm. We tested this hypothesis by asking participants to rate the intensity of pain evoked by various stimulus configurations at different body sites (for the precise methodology see free full text here).
Contrary to our hypothesis, all sites demonstrated the same magnitude of area- and distance-based spatial summation. Nonetheless, spatial summation is still likely to be a more relevant mechanism at some sites than others. For example, there is increased chance of spatial summation events in the spinal region, because there is significantly more tissue within relevant proximity compared to, say, a distal inter-phalangeal joint. In pure speculation: One could see a scenario where co-existing somatic and or/visceral conditions, particularly around the lumbar-pelvic region, might summate in such a way that the sum of (ordinarily more manageable) parts, results in a far more painful whole.
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Dr Daniel Harvie is an NHMRC Early Career Research Fellow based at The Hopkins Centre in the Menzies Health Institute QLD at Griffith University. His main focus is the investigation of central nervous system contributions to persistent pain, and the development of brain-based treatments for preventing and treating persistent pain, including those that involve sensory re-training, virtual reality, and education.