The everyday picture treats pain as a damage gauge: the more it hurts, the more harm has been done, so relief means fixing the tissue. The pain scientist Lorimer Moseley tells a story that breaks the gauge, in his talk "Why Things Hurt" [1]. Walking in the Australian bush, he felt something touch his leg and took it for a twig, as it had been countless times before. It was an eastern brown snake, and he nearly died. Six months later, walking in the bush again, something touched his leg and he was in screaming agony until a friend looked and found a little scratch from a twig.
Pain science, laid out in Butler and Moseley's Explain Pain [2], resolves this by keeping two things apart. Nociception is signalling from sensors in the tissues that something potentially damaging is happening. Pain is an experience the brain produces when it concludes the body needs protecting; in Moseley's words, "100% of the time, pain is a construct of the brain." The nociceptive signal is one piece of evidence, weighed with context: where you are, what this sensation meant last time. The venom set off plenty of danger signals, but the context said twig, so there was almost no pain. The second twig sent weak signals, but the brain now read that sensation in that place as a near-fatal bite.
Scans show the same gap. A review of 33 studies covering 3,110 people with no back pain found disc degeneration in 37% of 20-year-olds and 96% of 80-year-olds [3]. The authors conclude many such findings are part of normal ageing.
Damage still counts as an input: the same group found disc bulges and degeneration more common in adults aged 50 and under with back pain than in those without [4]. What structure doesn't do is set the volume alone. The output is closer to input scaled by the system's sensitivity, and that sensitivity moves with stress, sleep and expectation while the structure stays put. Persistent pain raises it further, through central sensitization: the circuits that produce pain get better at producing it. So a scan finding doesn't by itself explain a pain, and teaching patients this model, pain neuroscience education, is a treatment in its own right, with trials in chronic musculoskeletal pain reporting less pain and disability [5].
References
Why Things Hurt [link]
2011.Explain Pain Second Edition [link]
Butler, D. S. and Moseley, G. L., 2013. Noigroup Publications.Systematic Literature Review of Imaging Features of Spinal Degeneration in Asymptomatic Populations [link]
Brinjikji, W., Luetmer, P. H., Comstock, B., Bresnahan, B. W., Chen, L. E., Deyo, R. A., Halabi, S., Turner, J. A., Avins, A. L., James, K., Wald, J. T., Kallmes, D. F. and Jarvik, J. G., 2014. American Journal of Neuroradiology, Vol 36(4), pp. 811–816. American Society of Neuroradiology (ASNR). DOI: 10.3174/ajnr.a4173MRI Findings of Disc Degeneration are More Prevalent in Adults with Low Back Pain than in Asymptomatic Controls: A Systematic Review and Meta-Analysis [link]
Brinjikji, W., Diehn, F. E., Jarvik, J. G., Carr, C. M., Kallmes, D. F., Murad, M. H. and Luetmer, P. H., 2015. AJNR: American Journal of Neuroradiology, Vol 36(12), pp. 2394–2399.The efficacy of pain neuroscience education on musculoskeletal pain: A systematic review of the literature [link]
Louw, A., Zimney, K., Puentedura, E. J. and Diener, I., 2016. Physiotherapy Theory and Practice, Vol 32(5), pp. 332–355. Informa UK Limited. DOI: 10.1080/09593985.2016.1194646