Rehab Science · Chapter 2

How Pain Works

Pain is produced through a loop: sensors in tissues detect stimuli, ascending (afferent) pathways carry information to the brain, the brain builds a unique "neurosignature", and descending (efferent) pathways send back actions and can also block danger messages. Sensitivity is adjustable through neuroplasticity—up after injury and stress, down with graded exposure.

CompleteSource: Rehab Science · Ch 2 · PDF pp. 24–44

You will learn to

✓ Trace the pain loop from sensor to brain to response
✓ List the sensor types that can be sensitised
✓ Explain sensitisation and neuroplasticity in plain language
✓ Describe descending inhibition and why pain can be absent in danger

Understanding pain reduces pain

📖 Rehab Science

Nerves do not "make" pain—they carry information. People who learn how their nervous system works tend to feel less threatened and less fearful, and that alone can lower symptoms.

Source: Rehab Science · Ch 2 · PDF p. 24· 8 original research citationsshow sources ▾

[Ch 2·1] Melzack R. Pain and the neuromatrix in the brain. J Dent Educ. 2001;65(12):1378-1382. find original ↗
[Ch 2·2] Moseley GL. A pain neuromatrix approach to patients with chronic pain. Man Ther. 2003;8(3):130-140. find original ↗
[Ch 2·3] Watson JA, Ryan CG, Cooper L, et al. Pain neuroscience education for adults with chronic musculoskeletal pain: a mixed-methods systematic review and meta-analysis. J Pain. 2019;20(10):1140.e1-1140.e22. find original ↗
[Ch 2·4] Louw A, Zimney K, Puentedura EJ, Diener I. The efficacy of pain neuroscience education on musculoskeletal pain: a systematic review of the literature. Physiother Theory Pract. 2016;32(5):332-355. find original ↗
[Ch 2·5] Moseley GL, Nicholas MK, Hodges PW. A randomized controlled trial of intensive neurophysiology education in chronic low back pain. Clin J Pain. 2004;20(5):324-330. find original ↗
[Ch 2·6] Meeus M, Nijs J, Van Oosterwijck J, Van Alsenoy V, Truijen S. Pain physiology education improves pain beliefs in patients with chronic fatigue syndrome compared with pacing and self-management education: a double-blind randomized controlled trial. Arch Phys Med Rehabil. 2010;91(8):1153-1159. find original ↗
[Ch 2·7] Clarke CL, Ryan CG, Martin DJ. Pain neurophysiology education for the management of individuals with chronic low back pain: systematic review and meta-analysis. Man Ther. 2011;16(6):544-549. find original ↗
[Ch 2·8] Louw A, Diener I, Butler DS, Puentedura EJ. The effect of neuroscience education on pain, disability, anxiety, and stress in chronic musculoskeletal pain. Arch Phys Med Rehabil. 2011;92(12):2041-2056. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

The pain loop

📖 Rehab Science

Sensors in skin, muscles and joints report temperature, pressure and movement. The message travels up to the spinal cord and brain. If the brain decides action is needed, it sends a response back down—usually a movement.

Diagram · The pain loopOriginal illustration

Sensors detect stimuli; ascending (afferent) signals reach the brain, which weighs all inputs and may output pain, movement and/or a stress response. Descending (efferent) pathways carry actions and can inhibit danger messages. — Based on concepts in Rehab Science Ch 1–2 (PDF pp. 19–20, 25, 36, 42–43).

Source: Rehab Science · Ch 2 · PDF p. 25· 1 original research citationshow sources ▾

Also cited by the book on this page:

[Ch 2·1] Melzack R. Pain and the neuromatrix in the brain. J Dent Educ. 2001;65(12):1378-1382. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

Six kinds of sensor that can become sensitive

📖 Rehab Science

Think of the warning lights on a car dashboard. Your body has "lights" for temperature, pressure, movement, stress chemicals, immune chemicals and blood flow. Any of them can be turned up so that they switch on too easily.

Temperature · Pressure · Movement · Stress · Immunity · Blood flow
Movement sensitivity is the main target of the rehab exercises

Diagram · Your body’s dashboard lightsOriginal illustration

Six sensor systems feed the brain like warning lights on a car dashboard. Each can be ‘turned up’ (sensitised) so it lights up too easily. — Based on concepts in Rehab Science Ch 2 (PDF pp. 28–32).

Source: Rehab Science · Ch 2 · PDF pp. 28–32· 2 original research citationsshow sources ▾

Also cited by the book on these pages:

[Ch 2·9] Devor M. Sodium channels and mechanisms of neuropathic pain. J Pain. 2006;7(1 Suppl 1):S3-S12. find original ↗
[Ch 2·11] Devor M. Response of nerves to injury in relation to neuropathic pain. In: McMahon S, Koltzenburg M. Wall and Melzack’s Textbook of Pain. Philadelphia: Elsevier; 2013. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

Sensitisation and chronic pain

📖 Rehab Science

After an injury—especially when you are also stressed or sleeping badly—your nervous system can "turn up the volume" within days. If it stays turned up after the tissue has healed, everyday movements such as bending can keep hurting. Gentle, gradually increasing movement teaches the system that the movement is safe again.

Diagram · Sensitisation lowers the thresholdOriginal illustration

After injury and stress, less stimulus is needed to trigger a danger response. Graded exposure gradually raises the threshold back toward normal. — Based on concepts in Rehab Science Ch 2 (PDF pp. 32–35).

Source: Rehab Science · Ch 2 · PDF pp. 32, 33· 3 original research citationsshow sources ▾

[Ch 2·9] Devor M. Sodium channels and mechanisms of neuropathic pain. J Pain. 2006;7(1 Suppl 1):S3-S12. find original ↗
[Ch 2·10] Devor M, Govrin-Lippmann R, Angelides K. Na+ channel immunolocalization in peripheral mammalian axons and changes following nerve injury and neuroma formation. J Neurosci. 1993;13(5):1976-1992. find original ↗
[Ch 2·11] Devor M. Response of nerves to injury in relation to neuropathic pain. In: McMahon S, Koltzenburg M. Wall and Melzack’s Textbook of Pain. Philadelphia: Elsevier; 2013. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

Neuroplasticity — doors into the cell

📖 Rehab Science

Nerve cells have "doors" (receptors) that let chemical messengers in. With threat, the cell builds more doors and becomes easier to trigger. Gradual, safe exposure to a movement helps the cell close doors again.

Source: Rehab Science · Ch 2 · PDF pp. 33, 34, 35show sources ▾

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

Ascending pathways and nociception

📖 Rehab Science

A nerve rests at a set electrical level. When a stimulus is strong enough to reach a threshold, it fires a message upward. Special "danger detectors" only fire for strong stimuli—but even then the brain may answer with movement (shift in your chair) instead of pain.

Nociception ≠ pain
Resting ≈ −70 mV → threshold ≈ −55 mV → action potential

Diagram · From stimulus to signalOriginal illustration

A nerve rests near −70 mV. A strong enough stimulus pushes it to ~−55 mV (threshold), firing an action potential toward the spinal cord. Sub-threshold stimuli fade away. — Based on concepts in Rehab Science Ch 2 (PDF pp. 36–39).

Source: Rehab Science · Ch 2 · PDF pp. 36, 38, 39show sources ▾

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

How the brain creates a unique pain

📖 Rehab Science

There is no single "pain centre". Many brain areas work together, and the pattern they form is unique to you. Once a pattern exists, it is easier to trigger again—so recovery means retraining the pattern, not just following an exercise list.

Source: Rehab Science · Ch 2 · PDF pp. 40, 41· 5 original research citationsshow sources ▾

[Ch 2·1] Melzack R. Pain and the neuromatrix in the brain. J Dent Educ. 2001;65(12):1378-1382. find original ↗
[Ch 2·2] Moseley GL. A pain neuromatrix approach to patients with chronic pain. Man Ther. 2003;8(3):130-140. find original ↗
[Ch 2·12] Flor H. The functional organization of the brain in chronic pain. Prog Brain Res. 2000;129:313-322. find original ↗
[Ch 2·13] Apkarian AV, Bushnell MC, Treede RD, Zubieta JK. Human brain mechanisms of pain perception and regulation in health and disease. Eur J Pain. 2005;9(4):463-484. find original ↗
[Ch 2·14] Tracey I, Mantyh PW. The cerebral signature for pain perception and its modulation. Neuron. 2007;55(3):377-391. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

Descending pathways — action and inhibition

📖 Rehab Science

Messages coming down from the brain usually tell muscles to act—like pulling your hand out of ice water. The brain can also release its own pain-blocking chemicals, which is why athletes or soldiers sometimes feel nothing until they are safe.

Source: Rehab Science · Ch 2 · PDF pp. 42, 43, 44· 1 original research citationshow sources ▾

[Ch 2·15] Doidge N. The Brain That Changes Itself. New York: Penguin Books; 2007. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

When pain feels good

📖 Rehab Science

A deep massage can hurt yet feel good because you believe it is helping, and your brain releases endorphins that soften the danger messages.

Source: Rehab Science · Ch 2 · PDF p. 44· 1 original research citationshow sources ▾

Also cited by the book on this page:

[Ch 2·15] Doidge N. The Brain That Changes Itself. New York: Penguin Books; 2007. find original ↗

Page numbers are those of the uploaded PDF edition. "Find original" opens a PubMed search for the cited study.

Check your understanding

1. What is "nociception"?

2. How quickly can the nervous system become sensitised in a threatening situation?

3. Approximately what membrane voltage is the activation threshold of most nerves?