Peripheral Sensitization Explained | Physiology of Pain

Nociceptors exist everywhere except the brain. A-delta and C fibers carry pain signals. Peripheral sensitization occurs when inflammatory mediators (prostaglandins, serotonin) lower receptor thresholds. NSAIDs work by blocking cyclooxygenase enzymes, reducing prostaglandin production. Versatis (5% lidocaine patch) is a sodium channel blocker for post-herpetic neuralgia. Capsaicin cream (0.025%) depletes substance P for peripheral neuropathies. These drugs target the transduction and transmission stages of pain.

This video explains the pharmacology of pain management through two key mechanisms: NSAIDs work by inhibiting cyclooxygenase enzymes (COX-1 and COX-2) in the arachidonic acid cascade, reducing inflammatory prostaglandins that cause pain and swelling, but this inhibition also affects protective prostaglandins leading to side effects like gastric irritation and renal issues; local anesthetics block sodium channels in nerve cells, preventing the transmission of pain signals from nociceptors to the spinal cord by binding to sodium channel proteins and interrupting the action potential wave, with examples including lidocaine (rapid onset, short duration) and bupivacaine (slower onset, longer duration); pharmacokinetics encompasses the five essential principles of drug handling: liberation (drug release), absorption (entry into bloodstream), distribution (spread throughout body), metabolism (liver breakdown), and excretion (kidney elimination).

Pain signals travel from damaged tissue through sensory neurons via voltage-gated sodium channels to the spinal cord and cerebral cortex. Local anesthetics block these channels for immediate surgical pain relief. As procedures continue, prostaglandins and leukotriens are released as inflammatory mediators that amplify pain signals. NSAIDs like naproxen, ibuprofen, diclofenac, and ketoprofen inhibit prostaglandin production (and leukotriens for some agents) to reduce amplified postoperative pain. Local anesthetics become less effective as tissue pH becomes more acidic during inflammation.

Pain is transmitted through a pathway where peripheral stimuli activate free nerve endings, releasing histamine, serotonin, and prostaglandins that travel via Type C fibers to the posterior root ganglion, where substance P and glutamate are released at the first synapse; signals then ascend 2-3 spinal segments, cross contralaterally, and reach the thalamus before processing in the somatosensory cortex. Analgesics are classified into two main groups: opioids (central-acting) that bind to mu, delta, and kappa receptors via Gi protein-coupled mechanisms to block calcium entry and prevent neurotransmitter release, and NSAIDs (peripherally-acting) that inhibit cyclooxygenase enzymes to reduce prostaglandin synthesis. Opioids include natural (morphine, codeine), semisynthetic (heroin, buprenorphine), and synthetic (fentanyl, methadone) types, with varying potencies and clinical applications from acute pain management to addiction treatment.

Pain transmission begins when tissue injury triggers phospholipase A2 to release arachidonic acid, which is converted to prostaglandins by COX-1 and COX-2 enzymes. Prostaglandins sensitize nociceptors and drive inflammation. Signals travel through peripheral nerves to the spinal cord dorsal horn, where neurotransmitters (glutamate, substance P, CGRP) transmit pain. COX-2 activation in the spinal cord causes central sensitization, amplifying pain signals. In the brain, prostaglandins regulate temperature and pain perception. Three drug classes target this pathway at different points: paracetamol blocks central COX enzymes; NSAIDs inhibit both COX-1 and COX-2 peripherally and centrally; opioids bind to opioid receptors to block pain signal transmission.

The biopsychosocial model recognizes that chronic pain results from the interaction of three interconnected domains: biological factors (physical causes and nerve function), psychological factors (mental health, emotions, cognition), and social factors (relationships, work, societal attitudes). Effective chronic pain management requires addressing all three dimensions simultaneously, as ignoring any one component leads to incomplete treatment outcomes.

The biopsychosocial model explains chronic primary pain as an independent disease arising from the interaction of biological, psychological, and social factors, where these three dimensions overlap and influence each other; effective treatment requires addressing all three dimensions through multimodal, non-pharmaceutical strategies that consider the whole person rather than focusing solely on physical causes.

This segment introduces the biopsychosocial model (2007) as the modern approach to understanding chronic pain. Unlike the biomédical model, it considers physical, psychological, and social factors: habits, exercise, nutrition, rest, beliefs about pain, stress, anxiety, work environment, and support systems. The instructor explains that chronic back pain causes multiple changes in the body: muscle loss, joint stiffness, increased pain sensitivity, sleep disruption, fatigue, fear of movement, and emotional changes like anxiety and depression.

The biopsychosocial model of pain considers that pain is influenced by biological, psychological, and social factors. The video explains that emotional and psychological states can significantly affect pain perception and treatment outcomes. Psychological factors can contribute to the development of chronic pain conditions, and negative psychological states can negatively affect treatment outcomes. The nocebo effect demonstrates how negative expectations or suggestions can worsen pain symptoms. Effective pain management must address all three dimensions rather than focusing solely on physical symptoms.

The biopsychosocial model explains that pain perception involves biological (tissue damage), psychological (brain interpretation), and social (environmental context) components. Anxiety and negative social environments increase pain perception, while positive coaching and external focus techniques reduce it. This model supports early movement after injury, as immobilization and fear-based messaging can perpetuate chronic pain states. The eight-week isometric rehabilitation course is essential—many patients prematurely discontinue treatment after feeling better, risking recurrence.

Graded motor imagery is a strategy that can help retrain a sensitized nervous system. This approach involves imagining movements or using mirrors to trick the brain into performing more movements. It helps reduce nervous system sensitivity before manual therapy can be safely applied.

Research shows that normalizing neurologic adaptations requires strategically integrating multiple evidence-based techniques in the right order at the right time. Key techniques include: Pain Neuroscience Education (PNE), which is evidence-based and can lower pain levels on its own; Graded Motor Imagery (GMI) including laterality training, explicit motor imagery, and mirror therapy, which improves sensory processing and weakens automatic pain responses; Neurodynamic nerve drills, which are specific movements targeting nerves to reduce peripheral sensitivity; Sensory discrimination strategies using non-painful tactile inputs to improve brain-body communication; and Pacing, which finds the sweet spot between overdoing and underdoing to build activity tolerance while reducing pathway sensitivity. Introducing all techniques at once is a common mistake that can cause the pain system to react negatively. Instead, techniques should be introduced in phases: Phase 1 - Understanding neurologic adaptations; Phase 2 - Setting the foundation for recovery; Phase 3 - Expanding activity tolerance; Phase 4 - Normalizing hypersensitive pain pathways; Phase 5 - Rebuilding physical strength safely once sensitivity is reduced.

Advanced interventions for central sensitization include graded motor imagery progressing through lateralization, visualization, and mirror therapy. Traditional desensitization may heighten alarm responses, so graded approaches are preferred. Work injuries create higher threat levels for the nervous system because they threaten livelihood and earning capacity. Without addressing the heightened nervous system state first, functional training fails. Osteoarthritis is common but shouldn't be framed as devastating; education must be gentle. Complex Regional Pain Syndrome represents the highest end of the central sensitization spectrum. Sensory testing comparing opposite body sides helps identify dysfunction. Clear communication that this is a sensory system problem empowers patients to understand their condition.

Pacing differs fundamentally from graded exercise therapy (GET), which uses a fixed schedule of increasing aerobic activity. While some trials suggest GET was moderately effective, most patients report it causes significant long-term harm when pushed by therapists who don't understand PEM. The key difference is that GET uses rigid schedules while pacing uses the patient's body signals as the guide. Pacing is the crucial first step that stops constant crashes and stabilizes the nervous system. Once stabilized, patients can gradually expand their life. This approach works by retraining the limbic system, which can get stuck in high alert and cause fatigue, brain fog, and sensitivities. By combining pacing with calming exercises, patients teach their nervous system that normal activities are no longer threats, calming the fight-or-flight response and helping with PEM more than anything else.

Graded exposure involves identifying patient fears in a hierarchy and systematically exposing patients to feared activities to decrease fear responses. Pacing teaches patients to regulate their activity levels rather than pushing through pain. Both techniques address the physical behaviors that perpetuate chronic pain. Many patients lack awareness of how to pace themselves appropriately, requiring physiotherapists to help identify barriers and establish realistic activity goals.