Pain Anatomy, Physiology, and Pathophysiology
Core concepts
Trace the pain pathway: nociceptor transduction, primary afferent conduction (A-delta fast, C slow), dorsal horn synapses and ascending tracts (spinothalamic), and descending modulation from periaqueductal gray and rostral ventromedial medulla. Understand sensitization: peripheral (inflammatory mediators lower threshold) and central (wind-up, NMDA receptor involvement).
Mechanism categories
Nociceptive (tissue injury, responds to NSAIDs/opioids), neuropathic (nerve injury, responds to gabapentinoids/SNRIs/TCAs), and nociplastic (altered processing without clear tissue or nerve injury). Sympathetically maintained pain and visceral pain pathways are also testable.
Traps
- Assuming all pain is nociceptive; mechanism determines drug class.
- Confusing peripheral with central sensitization.
- Forgetting that opioid-induced hyperalgesia is distinct from tolerance.
- Overlooking referred pain patterns from viscera and joints.
Worked rule of thumb
Ask what mechanism fits: if the pain is burning, shooting, and dermatomal, think neuropathic and choose a neuropathic agent; if it is aching and mechanical, think nociceptive. Mixed mechanisms are common and justify combination therapy.
Clinical link
Mechanistic reasoning connects anatomy and pharmacology and is the thread running through the entire exam.