Neurology
Pain Pathway
Nociception, ascending tracts, and the brain's construction of pain perception
Pain is the conscious perception of nociceptive signaling — distinct from nociception itself, which is the neural process. Tissue damage activates nociceptors (free nerve endings expressing TRPV1, TRPA1, ASIC, P2X channels). Signals travel via Aδ fibers (sharp, fast pain) and C fibers (dull, slow burning pain) to the dorsal horn, synapse, decussate, then ascend the spinothalamic tract to the thalamus and cortex. Modulation occurs at the dorsal horn (gate control theory), brainstem (PAG, RVM), and cortex. Chronic pain involves central sensitization, neuroplasticity, and disinhibition. Treatment targets every level — NSAIDs peripherally, opioids centrally, anticonvulsants for neuropathic pain.
- Aδ fibersMyelinated, 5-30 m/s, sharp pain
- C fibersUnmyelinated, 0.5-2 m/s, dull/burning pain
- Main ascending tractSpinothalamic tract (decussates at cord level)
- Key receptorTRPV1 (capsaicin, heat > 43°C, H+)
- Endogenous opioidsEndorphins, enkephalins, dynorphins
- Neuropathic agentsGabapentin, pregabalin, duloxetine, TCAs
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Why the pain pathway matters
- Acute pain. Postoperative, trauma, MI all require targeted analgesia.
- Chronic pain. Affects 20% of adults; central sensitization needs distinct treatment.
- Opioid epidemic. Understanding tolerance, dependence, and alternatives is essential.
- Neuropathic pain. Diabetic, postherpetic, MS, sciatica need anticonvulsants/antidepressants.
- Anesthesia. Multimodal approach blocks pain at multiple levels.
- Cancer pain. WHO ladder guides escalation through analgesics.
- Pediatrics and elderly. Pain assessment is harder; under-treatment is common.
Common misconceptions
- Pain is in the wound. Pain is constructed in the brain — phantom limb proves it.
- Higher dose always more effective. Opioid-induced hyperalgesia paradoxically worsens pain.
- Chronic pain is acute pain that lasts. Different mechanism — central sensitization, not ongoing tissue damage.
- NSAIDs work for all pain. Poor for neuropathic pain; risky in elderly with renal/GI disease.
- Opioids should be avoided. Appropriate for acute and cancer pain; risk lies in chronic non-cancer use.
- Pain reports are unreliable. Self-report remains the gold standard; behavior alone underestimates pain.
Frequently asked questions
What's the difference between nociception and pain?
Nociception is the neural process of detecting and transmitting noxious stimuli — it occurs in unconscious patients. Pain is the conscious, emotional, multidimensional experience constructed by the brain from these signals. The IASP definition: 'an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.' This distinction matters — pain can exist without nociception (phantom limb, fibromyalgia) and nociception without pain (anesthesia).
How does the gate control theory work?
Melzack and Wall (1965) proposed that large-diameter Aβ fibers (touch, vibration) inhibit pain signaling at the dorsal horn through inhibitory interneurons. Rubbing an injury 'closes the gate' on C fiber input. This explains why TENS (transcutaneous electrical nerve stimulation), massage, and acupuncture provide analgesia. Spinal cord stimulators implanted for chronic pain exploit the same principle — stimulating dorsal columns reduces pain signal transmission.
How does central sensitization cause chronic pain?
Persistent C-fiber input causes glutamate-driven NMDA receptor activation in dorsal horn neurons, leading to long-term potentiation. Wind-up makes neurons fire more for the same input. AMPA receptor insertion, gene expression changes, and microglial activation maintain hyperexcitability. Result: allodynia (pain from non-painful stimuli), hyperalgesia (exaggerated pain), and pain spreading beyond the original injury. Targeted by gabapentinoids and ketamine.
What do opioids do at receptors?
Opioids bind μ, κ, and δ G-protein-coupled receptors. Activation closes voltage-gated Ca2+ channels (reducing neurotransmitter release) and opens K+ channels (hyperpolarizing neurons). μ-receptor effects: analgesia, euphoria, respiratory depression, miosis, constipation, tolerance. Endogenous ligands are endorphins, enkephalins, dynorphins. PAG (periaqueductal gray) opioid activation engages descending inhibition via the rostroventral medulla and locus coeruleus.
Why do NSAIDs work for inflammatory pain?
Tissue damage releases prostaglandins (PGE2) that sensitize nociceptors — making them fire to weaker stimuli. NSAIDs inhibit COX-1 and COX-2 enzymes that synthesize prostaglandins, reducing peripheral sensitization. COX-2 selective inhibitors (celecoxib) preserve gastric COX-1 to spare GI mucosa. Side effects: GI bleeding (COX-1), renal injury, cardiovascular events (COX-2). Acetaminophen acts centrally — exact mechanism still debated.
What is neuropathic pain?
Pain caused by lesion or disease of the somatosensory nervous system itself. Examples: diabetic neuropathy, postherpetic neuralgia, sciatica, trigeminal neuralgia, phantom limb. Mechanisms include ectopic firing from damaged axons, sodium channel upregulation (Nav1.7, Nav1.8), and central sensitization. Often poorly responsive to opioids. First-line: gabapentin/pregabalin (Cav α2δ ligands), duloxetine, TCAs (amitriptyline). Topical lidocaine for localized cases.
How is referred pain explained?
Visceral pain often refers to dermatomes sharing spinal level inputs — heart pain refers to left arm/jaw because cardiac and somatic afferents converge on the same dorsal horn neurons (T1-T5). The brain, lacking practice with visceral pain, misattributes signals to the more familiar somatic territory. Classic patterns: appendicitis to umbilicus then RLQ, gallbladder to right shoulder, diaphragmatic irritation to shoulder via phrenic nerve (C3-C5).