Pain pathways

Clinical relevance (where pain pathways matter in anaesthesia)

• Choice of analgesic technique depends on where you interrupt the pathway: peripheral transduction (NSAIDs/local infiltration), conduction (regional/neuraxial LA), spinal transmission (intrathecal/epidural opioids, α2 agonists), supraspinal modulation (opioids, ketamine), affective/cognitive components (multimodal, anxiolysis).
• Different fibre types explain symptom patterns and block characteristics.
  • Fast, sharp pain: Aδ fibres; slow, dull pain: C fibres.
  • Differential block: small myelinated and unmyelinated fibres are blocked earlier than large myelinated fibres (but clinical order depends on concentration, frequency-dependent block, and fibre location).
• Central sensitisation explains disproportionate pain, opioid escalation, and benefit of pre-emptive/multimodal strategies.
• Neuropathic pain arises from lesion/disease of the somatosensory system; mechanisms include ectopic firing, disinhibition, and altered descending control.

Structured pathway (from stimulus to perception)

• Transduction → Conduction → Transmission (spinal) → Ascending pathways → Thalamocortical processing → Perception/behaviour; with descending modulation at multiple levels.

Definitions and classification

• Pain: unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage (IASP).
• Nociception: neural processes of encoding noxious stimuli; may occur without pain (e.g., anaesthesia, spinal cord injury).
• Nociceptive pain: activation of nociceptors (somatic/visceral). Neuropathic pain: lesion/disease of somatosensory system. Nociplastic pain: altered nociception without clear tissue damage/lesion (e.g., fibromyalgia).

Nociceptors and transduction (periphery)

• Nociceptors are free nerve endings of primary afferent neurons (cell bodies in dorsal root ganglion for body; trigeminal ganglion for face).
• Modalities: mechanical, thermal, chemical; many are polymodal (especially C fibre nociceptors).
• Key transducer channels/receptors (examples):
  • TRPV1: heat (>~43°C), capsaicin, protons (acidosis).
  • TRPA1: irritants, oxidative stress; cold (context-dependent).
  • ASIC: acid-sensing ion channels (protons).
  • P2X/P2Y: ATP released from damaged cells.
• Inflammatory mediators lower threshold and increase firing (peripheral sensitisation).
  • Bradykinin, prostaglandins (PGE2), leukotrienes, histamine, serotonin (5-HT), cytokines (e.g., TNF-α, IL-1β), NGF.
  • Clinical: primary hyperalgesia at injury site; NSAIDs reduce prostaglandin-mediated sensitisation.

Primary afferent fibres (conduction)

• Aδ fibres: small, myelinated; faster conduction; sharp, well-localised pain; often mechanical/thermal.
• C fibres: small, unmyelinated; slow conduction; dull, burning/aching pain; polymodal; strong autonomic/emotional component.
• Aβ fibres: large, myelinated; normally low-threshold mechanoreceptors (touch); can contribute to allodynia after central sensitisation (Aβ input accessing pain pathways).
• Cell bodies in DRG; central processes enter spinal cord via dorsal roots and may ascend/descend 1–2 segments in Lissauer’s tract before synapsing.

Spinal cord processing (transmission and modulation)

• Primary afferents synapse in dorsal horn laminae (Rexed):
  • Lamina I (marginal zone): nociceptive-specific neurons; projects to spinothalamic/spinoparabrachial pathways.
  • Lamina II (substantia gelatinosa): interneurons; major site of modulation (opioid receptors, inhibitory control).
  • Lamina V: wide dynamic range (WDR) neurons; integrate nociceptive + non-nociceptive input; implicated in central sensitisation and referred pain.
• Neurotransmitters at primary afferent synapse:
  • Fast excitatory: glutamate (AMPA/kainate).
  • Slow/prolonged: substance P (NK1), CGRP; contribute to neurogenic inflammation and central sensitisation.
• Inhibitory interneurons: GABA and glycine; loss of inhibition contributes to allodynia/hyperalgesia.
• Gate control concept: non-nociceptive Aβ input can activate inhibitory interneurons in dorsal horn to reduce nociceptive transmission (basis for rubbing/TENS).
• Central sensitisation (wind-up): activity-dependent increased excitability of dorsal horn neurons.
  • Mechanisms: NMDA receptor activation (Mg2+ block removed by depolarisation), increased intracellular Ca2+, kinase activation, gene transcription changes; microglial/astrocyte mediators.
  • Clinical: secondary hyperalgesia (surrounding uninjured tissue), allodynia, opioid tolerance/hyperalgesia (multifactorial).
  • Drug targets: ketamine (NMDA antagonism), gabapentinoids (α2δ Ca2+ channel subunit), α2 agonists, neuraxial local anaesthetics/opioids.

Ascending pathways (spinal to brain)

• Second-order neurons decussate in anterior white commissure within 1–2 segments and ascend contralaterally in anterolateral system.
• Spinothalamic tract (STT): major pathway for pain and temperature to thalamus.
  • Lateral STT: sensory-discriminative aspects (location, intensity).
  • Anterior/medial components: affective-motivational aspects (less discrete; overlaps with other medial pain pathways).
• Spinoreticular: to reticular formation (arousal, autonomic responses) then intralaminar thalamic nuclei.
• Spinomesencephalic/spinoparabrachial: to PAG, parabrachial nucleus, amygdala/hypothalamus (affect, autonomic/endocrine responses).
• Trigeminal pain (face): primary afferents to spinal trigeminal nucleus; second-order neurons cross and ascend in trigeminothalamic tract to VPM thalamus.

Thalamocortical processing and perception

• Thalamus relays to cortical networks rather than a single “pain centre”.
• Sensory-discriminative: primary/secondary somatosensory cortex (S1/S2).
• Affective-motivational: anterior cingulate cortex (ACC), insula; limbic structures (amygdala).
• Cognitive-evaluative: prefrontal cortex; expectation/placebo/nocebo effects via descending control.

Descending modulation (inhibition and facilitation)

• Key pathway: cortex/limbic → periaqueductal grey (PAG) → rostral ventromedial medulla (RVM; incl. nucleus raphe magnus) → dorsal horn.
• Neurochemistry:
  • Serotonin (5-HT) from raphe nuclei; noradrenaline from locus coeruleus; both can inhibit or facilitate depending on receptor subtype and context.
  • Endogenous opioids (enkephalins, endorphins, dynorphins) act at μ/δ/κ receptors in PAG, RVM, and dorsal horn.
• Clinical pharmacology links:
  • Opioids: presynaptic inhibition of transmitter release (↓Ca2+), postsynaptic hyperpolarisation (↑K+).
  • SNRIs/TCAs: enhance descending inhibitory monoaminergic tone (useful in neuropathic pain).
  • α2 agonists (clonidine/dexmedetomidine): reduce neurotransmitter release and augment spinal inhibition.

Referred pain and visceral pain

• Visceral afferents travel with sympathetic (T1–L2) and parasympathetic (vagus, pelvic splanchnics S2–S4) pathways; sparse innervation and wide receptive fields → poor localisation.
• Referred pain: convergence of visceral and somatic afferents onto common dorsal horn neurons (often WDR) leads to mislocalisation to a somatic dermatome.
• Visceral pain often associated with autonomic features (nausea, sweating) due to strong connections with brainstem/hypothalamus.

Key terms (often tested)

• Hyperalgesia: increased pain from a stimulus that normally provokes pain.
• Allodynia: pain due to a stimulus that does not normally provoke pain (e.g., light touch).
• Primary hyperalgesia: at injury site (peripheral sensitisation). Secondary hyperalgesia: surrounding tissue (central sensitisation).
• Wind-up: frequency-dependent progressive increase in dorsal horn neuron response to repeated C-fibre stimulation (NMDA-dependent).