¶ A-delta fibres
¶ Definition
A-delta (Aδ) fibres are small-diameter (
μm), thinly myelinated sensory nerve fibres conducting at 12-30 m/s (typically ~15 m/s) that transmit "first pain" — the immediate, sharp, well-localized sensation upon acute injury. They are cutaneous afferents specialized for mechanical and thermal nociception, synapsing primarily in lamina I (marginal zone) of the dorsal horn. Their rapid conduction relative to unmyelinated C-fibres enables protective withdrawal reflexes before dull, aching "second pain" arrives.
¶ Picture It
Imagine a neighbourhood with two alarm systems. The Aδ-fibre system is the wireless smart alarm — it has a thin wire (thin myelin sheath) that sends signals in quick bursts (saltatory conduction), reaching the control centre (spinal cord) within milliseconds. When you step on a nail, the Aδ alarm screams "SHARP METAL IN FOOT!" — loud, immediate, precise location. This triggers an instant reflex: you yank your foot away before you even consciously register what happened. Meanwhile, the old landline alarm (C-fibres) is still dialling through copper wire (unmyelinated, slow), and will arrive seconds later with the message "dull aching pain now spreading." The Aδ system is your emergency broadcast — it gets you out of danger fast, but it doesn't linger. Once the threat is gone, it quiets. If this system breaks (as in small fibre neuropathy), you lose your "ouch!" reflex and can injure yourself repeatedly without noticing — like an alarm that never rings.
¶ Mechanism
Aδ fibres are Group III afferents in the Lloyd-Hunt classification, characterized by:
- Thin myelination: 1-5 layers of myelin (vs 20-40 in A-beta fibres), enabling moderate-speed saltatory conduction at 12-30 m/s (peak ~15 m/s)
- Nociceptor specialization:
- Central termination: Aδ terminals synapse in lamina I (marginal zone) and outer lamina II (substantia gelatinosa) of the dorsal horn, releasing glutamate (fast excitatory) and Substance P (slower neuromodulation)
- Ascending pathways: Second-order neurons cross midline and ascend via spinothalamic tract (neospinothalamic pathway) → lateral thalamus (VPL nucleus) → primary somatosensory cortex (S1) and insular cortex
- Withdrawal reflex circuit: Aδ input activates interneurons in lamina V → alpha motor neurons in ventral horn → immediate flexor muscle contraction (withdrawal) at spinal reflex latency (~50-80 ms) before cortical awareness
Threshold specificity: Aδ fibres have a high activation threshold (typically 45-47°C for heat, >5°C for cold, >10 mN for mechanical) — they are "silent" during normal sensory input and only fire when tissue damage is imminent or occurring.
Temporal coding: The latency difference between Aδ arrival (~80-150 ms from hand to cortex) and C-fibres arrival (~500-1000 ms) creates the characteristic "double pain" phenomenon: immediate sharp stab (Aδ) followed by dull burning ache (C-fibres).
¶ Clinical Significance
Protective role: Aδ fibres constitute the body's first-line warning system for acute tissue damage. Their rapid conduction enables reflexive avoidance before cortical processing — evolutionarily critical for survival (e.g., withdrawing from fire, sharp objects, predator bites). This is Metamodel 0 in action: the alarm that initiates immediate defensive behaviour.
Clinical loss syndromes:
- Small fibre neuropathy (SFN): Selective Aδ and C-fibre degeneration (e.g., in diabetes, chemotherapy, immune-mediated conditions) leads to loss of protective first pain → unnoticed burns, cuts, pressure ulcers. Diagnosed via intraepidermal nerve fibre density <5 fibres/mm (normal >7.6 fibres/mm at ankle).
- Congenital insensitivity to pain: Rare mutations in SCN9A gene (voltage-gated sodium channel Nav1.7) eliminate Aδ/C-fibre function → patients sustain severe injuries without awareness.
Sensitization and chronic pain:
- Central sensitization: Repeated Aδ input (e.g., chronic injury, inflammation) → NMDA receptor phosphorylation and wind-up in dorsal horn neurons → mechanical allodynia (normally innocuous Aβ touch now perceived as sharp pain via cross-talk in lamina II).
- Hyperalgesia: Inflammatory mediators (prostaglandins, bradykinin, NGF) sensitize Aδ nociceptors → lower activation threshold (e.g., heat pain at 40°C instead of 45°C).
cPNI intervention targets:
- Cold therapy: Preferentially blocks Aδ conduction (myelin-dependent conduction more sensitive to temperature than unmyelinated fibres) → acute pain relief without blocking motor function.
- Capsaicin: High-concentration application (8% patch) desensitizes TRPV1 on Aδ terminals → reduces mechanical hyperalgesia in neuropathic pain.
- Gate Control Theory: Activating A-beta fibres (non-nociceptive) via TENS, manual therapy, or vibration inhibits Aδ transmission at spinal level (via GABAergic interneurons in lamina II).
- Pain neuroscience education: Explaining the Aδ vs C-fibre timeline helps patients reframe pain ("the sharp stab is my alarm, not tissue damage") → reduces catastrophizing and fear-avoidance.
Evolutionary mismatch: Modern chronic pain syndromes (fibromyalgia, chronic low-grade inflammation) involve Aδ sensitization without ongoing tissue threat — the alarm stuck in "on" mode due to systemic inflammation (IL-1β, TNF-α) and central sensitization. This reflects Metamodel 1 dysfunction: the system designed for acute threats now responds to chronic, low-grade stressors.
¶ Key Facts
- Diameter: 1-5 μm (small, thinly myelinated)
- Conduction velocity: 12-30 m/s (typically ~15 m/s; ~10x faster than C-fibres, ~6x slower than A-beta fibres)
- Myelin thickness: 1-5 myelin layers (vs 20-40 in A-beta)
- Sensory modalities: Mechanical nociception (sharp pain), thermal nociception (heat >45°C, cold <5°C)
- Central termination: Lamina I (marginal zone) and outer lamina II (substantia gelatinosa) of dorsal horn
- Neurotransmitters: Glutamate (primary fast transmission) + Substance P (neuromodulation)
- Reflex latency: Spinal withdrawal reflex occurs at ~50-80 ms (before cortical awareness)
- Conscious perception latency: Sharp pain reaches cortex at ~80-150 ms from distal limb
- Key receptors: TRPV1 (heat), TRPA1 (cold/irritants), ASIC (acidosis), P2X3 (ATP)
- Clinical threshold for diagnosis: Intraepidermal nerve fibre density <5 fibres/mm indicates small fibre neuropathy (normal >7.6/mm at ankle)
- "Double pain" interval: 300-800 ms gap between Aδ "first pain" and C-fibre "second pain"
- Evolutionary function: Immediate threat detection and reflexive avoidance behaviour
¶ Connections
- C-fibres — unmyelinated, slower (0.5-2 m/s) fibres transmitting dull, burning "second pain" vs Aδ sharp "first pain"
- A-beta fibres — large, heavily myelinated non-nociceptive fibres (30-70 m/s) that can inhibit Aδ transmission via gate control
- first pain — the immediate, sharp, well-localized sensation transmitted by Aδ fibres
- nociceptors — specialized sensory receptors innervated by Aδ fibres (mechanical and thermal types)
- dorsal horn — first central synapse site for Aδ input, particularly lamina I and II
- lamina I — primary termination zone for Aδ nociceptive fibres (marginal zone)
- lamina II — substantia gelatinosa where Aδ terminals interact with inhibitory interneurons
- myelination — thin myelin sheath on Aδ enables moderate-speed saltatory conduction
- withdrawal reflex — spinal reflex arc activated by Aδ input enabling rapid protective movement
- TRPV1 — heat-sensitive ion channel (>42°C) expressed on Aδ thermal nociceptors
- TRPA1 — cold- and irritant-sensitive channel on Aδ polymodal nociceptors
- hyperalgesia — Aδ sensitization by inflammatory mediators lowers pain threshold
- allodynia — central sensitization allows normally innocuous A-beta input to activate pain pathways via Aδ convergence
- small fibre neuropathy — selective degeneration of Aδ and C-fibres causes loss of protective pain
- capsaicin — TRPV1 agonist that desensitizes Aδ terminals with prolonged exposure
- thermal nociception — detection of noxious heat (>45°C) and cold (<5°C) via Aδ fibres
- mechanical nociception — sharp, high-threshold mechanical pain transmitted by Type I Aδ fibres
- dorsal root ganglion — location of Aδ fibre cell bodies (pseudo-unipolar neurons)
- conduction velocity — Aδ speed (12-30 m/s) allows temporal distinction from C-fibre pain
- spinothalamic tract — ascending pathway carrying Aδ nociceptive signals to thalamus and cortex
- Substance P — neuropeptide co-released with glutamate from Aδ terminals in dorsal horn
- glutamate — primary fast excitatory neurotransmitter released by Aδ fibres
- central sensitization — chronic Aδ input leads to NMDA receptor phosphorylation and wind-up in dorsal horn
- gate control theory — A-beta activation inhibits Aδ transmission via GABAergic interneurons
- ASIC — acid-sensing ion channels on Aδ fibres detect tissue acidosis (pH <6.5)
- P2X3 Receptor — ATP-gated ion channel on Aδ terminals activated by cellular damage
- intraepidermal nerve fibre density — diagnostic measure for small fibre neuropathy (Aδ and C-fibre loss)
- cold therapy — preferentially blocks Aδ conduction via effects on myelinated nerve function
- NGF — nerve growth factor released during inflammation sensitizes Aδ nociceptors
- prostaglandins — inflammatory mediators that lower Aδ activation threshold
- bradykinin — inflammatory peptide that activates and sensitizes Aδ nociceptors
- fibromyalgia — condition involving Aδ sensitization without tissue damage (central sensitization)
- neuropathic pain — often involves Aδ fibre dysfunction or ectopic firing
- pain neuroscience education — explaining Aδ vs C-fibre timeline reduces pain catastrophizing
- insular cortex — receives Aδ input for affective-motivational dimension of pain
- lateral thalamus — relay station for Aδ sensory-discriminative pain information to S1 cortex
- NMDA receptor — glutamate receptor whose phosphorylation by chronic Aδ input drives central sensitization
¶ Module References
- Module 5