Dr. Elke Schaumberg — organized by lecture objectives
Everything in both lectures hangs on these four phases. Know them cold — they are the spine of the whole unit.
| Phase | One-line definition | Where it happens |
|---|---|---|
| Transduction | Noxious stimulus converted into an action potential | Peripheral nociceptor (free nerve ending) |
| Transmission | Relay of the signal from periphery → CNS → brain | 1° afferent → dorsal horn → spinothalamic tracts → brain |
| Modulation | Up- and down-regulation of nociceptive signaling | Dorsal horn (± descending brainstem input) |
| Perception | The pain experience / meaning-making | Brain “pain matrix” |
Conceptual anchor: Nociception ≠ pain. Nociception is the neural signaling; pain is the perceived, interpreted experience. Severe tissue damage can occur with little pain, and pain can occur with no tissue damage.
Overarching objective: Diagram the neuroanatomical mechanisms/pathways responsible for the four phases of nociception, including the relevant receptors involved.
| Feature | A-δ (delta) | Type C |
|---|---|---|
| Diameter | Small | Smallest |
| Myelination | Thinly myelinated | Unmyelinated |
| Conduction | Slow (faster than C) | Slowest |
| Activation threshold | High | Highest |
| Stimuli | Mechanical + thermal (mostly cold) | Polymodal: thermal + mechanical + chemical (kinins, prostaglandins) — mostly heat + inflammatory soup |
| Pain quality | Fast, sharp, localized | Slow, deep ache, poorly localized |
| Share of nociceptors | — | ~75% (predominate) |
Contrast with sensory (non-nociceptive) neurons — context only: - Aα: large, thickly myelinated, ~80–120 m/s → proprioception - Aβ: large, myelinated, ~25–70 m/s → low-threshold mechanoreceptors / fine touch - Both fast-conducting, respond to low-threshold stimuli.
⚠️ Slide 9 (“Sensory Fibers” resource) was explicitly marked NOT on exam.
Transduction of chemical nociception depends on the innate immune response producing an inflammatory soup:
Innate response sequence: macrophages engulf debris + release cytokines → damaged tissue releases prostaglandins → ↑ vascular permeability + WBC margination → local signs of inflammation (rubor, calor, tumor, dolor) → cytokines & prostaglandins activate nociceptors.
Where are nociceptors located? Skin, muscle, joint capsules/ligaments/tendons, outer 1/3 of the annulus fibrosus, posterior longitudinal ligament (PLL), organ capsules, some organs, eyes.
Acute pain in a healthy nervous system: activation is roughly proportional to tissue damage (more damage → more prostaglandins/cytokines → stronger, more frequent APs → stronger signal). BUT perception is not this simple — severe injury can occur without pain.
| Fiber | Neurotransmitter released | 2nd-order receptor(s) |
|---|---|---|
| A-δ | Glutamate (predominant, excitatory) | AMPA and NMDA |
| Type C | Substance P + glutamate | NK1 (Sub P), plus AMPA/NMDA |
AMPA receptor: - Ligand-gated ionotropic receptor (forms an ion channel pore). - Glutamate binds → Na⁺ influx → depolarization.
NMDA receptor (HIGH YIELD — classic exam target): - Blocked by Mg²⁺ at rest. - Requires glutamate AND glycine to bind. - Nearby AMPA-driven depolarization dislodges the Mg²⁺ plug. - Then allows Ca²⁺ influx. - Implicated in long-term potentiation (LTP) → link to sensitization.
NK1 receptor (Substance P): - Sub P amplifies the effect of AMPA/NMDA. - ↑ Ca²⁺ into the neuron → cascade producing (1) noxious signal transmission and (2) LTP via gene transcription → production of more receptors.
AMPA vs. NMDA in one line: AMPA = fast, Na⁺, immediate depolarization (works at low input). NMDA = needs strong/sustained depolarization to lose its Mg²⁺ block, gates Ca²⁺, drives plasticity/LTP.
⚠️ Common distractor check: NMDA gates Ca²⁺ (not Na⁺); Mg²⁺ blocks NMDA at rest/minor stimulus; glycine is a co-agonist at NMDA (it does not block AMPA). The direct downstream effect of Ca²⁺ influx = 2nd-messenger activation + gene transcription (not immediate AMPA depolarization).
| Lateral (neospinothalamic) | Medial (paleospinothalamic + spinoreticular + spinomesencephalic) | |
|---|---|---|
| Primary input | A-δ | Type C |
| Speed / localization | Fast, well-localized | Slow, poorly localized, dull/aching |
| Neuron structure | 3rd-order neuron pathway | NOT a strict 3-neuron pathway (multi-neuronal, divergent) |
| Perception dimension | Sensory-discriminative | Affective-motivational + cognitive-evaluative |
Lateral tract detail: 2nd-order neurons carrying A-δ input cross the midline at the segmental level via the anterior white commissure, ascend in the anterolateral column, are somatotopically ordered (caudal = more lateral), synapse on 3rd-order neurons in the VPL (ventral posterolateral) nucleus of the thalamus → project to S1/S2, parietal cortex. Modulation note: sends collaterals to the PAG as it ascends.
Medial/divergent detail: slow, diffuse; projections throughout the brainstem; autonomic + affective. - Spinoreticular tract: → medial thalamus → pain matrix; not somatotopic (poor localization); interferes with sleep & attention; superior colliculus turns eyes/head toward the stimulus; projects to hypothalamus and cingulate/limbic. Modulation collaterals into reticular formation, PAG, parabrachial nuclei.
The ascending anterolateral pathway conveys nociception, temperature, and crude touch — lateral (fast/discriminative) + medial (slow/affective) divisions as above.
The brain is the meaning-maker. Pain = interpretation of sensory info about the person’s internal world. Three dimensions:
Regions commonly activated & their roles: - S1 / S2 somatosensory cortex — sensory-discriminative. - Cingulate cortex — anterior = affective component; integrates sensory input into cognitive processing → motor/pain behaviors. - Insula — activated by noxious stimuli and even by viewing/imagining painful situations. - Amygdala (limbic) — “smoke detector”; emotional-affective dimension + modulation; learned fear, anxiety, depression; projects to PFC, hippocampus, hypothalamus. - Hippocampus — forms memories of painful stimuli. (Exam distractor: memory of pain = hippocampus, NOT motor cortex/PFC.) - Prefrontal cortex (PFC) — higher functions; learns about/attaches negative affect to nociception; anticipates pain AND can control (modulate) pain. - Also: basal ganglia, posterior parietal cortex, hypothalamus.
Three top-level objectives: (1) Describe modulation; (2) Describe peripheral & central sensitization from chronic pain; (3) Differentiate mechanism-based pain types.
Modulation = altering the transmission of nociceptive input. Two directions: - Bottom-up = spinal gate control (peripheral Aα/Aβ input). - Top-down = descending modulation from the brainstem.
EPSP vs. IPSP: - EPSP — NT depolarizes postsynaptic membrane (excitatory). - IPSP — NT hyperpolarizes it (inhibitory). - An AP fires only when EPSPs > IPSPs and threshold is reached.
How inhibition happens at the 1st/2nd-order synapse (mechanism — HIGH YIELD): - Interneurons release GABA; descending tracts release enkephalins & endorphins. - Presynaptic (on nociceptor): binds → ↓ Ca²⁺ influx → less glutamate released. - Postsynaptic (2nd-order neuron): opens K⁺ channels → hyperpolarization. - Net result: harder to generate an AP (less glutamate + hyperpolarized 2nd-order neuron).
Descending pathways arise from supraspinal sites, project to the dorsal horn (synapse in lamina II), and are bidirectional with pain-matrix centers. Primary descending NTs at the cord = serotonin & norepinephrine.
| Source | Location | Neurotransmitter(s) | Notes |
|---|---|---|---|
| PAG (periaqueductal grey) | Midbrain | Endorphins/enkephalins (opioids) | Anti-nociceptive; receives collaterals from lateral spinothalamic tract + thalamus/hypothalamus/cortex; activates the NRM |
| RVM / NRM (nucleus raphe magnus) | Rostral ventromedial medulla | Serotonin | PAG → NRM → dorsal horn; serotonin stimulates inhibitory interneurons → blocks transmission; stimulation → powerful analgesia, ↓ hyperalgesia |
| Locus coeruleus (LC) | Pons | Norepinephrine | Diffuse projections (incl. dorsal horn); excited by spinothalamic input; mostly inhibitory (antinociceptive) but also facilitatory (pronociceptive) cell clusters |
Exam quick-hits: endogenous opioids in descending pathway → PAG; NT of LC → norepinephrine; region receiving spinothalamic input to initiate descending modulation → PAG; opioid receptors activated by PAG opioids → μ and κ.
The dorsal horn does not just relay — it integrates peripheral input, local interneuron activity, and descending brainstem input; it computes EPSP vs. IPSP balance; and it is where plasticity/sensitization (LTP) takes hold. It actively decides what ascends → functionally a decision-making hub, i.e., a “second brain” for pain.
Definition: “Increased responsiveness and reduced threshold of nociceptive neurons in the periphery to stimulation of their receptive fields” — ↑ sensitivity after injury/cell damage → primary hyperalgesia / primary allodynia.
Mechanisms: 1. At the injury site: mast-cell degranulation, inflammatory secretion, enzyme induction (COX-2) → high-threshold afferents become low-threshold; nociceptors start detecting innocuous input. Almost exclusively Type C fibers. 2. Upregulation of new receptors — 2nd-messenger system → gene transcription → more receptors on the neuron surface → sensitive to less inflammatory soup. 3. Neurogenic inflammation (via antidromic conduction / “axon reflex”) — release of substance P & CGRP from the peripheral nerve terminal → vasodilation + ↑ vascular permeability → recruits neutrophils/macrophages/lymphocytes, more mast-cell degranulation → perpetuates inflammation.
Orthodromic vs. antidromic: ortho = periphery→cell body→dorsal horn (normal). Anti = cell body→periphery (“axon reflex”) → releases Sub P & CGRP → neurogenic inflammation. Key idea: inflammation causes pain AND activated nociceptors can cause inflammation (bidirectional).
Clinical presentation of peripheral sensitization: - ↓ threshold to noxious stimuli → primary hyperalgesia/allodynia at the injury site - ↑ spontaneous activity - Slight increase in receptive field - Heat sensitivity — and heat sensitivity is present ONLY in peripheral sensitization (a clean discriminator vs. central).
Driven by the AMPA → NMDA mechanism under high, sustained input:
Result = hyperalgesia + heightened response to noxious AND tactile stimuli.
LTP = the nociplastic pain mechanism: persistent synaptic strengthening → “enhanced” transmission to the pain matrix → less input required to activate the pathway.
Clinical presentation of central sensitization: - ↑ spontaneous activity; ↓ activation threshold (noxious and innocuous) - Hyperalgesia + allodynia - Enlarged receptive fields (secondary hyperalgesia) - 2-point discrimination deficit
Clinical features (chronic pain profile): pain persists beyond expected healing time; disproportionate to injury; widespread, non-anatomical distribution; history of failed interventions; negative affect/poor self-efficacy/catastrophizing/fear-avoidance; unresponsive to NSAIDs/COX-2 but responsive to anti-epileptics, antidepressants, opioids; easily provoked/slow to settle; constant/unremitting.
| Peripheral | Central | |
|---|---|---|
| Site | Restricted to injury site | Non-inflamed tissue too |
| Marker | Primary hyperalgesia | Secondary hyperalgesia |
| Pathology present? | Yes (tissue damage/inflammation) | May be none |
| Pain elicited by | Nociceptor activation | Nociceptors + low-threshold Aβ mechanoreceptors |
| Dominant sensitivity | Thermal (heat) | Mechanical (Aβ-mediated pain) |
| Main fiber | Type C | 2nd-order + WDR neurons |
⚠️ Explicitly NOT on exam: Slide 61 (detailed WDR neuron convergence / wind-up) and Slide 66 (descending-pathway ON/OFF-RVM changes in chronic pain) were marked off the test. Know that WDR “wind-up” exists and helps explain secondary hyperalgesia/allodynia, but the detailed mechanism isn’t tested.
Classifying the mechanism drives (1) PT interventions and (2) pharmacologic management.
| Type | Key features |
|---|---|
| Nociceptive | Inflammation-mediated; proportional to injury; diffuse → focused |
| Nociplastic | Central mediation; not proportional to tissue damage; dorsal-horn + descending pathway changes; secondary hyperalgesia, expanding receptive fields, hyperalgesia/allodynia; psychosocial/pain-matrix involvement |
| Neuropathic | Follows a nerve/dermatome/brain-representation distribution |
| Mixed | Combination of the above |
Nociplastic = overarching umbrella term across diverse conditions that share the common mechanism of amplified nociceptive processing: e.g., fibromyalgia, osteoarthritis, RA, TMJ, IBS, chronic neck pain/whiplash, low back pain.
Notes generated from the course lecture decks. Flagged “NOT on exam” items reflect the instructor’s own slide annotations.