The progressive amplification of physiological responses to repeated or sustained stimuli through neuroplastic and immunological mechanisms that lower activation thresholds and enhance signal transmission. In stress systems, sensitisation manifests as escalating HPA axis and sympathetic nervous system reactivity; in pain systems, it involves peripheral and central mechanisms that reduce nociceptor firing thresholds and amplify ascending nociceptive signals while diminishing descending inhibitory control.
Imagine a home security system that learns from experience—but learns the wrong lesson. The first time a cat walks past, the motion sensor triggers at 50kg of movement. But after several false alarms where the homeowner panics, the system "learns" to be more vigilant: it recalibrates to trigger at 30kg, then 20kg, then 10kg. Eventually, even a mouse or a falling leaf sets off full sirens and floodlights. The system isn't broken—it's doing exactly what it was designed to do (protect the home), but it's become hypersensitive through repeated activation. Each alarm response reinforces the lowered threshold. Now imagine two homes: one where the security company reviews footage and says "just cats, reset to normal" (habituation), and another where every alarm triggers more sensitive settings (sensitisation). The sensitised home eventually responds to wind, shadows, and vibrations—stimuli that would never have triggered the original system. This is adaptive if burglars are actually increasing, but becomes a burden if they're not. The homeowner can't sleep, neighbours complain, and the system consumes excessive power. To reverse it, you can't just "fix" the sensors—you need to provide enough safety signals (review footage, install better locks, station a guard) that the system can gradually recalibrate its threat threshold downward.
Sensitisation operates through distinct but interconnected peripheral and central mechanisms:
At the tissue level, injury triggers release of inflammatory mediators creating an "inflammatory soup" (bradykinin, prostaglandins, IL-1β, TNF-α, NGF, ATP, H+, K+) → these molecules bind receptors on nociceptor terminals (TRPV1, TRPA1, purinergic receptors, bradykinin B2 receptors) → activation of intracellular signaling cascades (PKA, PKC, phospholipase C) → phosphorylation of TRP channels and voltage-gated sodium channels (Nav1.7, Nav1.8, Nav1.9) → reduced activation threshold for action potential generation (may drop from -55mV to -65mV) → increased spontaneous firing and enhanced responses to subsequent stimuli.
NGF plays a crucial role: NGF → binds TrkA Receptor on nociceptor terminals → retrograde transport to dorsal root ganglion → increased expression of substance P, CGRP, and TRPV1 receptors → phenotypic switch in sensory neurons → long-term hyperexcitability.
At the spinal dorsal horn, repeated C-fiber input triggers:
- Temporal summation (wind-up): repeated C-fiber stimulation at >0.3Hz → progressive increase in dorsal horn neuron response due to NMDA receptor activation → removal of Mg²⁺ block → Ca²⁺ influx → activation of PKC, calcium-calmodulin kinase II (CaMKII) → phosphorylation of AMPA receptors → insertion of additional AMPA receptors at synapses → enhanced synaptic efficacy
- Reduced inhibition: decreased GABAergic and glycinergic inhibition in dorsal horn → disinhibition of excitatory interneurons → expansion of receptive fields
- Microglial activation: release of BDNF, TNF-α, IL-1β → further enhancement of synaptic transmission
- Phenotypic switch: Aβ fibers (normally non-nociceptive) begin expressing substance P and form synapses in superficial dorsal horn → allodynia (light touch now activates pain pathways)
At higher centers:
In non-habituator phenotypes:
- Repeated stressors → progressive increase in HPA axis reactivity
- Enhanced CRH expression in paraventricular nucleus
- Increased ACTH response to same CRH stimulus
- Greater cortisol secretion with each stressor exposure
- Sympathetic nervous system sensitisation: enhanced noradrenaline release → upregulation of adrenergic receptors in target tissues → amplified cardiovascular and metabolic responses
- Amygdalar sensitisation: increased basolateral amygdala reactivity → enhanced threat detection → feed-forward amplification of stress responses
graph TD
A[Repeated Noxious Stimulus] --> B[Peripheral Tissue]
B --> C[Inflammatory Soup Release]
C --> D["IL-1β, TNF-α, NGF, PGE2, Bradykinin"]
D --> E[TRP Channel Phosphorylation]
D --> F[Nav Channel Sensitisation]
E --> G[Lowered Firing Threshold]
F --> G
G --> H[Increased C-fiber Input to Dorsal Horn]
H --> I[Temporal Summation]
H --> J[NMDA Receptor Activation]
J --> K["Ca²⁺ Influx"]
K --> L[PKC & CaMKII Activation]
L --> M[AMPA Receptor Phosphorylation]
M --> N[Enhanced Synaptic Transmission]
H --> O[Microglial Activation]
O --> P[BDNF & Cytokine Release]
P --> N
N --> Q[Reduced GABAergic Inhibition]
Q --> R[Dorsal Horn Hyperexcitability]
R --> S[Expanded Receptive Fields]
S --> T[Ascending to Thalamus]
T --> U[Reduced Thalamic Filtering]
U --> V[Enhanced Cortical Activation]
V --> W[ACC/Insula Amplification]
W --> X[Descending Facilitation from RVM]
X --> R
Y[Repeated Stress] --> Z[Amygdala Sensitisation]
Z --> AA[Enhanced CRH Expression]
AA --> AB[Increased HPA Reactivity]
AB --> AC[Progressive Cortisol Amplification]
AC --> AD[Glucocorticoid Effects on Hippocampus]
AD --> AE[Reduced Negative Feedback]
AE --> AB
Sensitisation represents a fundamental shift in clinical reasoning: the amplified response is not pathological malfunction but adaptive over-calibration for threat detection and tissue protection. This reframing has profound therapeutic implications.
Pain Conditions: Central sensitisation explains why chronic pain patients experience allodynia (cotton swab on skin triggers pain) and secondary hyperalgesia (pain spreads beyond injured tissue). The nervous system isn't "broken"—it has learned to protect more aggressively. In fibromyalgia, widespread pain results from generalised central sensitisation (MRI studies show altered pain matrix connectivity). In chronic low back pain, sensitisation explains why 85% show no structural pathology on imaging—the amplification is in the nervous system, not the tissue. Clinical threshold: Conditioned Pain Modulation scores <-10% indicate impaired descending inhibition and active central sensitisation.
Stress Phenotypes: The non-habituator phenotype shows progressive HPA axis and sympathetic nervous system sensitisation. Each stressor produces larger cortisol and catecholamine responses than the previous one. This contrasts with habituators who show progressively smaller responses. Non-habituators require different interventions: not just stress reduction, but active recalibration through safety signaling, predictability, and controllability. Morning cortisol >15 μg/dL in response to minor stressors suggests sensitised HPA reactivity.
Evolutionary Context: Sensitisation is adaptive during wound healing—the dolor of classical inflammation prevents further tissue damage by making movement painful. Post-mortem examination of wild primates shows extensive healed fractures and soft tissue injuries, yet these animals displayed normal function before death, suggesting successful desensitisation once healing completed. In modern humans, chronic low-grade inflammation and psychological stress maintain sensitisation long after adaptive utility ends—an evolutionary mismatch.
Intervention Implications:
- Top-down approaches: Pain neuroscience education (PNE) recalibrates threat perception by explaining pain biology → reduces anterior cingulate and insula activation
- Bottom-up approaches: Graduated exposure, movement, and sensory discrimination training → promotes neuroplasticity toward desensitisation
- Neuroimmune modulation: Omega-3 fatty acids (EPA >2g/day) shift toward specialized pro-resolving mediators → reduced microglial activation → decreased facilitation
- Stress system recalibration: Establishing safety, predictability, and control → reduces amygdalar drive → allows HPA axis habituation
- Sleep optimisation: REM sleep deprivation impairs descending pain modulation → maintaining 7-9 hours with adequate REM prevents sensitisation maintenance
Comorbidity Patterns: The shared mechanisms between stress and pain sensitisation explain high comorbidity: 60-80% of chronic pain patients meet criteria for anxiety disorders or depression. Both involve amygdalar hyperreactivity, reduced prefrontal inhibition, and altered monoaminergic signaling. Treatment must address both systems simultaneously.
Exam Alert: Distinguish sensitisation (progressively larger responses) from habituation (progressively smaller responses). Know that sensitisation involves reduced thresholds and enhanced transmission, whereas habituation involves synaptic depression and increased inhibition. Recognise that both are adaptive—context determines whether the adaptation remains appropriate.
- Sensitisation produces progressively amplified responses to repeated identical stimuli—opposite pattern from habituation
- Peripheral sensitisation can reduce nociceptor firing threshold from -55mV to -65mV through TRP and Nav channel phosphorylation
- Central sensitisation requires NMDA receptor activation and removal of Mg²⁺ block—temporal summation threshold is >0.3Hz C-fiber input
- Non-habituator phenotype shows stress system sensitisation with escalating cortisol and catecholamine responses to repeated stressors
- Inflammatory mediators (IL-1β, TNF-α, NGF, PGE2) directly sensitise TRPV1 and TRPA1 channels—the basis of sensoimmunology
- Wind-up in dorsal horn requires only 0.5-2 seconds of sustained C-fiber input to initiate—demonstrates rapid plasticity
- Microglial BDNF release enhances synaptic transmission by 200-300% in sensitised dorsal horn neurons
- Descending facilitation from RVM mediated by 5-HT3 receptors amplifies pain signals—opposite role of 5-HT in periphery
- Conditioned pain modulation (CPM) scores <-10% indicate impaired descending inhibition and active central sensitisation
- Post-mortem studies of wild primates show extensive healed trauma without observable pain behaviour—evidence that sensitisation normally reverses
- Allodynia results from phenotypic switch where Aβ fibres begin expressing substance P and synapsing in superficial dorsal horn (normally only C and Aδ)
- HPA axis sensitisation in non-habituators requires different treatment than habituators—safety signals and predictability vs exposure
- EPA >2g/day shifts lipid mediator balance toward resolvins and away from pro-inflammatory prostaglandins—reduces microglial contribution to sensitisation
- Sleep deprivation for even one night reduces descending pain modulation by 30-40%—maintaining REM sleep is protective
- habituation — opposite adaptation pattern involving progressive reduction of responses through synaptic depression and enhanced inhibition
- non-habituator — phenotype characterised by stress system sensitisation with escalating HPA and SNS responses to repeated stressors
- HPA axis — shows progressive sensitisation in non-habituators with enhanced CRH expression and ACTH responsiveness
- sympathetic nervous system — demonstrates sensitisation through enhanced noradrenaline release and receptor upregulation in stress-sensitised individuals
- chronic pain — maintained by peripheral and central sensitisation mechanisms that amplify nociceptive signals and reduce inhibition
- allodynia — results from central sensitisation lowering threshold so that Aβ fiber input triggers pain responses
- hyperalgesia — amplified pain response to noxious stimuli due to enhanced nociceptor sensitivity and spinal facilitation
- nociceptor — peripheral sensitisation lowers firing threshold through inflammatory mediator-induced phosphorylation of ion channels
- TRP channels — TRPV1 and TRPA1 sensitised by inflammatory cytokines through PKA and PKC-mediated phosphorylation
- wind-up — form of rapid sensitisation in dorsal horn requiring sustained C-fiber input and NMDA receptor activation
- dorsal horn — site of central sensitisation involving NMDA receptor activation, microglial BDNF release, and reduced inhibition
- thalamus — reduced filtering in sensitised pain system allows greater cortical activation from same peripheral input
- descending facilitation — serotonergic and noradrenergic pathways from rostroventral medulla amplify spinal nociceptive transmission in sensitisation
- descending pain modulation — impaired in sensitisation states, reducing endogenous opioid and monoaminergic inhibition
- periaqueductal gray — reduced descending inhibition from PAG-RVM pathway contributes to maintenance of sensitisation
- rostroventral medulla — switches from descending inhibition to facilitation in sensitised states through 5-HT3 receptor activation
- anterior cingulate cortex — shows enhanced activation in sensitisation, amplifying emotional-affective dimension of pain
- insula — increased activity in sensitisation amplifies interoceptive awareness and pain unpleasantness
- amygdala — sensitisation involves enhanced basolateral amygdala reactivity driving both stress and pain amplification
- neuroplasticity — sensitisation represents maladaptive plasticity with enhanced synaptic transmission and reduced inhibition
- inflammation — inflammatory mediators (IL-1β, TNF-α, NGF, PGE2) drive both peripheral and central sensitisation
- cytokines — pro-inflammatory cytokines directly sensitise nociceptors and enhance dorsal horn synaptic transmission
- BDNF — released by microglia in sensitised dorsal horn, enhances synaptic transmission and maintains hyperexcitability
- NGF — binds TrkA receptors causing phenotypic changes in nociceptors with increased substance P and TRPV1 expression
- substance P — upregulated in sensitised nociceptors, mediates enhanced neurotransmission in dorsal horn
- CGRP — increased expression in sensitised primary afferents contributes to enhanced nociceptive transmission
- NMDA receptor — activation required for central sensitisation through removal of Mg²⁺ block and Ca²⁺ influx
- microglia — activation in dorsal horn releases BDNF, IL-1β, and TNF-α that enhance synaptic transmission
- chronic stress — produces progressive sensitisation of HPA and SNS in non-habituator phenotypes
- cortisol — shows escalating secretion in stress-sensitised individuals despite repeated identical stressors
- CRH — enhanced expression in paraventricular nucleus drives HPA sensitisation
- noradrenaline — enhanced release and receptor upregulation in sympathetic sensitisation
- fibromyalgia — prototypical central sensitisation syndrome with widespread pain and allodynia
- pain neuroscience education — reduces sensitisation by recalibrating threat perception and reducing cortical pain matrix activation
- omega-3 — EPA >2g/day shifts toward specialized pro-resolving mediators, reducing microglial contribution to sensitisation
- specialized pro-resolving mediators — resolvins and protectins reduce microglial activation and promote resolution of sensitisation
- sleep — REM sleep deprivation impairs descending modulation and maintains sensitisation
- anxiety disorders — 60-80% comorbidity with chronic pain due to shared amygdalar and monoaminergic sensitisation mechanisms
- sensoimmunology — inflammatory cytokines sensitise TRP receptors, linking immune activation to sensory amplification
- evolutionary mismatch — chronic sensitisation maintained by modern stressors exceeds adaptive utility designed for acute threat
- Module 3 (Neuroendocrinology: stress system sensitisation, non-habituator phenotype)
- Module 5 (Pain: peripheral and central sensitisation mechanisms, allodynia, hyperalgesia)