Voltage-gated sodium channels (Nav channels) are transmembrane proteins containing four homologous domains (I-IV), each with six α-helical segments (S1-S6), that open in response to membrane depolarization. When threshold potential (~-55mV) is reached, these channels undergo rapid conformational change allowing selective Na⁺ influx, generating the upstroke of the action potential. Nine subtypes (Nav1.1-1.9) exist with tissue-specific expression, kinetics, and pharmacological properties—Nav1.7, Nav1.8, and Nav1.9 in nociceptors are critical for pain signaling.
Think of sodium channels as spring-loaded trapdoors in the floor of a nightclub, with bouncers (voltage sensors) monitoring the crowd's energy level. At rest, all trapdoors are locked shut. When the energy in the room (membrane voltage) rises to a critical threshold, the bouncers suddenly release all the trapdoors simultaneously—Na⁺ ions rush up through the floor like eager fans storming the stage, instantly electrifying the space (depolarization). But here's the key: each trapdoor has a secondary safety mechanism—within 1-2 milliseconds, an automatic lock clicks into place (inactivation gate swings shut), preventing more ions from entering even though the trapdoor itself is technically still "open." The club can't get more excited; it's maxed out. Only after the crowd calms down (repolarization completes) do the trapdoors reset to their closed-but-ready state. Different nightclub venues (neuron types) have trapdoors with different sensitivities: some open easily with just a whisper of excitement (hyperexcitable nociceptors with sensitized Nav1.7), while others need a massive energy spike to trigger (healthy sensory neurons with normal thresholds).
¶ Channel Structure and Gating
Voltage-gated sodium channels are large glycoproteins (~260 kDa α-subunit) with four homologous domains (I-IV), each containing six transmembrane segments (S1-S6). The S4 segments contain positively charged arginine/lysine residues every third position, functioning as voltage sensors. The pore-forming region lies between S5-S6 segments, with a selectivity filter (DEKA motif: Asp-Glu-Lys-Ala) that discriminates Na⁺ (0.95 Å radius) from other cations.
Resting State (-70mV):
- Activation gate closed
- Inactivation gate (hinged lid) open
- Na⁺ gradient: ~140 mM extracellular, ~12 mM intracellular
- Electrochemical driving force: ~90 mV
Activation (threshold reached ~-55mV):
- Voltage sensors (S4 helices) detect depolarization
- Outward S4 movement triggers conformational change
- Activation gate opens within 0.1-0.5 ms
- Na⁺ influx: 10⁷ ions/second/channel
- Membrane rapidly depolarizes to +40 mV (sodium equilibrium potential)
- Peak current density: 50-200 pA/pF
Inactivation (within 1-2 ms):
- Cytoplasmic inactivation gate (IFM motif: Ile-Phe-Met in domain III-IV linker) swings into pore
- Channel enters non-conducting state despite continued depolarization
- This "ball-and-chain" mechanism is voltage- and time-dependent
- Absolute refractory period begins
Deactivation (return to resting):
- Requires repolarization below -70 mV
- Inactivation gate dissociates
- Activation gate closes
- Channel returns to closed-resting state
- Recovery from inactivation: 2-10 ms (voltage-dependent)
graph TD
A["Resting State<br/>-70mV<br/>Activation gate closed<br/>Inactivation gate open"] -->|Depolarization to -55mV| B["Open State<br/>Na+ influx<br/>107 ions/s/channel"]
B -->|1-2 ms| C["Inactivated State<br/>Ball-and-chain blocks pore<br/>Absolute refractory period"]
C -->|Repolarization below -70mV| A
B -->|"Peak +40mV"| D["Membrane at ENa<br/>Na+ driving force = 0"]
C -.->|"Cannot reopen<br/>during depolarization"| E["Refractory Period<br/>Prevents backward propagation"]
Nav1.7 (SCN9A gene):
- Threshold: -40 mV (relatively hyperpolarized)
- Slow inactivation: hours to days
- "Threshold channel" for nociceptor excitability
- Activation: τ = 0.5 ms; Inactivation: τ = 1.5 ms
- TTX-sensitive (IC50 = 10 nM)
Nav1.8 (SCN10A gene):
- Threshold: -20 mV (more depolarized)
- Fast repriming kinetics (3 ms at -70 mV)
- Produces 80-90% of current during nociceptor firing
- TTX-resistant (IC50 > 100 μM)
- Critical for action potential upstroke amplitude
Nav1.9 (SCN11A gene):
- Threshold: -60 mV (most hyperpolarized)
- Ultra-slow inactivation (τ = 100-200 ms)
- Persistent "window current" near resting potential
- Sets resting membrane potential in nociceptors
- TTX-resistant
Inflammatory mediators alter channel kinetics:
PGE2 → EP receptor → PKA activation:
- Phosphorylates Nav1.8 at serine residues
- Shifts activation curve 10 mV hyperpolarized
- Increases peak current amplitude 200-300%
NGF → TrkA receptor → MAPK pathway:
- Increases Nav1.8/1.9 transcription (2-4 fold within 6 hours)
- Reduces inactivation kinetics
- Results in hyperexcitability (threshold drops from -55 to -65 mV)
TNF-α → TNFR1 → p38 MAPK:
- Reduces Nav1.7 slow inactivation
- Enhances resurgent currents
- Produces spontaneous activity
Gain-of-Function (hyperexcitability):
- Erythromelalgia (Nav1.7 mutations): Channels activate at more hyperpolarized potentials (-50 mV instead of -40 mV), causing spontaneous firing. Patients experience severe burning pain in extremities triggered by warmth. Clinical threshold: attacks occur when skin temperature exceeds 32°C.
- Paroxysmal extreme pain disorder (Nav1.7): Impaired inactivation causes prolonged depolarization, affecting trigeminal/autonomic neurons
- Small fiber neuropathy (Nav1.7, Nav1.8): Multiple gain-of-function mutations cause spontaneous nociceptor firing
Loss-of-Function (pain insensitivity):
- Congenital insensitivity to pain (Nav1.7 nonsense mutations): Complete absence of nociception; patients sustain repeated tissue damage; anosmia also present (olfactory neurons also express Nav1.7)
After tissue injury or inflammation, the threshold for action potential generation drops from ~-55 mV to ~-65 mV due to:
- Increased Nav channel density at nerve terminals (2-3 fold increase within 24 hours)
- Hyperpolarized activation curves (10-15 mV shift)
- Ectopic channel expression in axons/cell bodies (normally only at terminals/nodes)
- Formation of "hot spots" with abnormally high channel clusters
This connects to the Metamodel 5 (Pain) framework: peripheral sensitization creates an amplified input signal that drives central sensitization, explaining why localized inflammation produces widespread pain hypersensitivity.
Local Anesthetics:
- Lidocaine blocks sodium channels by entering the pore from inside (use-dependent block)
- Higher affinity for open/inactivated states (10-100x)
- Preferentially blocks high-frequency firing (pain signals) over low-frequency (normal sensation)
- Clinical concentration for nerve block: 1-2% solution (30-60 mM)
Anticonvulsants:
- Carbamazepine: Stabilizes inactivated state; IC50 = 200 μM; used for trigeminal neuralgia
- Lamotrigine: Similar mechanism; used for neuropathic pain
- Both produce frequency-dependent block (protect against ectopic firing without blocking normal signaling)
Evolutionary Mismatch Context:
The modern epidemic of chronic pain partly reflects sodium channel dysregulation in response to inflammatory signals that would have been acute/transient in ancestral environments. Persistent low-grade inflammation (metaflammation) from sedentary behavior, processed foods, and chronic stress creates sustained Nav channel sensitization—a system designed for brief tissue injury responses now runs continuously.
- Intraepidermal nerve fiber density <5 fibers/mm indicates small fiber neuropathy (often Nav-mediated)
- Quantitative sensory testing showing hyperalgesia to pinprick = peripheral sensitization (Nav1.7/1.8-dependent)
- Thermal thresholds <40°C for heat pain = Nav channel sensitization
- Nav channels contain ~2,000 amino acids forming four domains with 24 transmembrane segments total
- Approximately 1,000,000 sodium channels per motor neuron; 100-1,000 per nociceptor terminal
- Channel density at nodes of Ranvier: 1,000-2,000 channels/μm² (50x higher than internodal regions)
- Single-channel conductance: 15-20 pS (picosiemens)
- Mean open time: 0.7 ms; mean closed time (resting): infinite until depolarized
- Na⁺ selectivity: 12:1 over K⁺; 20:1 over Ca²⁺ (achieved by DEKA selectivity filter)
- Threshold for activation varies by subtype: Nav1.9 at -60 mV, Nav1.7 at -40 mV, Nav1.8 at -20 mV
- Action potential upstroke velocity directly proportional to Nav current density: 200-500 V/s in myelinated neurons
- Tetrodotoxin (TTX) blocks Nav1.1-1.7 with IC50 = 5-10 nM but not Nav1.8/1.9 (IC50 > 100 μM)
- Nav1.7 haploinsufficiency (one working copy) raises pain threshold 2-3 fold without eliminating pain
- Inflammatory mediators can reduce activation threshold by 10-15 mV within 30 minutes
- Nav1.8 produces 80-90% of the inward current during sustained high-frequency firing in nociceptors
- Recovery from inactivation accelerates with hyperpolarization: 10 ms at -70 mV vs 100 ms at -60 mV
- Mutations in SCN9A (Nav1.7 gene) account for ~30% of inherited pain disorders
- Action potential — Sodium channel opening creates the rapid upstroke (depolarization phase), determining amplitude and velocity of propagation
- Potassium channels — Open after sodium channels to repolarize membrane; balance determines action potential duration and refractory period
- Resting membrane potential — Sodium channels remain closed at rest (-70 mV); threshold must be reached (-55 mV) to trigger opening
- Depolarization — Na⁺ influx through channels causes membrane depolarization from -70 to +40 mV within 1 ms
- Repolarization — Sodium channel inactivation (plus potassium channel opening) enables membrane to return to negative potential
- Threshold — Voltage at which enough sodium channels open to generate regenerative depolarization (~-55 mV in most neurons)
- Subthreshold stimuli — Depolarizations below -55 mV activate insufficient sodium channels to reach threshold; they decay passively
- Nociception — Nav1.7, Nav1.8, Nav1.9 generate action potentials in nociceptors; their sensitization underlies pain hypersensitivity
- Nociceptors — Express specific sodium channel subtypes: Nav1.7 (threshold), Nav1.8 (upstroke), Nav1.9 (resting potential modulation)
- Hyperalgesia — Inflammatory mediators (PGE2, NGF, TNF-α) lower sodium channel activation threshold by 10-15 mV, causing heightened pain sensitivity
- Peripheral sensitization — Increased Nav channel density and altered gating kinetics amplify nociceptor excitability after tissue injury
- Local anesthetics — Lidocaine and bupivacaine block sodium channels by entering open pore, preventing action potential propagation
- Carbamazepine — Anticonvulsant that stabilizes sodium channel inactivated state; used for trigeminal neuralgia and neuropathic pain
- Inflammation — PGE2, bradykinin, NGF, and protons all modulate sodium channel properties to increase nociceptor excitability
- Nerve injury — Triggers ectopic sodium channel expression in axons and DRG cell bodies, causing spontaneous pain signals
- Nodes of Ranvier — High-density clusters (1,000-2,000/μm²) of Nav1.6 enable saltatory conduction in myelinated axons
- Myelin — Insulation that restricts sodium channels to nodes, increasing conduction velocity 10-50 fold via saltatory propagation
- Chronic pain — Sustained Nav channel dysregulation (altered expression, phosphorylation, trafficking) contributes to persistent hyperexcitability
- A-delta fibres — Myelinated nociceptors relying on nodal Nav1.6 clusters for fast (5-30 m/s) sharp pain transmission
- C tactile fibres — Unmyelinated afferents expressing Nav1.8/1.9 for slow (0.5-2 m/s) pleasant touch signaling
- Allodynia — Nav channel sensitization causes normally innocuous stimuli to activate nociceptors, producing pain from light touch
- CGRP — Released by Nav-mediated action potentials in nociceptor terminals; triggers neurogenic inflammation and sensitization
- NGF — Nerve Growth Factor increases Nav1.8/1.9 transcription via TrkA → MAPK pathway, enhancing nociceptor excitability
- Dorsal root ganglion — Cell bodies of sensory neurons where Nav channels are synthesized, then transported to peripheral terminals
- TNF-α — Pro-inflammatory cytokine that reduces Nav1.7 slow inactivation via p38 MAPK, increasing spontaneous firing
- ATP — Required for sodium-potassium pump that maintains Na⁺ gradient; pump failure causes depolarization and Nav channel inactivation
- Inflammatory phase — Acute response where Nav channel sensitization serves adaptive function (protective pain), but chronicity becomes maladaptive
- Resolution of inflammation — SPMs (resolvins, maresins) reverse Nav channel sensitization by reducing phosphorylation and normalizing kinetics
- Small fiber neuropathy — Condition caused by Nav1.7/1.8 mutations or acquired Nav dysfunction; diagnosed by reduced intraepidermal nerve fiber density
- Metabolic syndrome — Associated with Nav channel dysfunction in peripheral nerves due to AGEs, oxidative stress, and chronic inflammation