Nerves & Neuromuscular Junction
Nerves & Neuromuscular Junction
What Do You Need to Know?
- Nerve structure — endoneurium, perineurium, epineurium; Schwann cells (PNS) vs oligodendrocytes (CNS)
- Fiber classification — Erlanger-Gasser (Aα through C) and Lloyd-Hunt systems; velocity ≈ 6 × diameter
- Action potential — resting membrane potential, Na⁺/K⁺ channels, refractory periods, saltatory conduction
- Synaptic transmission — EPSPs vs IPSPs, temporal vs spatial summation
- NMJ anatomy & physiology — P/Q-type Ca²⁺ channels, SNARE proteins, safety factor
- NMJ disorders — MG (postsynaptic, decrement), LEMS (presynaptic, increment), Botulism (SNARE cleavage)
- Nerve injury — Seddon vs Sunderland classification, Wallerian degeneration timeline
- Channelopathies — hyperkalemic PP (Na⁺ channel, myotonia), hypokalemic PP (Ca²⁺ channel, no myotonia)
Nerve Structure
Neuron Components
- Soma (cell body) — contains nucleus, Nissl substance (rough ER), site of protein synthesis
- Axon — single process for impulse conduction; axon hillock has lowest threshold for AP generation
- Dendrites — multiple branching processes; receive synaptic input
- Axonal transport:
- Anterograde (soma → terminal): kinesin; fast (200–400 mm/day) for vesicles, slow (1–5 mm/day) for cytoskeletal proteins
- Retrograde (terminal → soma): dynein; carries growth factors, viruses (rabies, herpes)
Peripheral Nerve Connective Tissue Layers
| Layer | Surrounds | Clinical Significance |
|---|---|---|
| Endoneurium | Individual nerve fibers | Must be intact for accurate regeneration; contributes to blood-nerve barrier |
| Perineurium | Fascicles (fiber bundles) | Main component of blood-nerve barrier; provides tensile strength |
| Epineurium | Entire nerve trunk | Contains vasa nervorum; target of surgical repair |
Myelinating Cells
| Feature | PNS — Schwann Cells | CNS — Oligodendrocytes |
|---|---|---|
| Cell-to-axon ratio | 1 Schwann cell : 1 internode | 1 oligodendrocyte : up to 50 internodes |
| Regeneration support | Good — forms bands of Büngner | Poor — inhibitory environment (Nogo, MAG) |
| Diseases | GBS, CIDP, CMT | MS, leukodystrophies |
Board Pearl
The perineurium is the primary barrier of the blood-nerve barrier. It is the structure that must be breached in perineuritis (e.g., leprosy). Schwann cells myelinate one internode each; oligodendrocytes myelinate up to 50 — explaining why CNS demyelination is more devastating.
Nerve Fiber Classification
Erlanger-Gasser Classification
| Fiber Type | Diameter (μm) | Velocity (m/s) | Myelination | Function |
|---|---|---|---|---|
| Aα | 12–20 | 70–120 | Heavy | Motor to skeletal muscle; proprioception (Ia, Ib) |
| Aβ | 5–12 | 30–70 | Heavy | Touch, pressure (type II afferents) |
| Aγ | 3–6 | 15–30 | Medium | Motor to muscle spindle (intrafusal fibers) |
| Aδ | 2–5 | 12–30 | Light | Fast pain, temperature, crude touch (type III) |
| B | 1–3 | 3–15 | Light | Preganglionic autonomic |
| C | 0.5–1.5 | 0.5–2 | Unmyelinated | Slow pain, temperature, postganglionic autonomic (type IV) |
Lloyd-Hunt Classification (Sensory Only)
| Lloyd-Hunt | Erlanger-Gasser Equivalent | Function |
|---|---|---|
| Ia | Aα | Muscle spindle primary endings (stretch) |
| Ib | Aα | Golgi tendon organs (tension) |
| II | Aβ | Muscle spindle secondary endings; touch, pressure |
| III | Aδ | Deep pressure, pain |
| IV | C | Slow pain, temperature |
Key Rule: Velocity ≈ 6 × Diameter
- Large myelinated fibers (Aα) → affected first by compression and ischemia
- Small unmyelinated fibers (C) → affected first by metabolic/toxic neuropathies (e.g., diabetes)
- Local anesthetics → block small fibers first (pain before motor)
Board Pearl
Conduction velocity ≈ 6 × fiber diameter. Compression/ischemia affects large fibers first (proprioception, motor). Metabolic/toxic neuropathies affect small fibers first (pain, temperature, autonomic). This distinction explains why diabetic neuropathy presents with burning pain while carpal tunnel presents with numbness.
Action Potential
Resting Membrane Potential
- Value: approximately −70 mV (range −60 to −90 mV depending on cell type)
- Maintained by: Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in) and K⁺ leak channels
- Nernst equation: determines equilibrium potential for each ion (EK ≈ −90 mV, ENa ≈ +60 mV)
- Goldman equation: accounts for permeability of all ions; at rest, membrane is most permeable to K⁺
Phases of the Action Potential
| Phase | Ion Channel Activity | Membrane Potential |
|---|---|---|
| Resting | K⁺ leak channels open; voltage-gated channels closed | −70 mV |
| Depolarization | Voltage-gated Na⁺ channels open (activation gate) | Rises toward +30 mV |
| Repolarization | Na⁺ channels inactivate (h-gate); voltage-gated K⁺ channels open | Falls back toward −70 mV |
| Hyperpolarization | K⁺ channels remain briefly open | Transiently below −70 mV (undershoot) |
Refractory Periods
| Period | Mechanism | Clinical Relevance |
|---|---|---|
| Absolute refractory | Na⁺ channels inactivated (h-gate closed); no stimulus can trigger AP | Ensures unidirectional propagation; limits maximum firing rate |
| Relative refractory | Some Na⁺ channels recovered; K⁺ channels still open; suprathreshold stimulus needed | Can fire but requires stronger stimulus; underlies frequency coding |
Saltatory Conduction & Velocity Factors
- Saltatory conduction: AP jumps node to node (nodes of Ranvier) in myelinated fibers → greatly increases speed
- Nodes of Ranvier: high density of voltage-gated Na⁺ channels
- Paranodal region: K⁺ channels normally covered by myelin; exposed in demyelination → K⁺ leak → conduction failure
- Factors increasing velocity: increased myelination, larger axon diameter, higher temperature
- Factors decreasing velocity: demyelination, cooling (hypothermia), smaller fiber size
Clinical Pearl
Demyelination exposes paranodal K⁺ channels → hyperpolarization → conduction block. This is why 4-aminopyridine (K⁺ channel blocker) can temporarily improve symptoms in MS by prolonging the AP at demyelinated segments.
Synaptic Transmission
Presynaptic & Postsynaptic Components
- Presynaptic terminal: contains synaptic vesicles, mitochondria, voltage-gated Ca²⁺ channels
- Synaptic cleft: 20–40 nm; contains degradative enzymes and extracellular matrix
- Postsynaptic membrane: neurotransmitter receptors (ionotropic and metabotropic), scaffolding proteins
EPSPs vs IPSPs
| Feature | EPSP | IPSP |
|---|---|---|
| Effect | Depolarization (toward threshold) | Hyperpolarization (away from threshold) |
| Ions | Na⁺ influx (or Ca²⁺) | Cl⁻ influx or K⁺ efflux |
| Neurotransmitters | Glutamate, acetylcholine | GABA, glycine |
| Graded? | Yes — not all-or-none | Yes — not all-or-none |
Summation
- Temporal summation: rapid, repeated firing of a single presynaptic neuron → cumulative EPSPs
- Spatial summation: simultaneous input from multiple presynaptic neurons → combined EPSPs at axon hillock
- If summed EPSPs reach threshold at the axon hillock → AP is generated
Neuromuscular Junction
NMJ Anatomy
- Presynaptic terminal: ACh-containing vesicles, P/Q-type voltage-gated Ca²⁺ channels, SNARE complex
- Synaptic cleft: 200–500 Å (20–50 nm); contains acetylcholinesterase (AChE) anchored to basal lamina
- Motor end plate (postsynaptic): junctional folds with nicotinic AChR concentrated at crests; Na⁺ channels at depths of folds
Steps of Normal Transmission
- AP arrives at presynaptic nerve terminal
- P/Q-type voltage-gated Ca²⁺ channels open → Ca²⁺ influx
- Ca²⁺ triggers SNARE-mediated vesicle fusion with presynaptic membrane
- ACh released into synaptic cleft (quantal release — each vesicle = 1 quantum ≈ 5,000–10,000 ACh molecules)
- ACh binds nicotinic AChR (2 ACh molecules per receptor needed)
- Na⁺ influx through receptor → end-plate potential (EPP)
- EPP exceeds threshold → muscle fiber AP → contraction
- AChE rapidly hydrolyzes ACh → choline recycled into nerve terminal
SNARE Proteins
| SNARE Protein | Location | Targeted By |
|---|---|---|
| Synaptobrevin (VAMP) | Vesicle membrane (v-SNARE) | Tetanus toxin; Botulinum toxin B, D, F, G |
| SNAP-25 | Presynaptic membrane (t-SNARE) | Botulinum toxin A, C, E |
| Syntaxin | Presynaptic membrane (t-SNARE) | Botulinum toxin C |
Safety Factor
- Definition: EPP amplitude is normally 3–4× greater than threshold needed for muscle AP
- Ensures reliable transmission even with moderate receptor loss or reduced release
- In MG: reduced AChR → decreased safety factor → transmission failure with repetitive use (fatigable weakness)
- In LEMS: reduced ACh release initially, but Ca²⁺ accumulates with repetitive stimulation → facilitation
Board Pearl
Botulinum toxin type A cleaves SNAP-25; tetanus toxin cleaves synaptobrevin (VAMP). Both block vesicle fusion. Botulism causes flaccid paralysis (blocks release at NMJ). Tetanus causes spastic paralysis (blocks inhibitory interneuron release in the spinal cord — the toxin travels retrograde).
NMJ Disorders
Major NMJ Disorders — Comparison
| Feature | Myasthenia Gravis | Lambert-Eaton | Botulism |
|---|---|---|---|
| Site | Postsynaptic | Presynaptic | Presynaptic |
| Target | Nicotinic AChR | P/Q-type VGCC | SNARE proteins |
| Antibodies | AChR (85%), MuSK (5–8%), LRP4 | VGCC (P/Q-type) | None (toxin-mediated) |
| Weakness pattern | Ocular → bulbar → proximal limbs; fatigable | Proximal legs > arms; improves transiently with use | Descending: cranial nerves → limbs → respiratory |
| Reflexes | Normal | Reduced/absent (improve post-exercise) | Reduced/absent |
| Autonomic | Spared | Prominent (dry mouth, constipation, impotence) | Prominent (dilated pupils, dry mouth, ileus) |
| RNS (2–3 Hz) | Decrement >10% | Low baseline CMAP; may decrement | Low baseline CMAP; may decrement |
| Post-exercise / high-freq RNS | Brief repair of decrement | Increment >100% | Small increment (20–40%) |
| Association | Thymoma (10–15%), thymic hyperplasia | Small cell lung cancer (50–60%) | Contaminated food, wounds, infant honey |
Myasthenia Gravis — Key Details
- AChR antibodies (85%): complement-mediated destruction of postsynaptic membrane
- MuSK antibodies (5–8%): more bulbar involvement, muscle atrophy, poor response to pyridostigmine
- LRP4 antibodies: rare, milder phenotype
- Drugs that worsen MG: aminoglycosides, fluoroquinolones, beta-blockers, magnesium, D-penicillamine
- Myasthenic crisis: FVC <15–20 mL/kg → intubate; treat with IVIG or plasmapheresis
Congenital Myasthenic Syndromes
- Genetic (not autoimmune) — no antibodies; no response to immunotherapy
- Presynaptic: ChAT deficiency (choline acetyltransferase)
- Synaptic: AChE deficiency (endplate acetylcholinesterase)
- Postsynaptic: AChR subunit mutations (most common — slow-channel and fast-channel syndromes)
- Treatment varies by subtype: pyridostigmine for most; fluoxetine or quinidine for slow-channel CMS
Board Pearl
Do not give pyridostigmine to patients with AChE deficiency CMS or slow-channel CMS — it will worsen symptoms. Do not give immunotherapy for congenital myasthenic syndromes (they are not autoimmune).
Board Pearl
MG fatigues (gets worse with use); LEMS facilitates (gets better with use). Both cause proximal weakness. LEMS has prominent autonomic symptoms; MG does not. Always screen LEMS patients for small cell lung cancer. An increment >100% on post-exercise RNS is the hallmark of LEMS.
Nerve Injury Classification
Seddon vs Sunderland Classification
| Seddon | Sunderland Grade | Structure Injured | Pathology | Recovery |
|---|---|---|---|---|
| Neurapraxia | I | Myelin only | Local demyelination; axon intact | Complete; weeks to 3 months |
| Axonotmesis | II | Axon (endoneurium intact) | Wallerian degeneration distally | Good; 1 mm/day |
| III | Axon + endoneurium | Wallerian degeneration; intrafascicular scarring | Variable; misdirected regeneration | |
| IV | Axon + endoneurium + perineurium | Only epineurium intact | Poor without surgery | |
| Neurotmesis | V | Complete nerve transection | Total disruption | None without surgical repair |
Wallerian Degeneration Timeline
- 0–48 hours: axon and myelin begin to degenerate distal to injury
- 3–5 days: Schwann cells proliferate, macrophages infiltrate to phagocytose debris
- 7–10 days: Wallerian degeneration complete; NCS shows absent/reduced distal CMAP and SNAP
- 2–3 weeks: fibrillations and positive sharp waves appear in proximal muscles on EMG
- 4–5 weeks: fibrillations appear in distal muscles
- Schwann cells form bands of Büngner to guide axonal regrowth
Recovery Patterns
- Regeneration rate: ∼1 mm/day (∼1 inch/month)
- Neurapraxia: full recovery in weeks to 3 months; no fibrillations on EMG
- Axonotmesis: recovery depends on distance from target; motor end plates degenerate by 12–18 months — sets time limit
- Signs of reinnervation: advancing Tinel sign, nascent (small, polyphasic) motor unit potentials on EMG
- Younger age, proximal injury, short distance to target → better prognosis
Board Pearl
Neurapraxia = conduction block without Wallerian degeneration. No fibrillations on EMG, full recovery expected. Axonotmesis shows fibrillations at 2–5 weeks. Key timing: NCS changes at 7–10 days; proximal fibs at 2–3 weeks; distal fibs at 4–5 weeks. Regeneration rate is 1 mm/day.
Channelopathies
Sodium Channelopathies (SCN4A — Nav1.4)
| Disorder | Mechanism | Clinical Features | Triggers |
|---|---|---|---|
| Hyperkalemic Periodic Paralysis | Gain of function → prolonged Na⁺ channel opening → persistent depolarization | Episodic weakness (minutes to hours); myotonia common | High K⁺, fasting, rest after exercise, cold |
| Paramyotonia Congenita | Impaired fast inactivation | Paradoxical myotonia (worsens with activity); cold-induced stiffness then weakness | Cold, exercise |
| Sodium Channel Myotonia | Delayed Na⁺ channel inactivation | Myotonia without paralysis; K⁺-aggravated | K⁺ loading |
Calcium Channelopathies
| Disorder | Gene / Channel | Mechanism | Clinical Features |
|---|---|---|---|
| Hypokalemic Periodic Paralysis | CACNA1S (Cav1.1) — 70%; SCN4A — 10% | Loss of function → reduced excitability during low K⁺ | Episodic weakness (hours to days); NO myotonia; carbs and rest trigger attacks |
| Absence Epilepsy (Childhood) | CACNA1A / CACNA1H (T-type Ca²⁺ channels) | Abnormal thalamocortical oscillation via low-threshold T-type Ca²⁺ channels | Staring spells with 3 Hz spike-and-wave on EEG; treated with ethosuximide (blocks T-type channels) |
| Malignant Hyperthermia | RYR1 (ryanodine receptor) | Uncontrolled Ca²⁺ release from sarcoplasmic reticulum | Triggered by volatile anesthetics/succinylcholine; rigidity, hyperthermia, rhabdomyolysis; treat with dantrolene |
Potassium Channelopathies
- Episodic Ataxia Type 1 (EA1): KCNA1 (Kv1.1) mutations → brief episodes of ataxia with myokymia; responds to carbamazepine
- Benign Familial Neonatal Seizures: KCNQ2/KCNQ3 → reduced M-current → seizures in first week of life; self-limited
- Andersen-Tawil Syndrome: KCNJ2 (Kir2.1) → periodic paralysis + cardiac arrhythmias (prolonged QT) + dysmorphic features
Periodic Paralysis Comparison
| Feature | Hypokalemic PP | Hyperkalemic PP |
|---|---|---|
| Channel | CACNA1S (Ca²⁺) or SCN4A (Na⁺) | SCN4A (Na⁺) |
| K⁺ during attack | Low (<3.5 mEq/L) | High or normal |
| Triggers | Carbs, rest after exercise, insulin, stress | Fasting, rest after exercise, cold, K⁺ load |
| Myotonia | Absent | Often present (clinical or EMG) |
| Attack duration | Hours to days | Minutes to hours (shorter) |
| Acute treatment | K⁺ replacement | Carbohydrate load, inhaled β-agonist, calcium gluconate |
| Prophylaxis | Acetazolamide, K⁺-sparing diuretics | Acetazolamide or dichlorphenamide |
Clinical Pearl
Thyrotoxic periodic paralysis mimics hypokalemic PP but is acquired (not inherited). It is most common in Asian males. Treat the hyperthyroidism; avoid high-dose IV K⁺ (risk of rebound hyperkalemia as K⁺ re-enters cells).
Board Pearl
HypoKPP = no myotonia; HyperKPP = myotonia common. Both worsen with rest after exercise. Acetazolamide (carbonic anhydrase inhibitor) is prophylactic for both types. Absence seizures involve T-type Ca²⁺ channels — treat with ethosuximide. Malignant hyperthermia = RYR1 mutation; treat with dantrolene immediately.
Quick Reference
High-Yield Summary Table
| Topic | Key Fact | Board Buzzword |
|---|---|---|
| Nerve layers | Endo → Peri → Epi (inside out) | Perineurium = blood-nerve barrier |
| Myelination | Schwann cell = 1:1; oligodendrocyte = 1:50 | Bands of Büngner (regeneration) |
| Fiber velocity | Velocity ≈ 6 × diameter | Aα fastest; C slowest |
| Resting potential | −70 mV; maintained by Na⁺/K⁺-ATPase | K⁺ leak channels set resting potential |
| Saltatory conduction | AP jumps between nodes of Ranvier | Paranodal K⁺ exposed in demyelination |
| NMJ transmission | P/Q Ca²⁺ channels → SNARE fusion → ACh release | Safety factor = 3–4× threshold |
| MG | Postsynaptic AChR Ab; fatigable; decrement on RNS | Thymoma, ptosis, bulbar weakness |
| LEMS | Presynaptic VGCC Ab; facilitates; increment >100% | SCLC, dry mouth, proximal legs |
| Botulism | SNAP-25 cleavage; descending paralysis | Dilated pupils, autonomic, infant honey |
| Neurapraxia | Demyelination only; conduction block; no fibs | Full recovery, Sunderland I |
| Axonotmesis | Axon loss; Wallerian degeneration; fibs at 2–5 wk | 1 mm/day regeneration |
| HyperKPP | SCN4A gain of function; myotonia + weakness | Fasting, cold, K⁺ triggers |
| HypoKPP | CACNA1S; NO myotonia; carbs trigger | Hours-long attacks; K⁺ replacement |
| Absence seizures | T-type Ca²⁺ channels; thalamocortical | 3 Hz spike-and-wave; ethosuximide |
Clinical Pearl
For any patient with acute weakness: (1) check respiratory function (FVC) before anything else, (2) distinguish UMN from LMN, and (3) if NMJ disorder suspected, check RNS pattern — decrement alone suggests MG; low CMAP with large increment suggests LEMS or botulism.
Board Pearl
Never give succinylcholine to patients with known or suspected hyperkalemic periodic paralysis, malignant hyperthermia susceptibility, or denervation injuries — risk of fatal hyperkalemia or MH crisis. Avoid aminoglycosides, magnesium, and fluoroquinolones in myasthenia gravis — may precipitate crisis.
References
- Kandel ER, Schwartz JH, Jessell TM, et al. Principles of Neural Science. 6th ed. McGraw-Hill; 2021.
- Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 4th ed. Elsevier; 2021.
- Aminoff MJ, Josephson SA. Aminoff’s Neurology and General Medicine. 6th ed. Academic Press; 2021.
- Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor’s Principles of Neurology. 12th ed. McGraw-Hill; 2023.
- Engel AG. Congenital myasthenic syndromes in 2018. Curr Neurol Neurosci Rep. 2018;18(8):46.
- Statland JM, Bhatt T. Channelopathies: Episodic and electrical diseases of the nervous system. In: Daroff RB, et al., eds. Bradley and Daroff’s Neurology in Clinical Practice. 8th ed. Elsevier; 2022.