Nerves & Neuromuscular Junction

Nerve Structure Fiber Types NMJ NMJ Disorders Nerve Injury EDX Correlates Channelopathies Summary

🔬 Nerve Structure & Organization

Peripheral Nerve Anatomy

Layer	Description	Clinical Significance
Endoneurium	Surrounds individual nerve fibers	Must be intact for regeneration; contains blood-nerve barrier
Perineurium	Surrounds fascicles (bundles of fibers)	Main component of blood-nerve barrier; determines nerve tensile strength
Epineurium	Surrounds entire nerve	Contains vasa nervorum; surgical repair target

Myelination

Feature	PNS (Schwann Cells)	CNS (Oligodendrocytes)
Cell:Axon ratio	1 Schwann cell : 1 internode	1 oligodendrocyte : up to 50 internodes
Regeneration	Good (Schwann cells guide regrowth)	Poor (inhibitory environment)
Diseases	GBS, CIDP, CMT	MS, leukodystrophies

Nodes of Ranvier

Location: Gaps between myelin segments
Function: Site of saltatory conduction; high concentration of voltage-gated Na+ channels
Paranodal region: Contains K+ channels; exposed in demyelination → conduction block

💎 Board Pearl

Saltatory conduction allows action potentials to “jump” between nodes of Ranvier, greatly increasing conduction velocity. Demyelination exposes paranodal K+ channels → hyperpolarization → conduction failure.

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⚡ Nerve Fiber Classification

Erlanger-Gasser Classification (Sensory & Motor)

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)
Aγ	3-6	15-30	Medium	Motor to muscle spindle (intrafusal)
Aδ	2-5	12-30	Light	Fast pain, temperature, 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)

💎 Board Pearl

Conduction velocity ≈ 6 × diameter (in μm). Large, myelinated fibers (Aα) are affected first by compression/ischemia. Small unmyelinated fibers (C) are affected first by metabolic/toxic neuropathies (diabetes).

Clinical Correlates of Fiber Type Involvement

Fiber Type Affected	Clinical Features	Example Conditions
Large fiber	Loss of proprioception, vibration; sensory ataxia; areflexia	GBS, CIDP, B12 deficiency, Friedreich ataxia
Small fiber	Burning pain, loss of pinprick/temperature; autonomic dysfunction; preserved reflexes	Diabetic neuropathy (early), amyloid, Fabry disease
Motor fiber	Weakness, atrophy, fasciculations	ALS, MMN, lead toxicity
Autonomic fiber	Orthostatic hypotension, anhidrosis, GI/GU dysfunction	Diabetes, amyloid, autoimmune autonomic ganglionopathy

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🔗 Neuromuscular Junction

NMJ Anatomy

Presynaptic terminal: Contains ACh vesicles, voltage-gated Ca2+ channels (P/Q type)
Synaptic cleft: Contains acetylcholinesterase (AChE)
Postsynaptic membrane: Contains nicotinic ACh receptors (nAChR) on junctional folds

Neuromuscular Transmission Steps

Action potential arrives at nerve terminal
Ca2+ influx through P/Q-type voltage-gated calcium channels
ACh vesicle fusion with presynaptic membrane (SNARE proteins)
ACh release into synaptic cleft (quantal release)
ACh binds to postsynaptic nicotinic receptors
Na+ influx → end-plate potential (EPP)
If EPP exceeds threshold → muscle action potential → contraction
ACh hydrolysis by acetylcholinesterase

Safety Factor

Definition: EPP amplitude is normally 3-4x greater than threshold needed for muscle AP

Clinical significance:

Ensures reliable transmission even with some receptor loss
In myasthenia gravis: reduced AChR → decreased safety factor → transmission failure with repetitive use
In Lambert-Eaton: reduced ACh release → facilitation with exercise (increased Ca2+ accumulation)

💎 Board Pearl

P/Q-type Ca2+ channels are the target in Lambert-Eaton myasthenic syndrome. SNARE proteins (synaptobrevin, SNAP-25, syntaxin) are targeted by botulinum toxin and tetanus toxin.

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⚠️ Neuromuscular Junction Disorders

Comparison of Major NMJ Disorders

Feature	Myasthenia Gravis	Lambert-Eaton	Botulism
Target	Postsynaptic AChR	Presynaptic P/Q Ca2+ channels	Presynaptic SNARE proteins
Mechanism	Antibody blocks/destroys AChR	Antibody reduces Ca2+ influx → less ACh release	Toxin cleaves SNAREs → blocks ACh release
Weakness pattern	Ocular, bulbar, proximal; fatigable	Proximal legs > arms; improves with exercise	Descending: cranial → limbs → respiratory
Reflexes	Normal	Reduced/absent (improve post-exercise)	Reduced/absent
Autonomic	Usually spared	Dry mouth, constipation, impotence	Prominent (dilated pupils, dry mouth, ileus)
RNS pattern	Decrement at low-frequency (2-3 Hz)	Low baseline CMAP; increment >100% post-exercise	Low baseline CMAP; small increment post-exercise
Association	Thymoma (10-15%); thymic hyperplasia	Small cell lung cancer (50-60%)	Contaminated food, wound, infant (honey)

Myasthenia Gravis – Details ▼

Antibodies:

AChR antibodies: 85% of generalized MG
MuSK antibodies: 5-8%; more bulbar, muscle atrophy
LRP4 antibodies: Rare; milder phenotype
Seronegative: ~10%

Clinical features:

Fatigable weakness (worse with activity, better with rest)
Ptosis, diplopia (ocular MG in 50% at onset)
Bulbar: dysarthria, dysphagia, facial weakness
Limb weakness (proximal > distal)

Myasthenic crisis: Respiratory failure requiring intubation; triggered by infection, surgery, medications

Drugs that worsen MG: Aminoglycosides, fluoroquinolones, beta-blockers, magnesium, neuromuscular blockers

Lambert-Eaton Myasthenic Syndrome – Details ▼

Key features:

Proximal leg weakness → arms → bulbar (opposite of MG)
Facilitation: Strength improves briefly after sustained contraction
Autonomic symptoms prominent (dry mouth in >80%)
Reflexes absent but may appear after exercise

Cancer association:

50-60% have small cell lung cancer (SCLC)
Cancer may present years after LEMS diagnosis
Screen with CT chest; repeat if initially negative

Treatment: 3,4-diaminopyridine (blocks K+ channels → prolongs depolarization → more Ca2+ entry)

💎 Board Pearl

MG = fatigable (gets worse with use). LEMS = facilitates (gets better with use). Both have proximal weakness. LEMS has autonomic symptoms; MG does not. Always screen LEMS for SCLC!

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🔧 Nerve Injury & Regeneration

Seddon Classification

Type	Pathology	Recovery	EMG/NCS
Neurapraxia	Local demyelination; axon intact	Complete; weeks to 3 months	Conduction block; no denervation
Axonotmesis	Axon disrupted; endoneurium intact	Good; 1 mm/day (1 inch/month)	Wallerian degeneration; fibs/PSWs; reinnervation potentials
Neurotmesis	Complete nerve transection	Poor; requires surgery	Complete denervation; no recovery without repair

Sunderland Classification (More Detailed)

Grade	Seddon Equivalent	Injury	Prognosis
I	Neurapraxia	Myelin only	Excellent
II	Axonotmesis	Axon (endoneurium intact)	Good
III	Axonotmesis	Axon + endoneurium	Variable
IV	Axonotmesis	Axon + endo + perineurium	Poor
V	Neurotmesis	Complete transection	None without surgery

Wallerian Degeneration

Definition: Degeneration of axon and myelin distal to site of injury
Timeline:
- Begins within 24-48 hours
- Complete by 7-10 days
- NCS shows absent/reduced responses distally by day 7-10
- EMG shows fibrillations/PSWs by 2-3 weeks (proximal) to 4-5 weeks (distal)
Schwann cells: Proliferate, phagocytose debris, form “bands of Büngner” to guide regeneration

Nerve Regeneration

Rate: ~1 mm/day or ~1 inch/month
Factors affecting recovery:
- Age (younger = better)
- Distance from target muscle
- Time since injury (motor end plates degenerate after 12-18 months)
- Accuracy of reinnervation
Signs of reinnervation: Tinel’s sign (advancing), nascent motor unit potentials on EMG

💎 Board Pearl

Neurapraxia = conduction block without Wallerian degeneration. No fibrillations on EMG. Full recovery expected. In axonotmesis, fibs/PSWs appear in 2-5 weeks (earliest in proximal muscles).

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📊 Electrodiagnostic Correlates

Nerve Conduction Study (NCS) Basics

Parameter	What It Measures	Abnormal In
Amplitude	Number of functioning axons	Axonal loss, conduction block
Conduction velocity	Speed of fastest fibers (myelination)	Demyelination
Distal latency	Time from distal stim to response	Distal demyelination

Demyelinating vs Axonal Patterns

Feature	Demyelinating	Axonal
Conduction velocity	Markedly slow (<70% LLN)	Normal or mildly slow
Distal latency	Prolonged (>130% ULN)	Normal or mildly prolonged
Amplitude	May be preserved (early) or low	Low (proportional to axon loss)
Temporal dispersion	Present	Absent
Conduction block	Present	Absent
F-wave latency	Prolonged or absent	Normal or mildly prolonged
EMG fibrillations	Less prominent (unless secondary axonal loss)	Prominent
Examples	GBS (AIDP), CIDP, CMT1	Diabetic neuropathy, AMAN, CMT2

Key EDX Findings

Conduction Block ▼

Definition: >50% reduction in proximal vs distal CMAP amplitude (excluding common entrapment sites)

Significance:

Indicates focal demyelination
Axon is intact but impulse cannot pass through demyelinated segment
Causes weakness WITHOUT atrophy (no Wallerian degeneration)
Potentially reversible with remyelination

Classic conditions:

GBS (acute)
CIDP (chronic)
Multifocal motor neuropathy (MMN)
Hereditary neuropathy with liability to pressure palsies (HNPP)

Temporal Dispersion ▼

Definition: Increased CMAP duration with proximal stimulation (>30% increase)

Mechanism: Different degrees of demyelination cause different conduction velocities → desynchronization of impulses

Significance: Feature of acquired demyelinating neuropathies (not seen in uniform hereditary demyelination like CMT1A)

F-Waves and H-Reflex ▼

F-Wave

Pathway: Motor nerve → anterior horn → same motor nerve back (antidromic → orthodromic)
Tests: Entire motor nerve including proximal segments and roots
Abnormal in: Proximal demyelination (GBS), radiculopathy
Features: Variable latency and morphology; small amplitude

H-Reflex

Pathway: Ia afferent → spinal cord → alpha motor neuron → muscle (monosynaptic reflex)
Essentially: Electrical equivalent of ankle jerk (S1 root)
Tests: S1 nerve root function; only reliably obtained in tibial nerve/soleus
Abnormal in: S1 radiculopathy, polyneuropathy

💎 Board Pearl

Conduction block = demyelinating. Low amplitudes everywhere = axonal. Temporal dispersion indicates acquired (non-uniform) demyelination. F-waves test proximal nerve segments not accessible to routine NCS.

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⚡ Channelopathies

Sodium Channelopathies

Disorder	Gene/Channel	Mechanism	Clinical Features
Hyperkalemic Periodic Paralysis	SCN4A (Nav1.4)	Gain of function → prolonged depolarization	Attacks with high K+, fasting, rest after exercise; myotonia common
Paramyotonia Congenita	SCN4A (Nav1.4)	Impaired fast inactivation	Cold-induced myotonia; “paradoxical” myotonia (worsens with activity)
Sodium Channel Myotonia	SCN4A (Nav1.4)	Delayed inactivation	Myotonia without weakness; K+-aggravated

Calcium Channelopathies

Disorder	Gene/Channel	Mechanism	Clinical Features
Hypokalemic Periodic Paralysis	CACNA1S (Cav1.1) – 70% SCN4A – 10%	Loss of function → reduced excitability	Attacks with low K+, carbs, rest after exercise; NO myotonia
Malignant Hyperthermia	RYR1 (ryanodine receptor)	Uncontrolled Ca2+ release from SR	Triggered by volatile anesthetics, succinylcholine; rigidity, hyperthermia, rhabdomyolysis

Chloride Channelopathies

Disorder	Gene/Channel	Clinical Features
Myotonia Congenita (Thomsen/Becker)	CLCN1 (ClC-1)	Myotonia (muscle stiffness); improves with activity (“warm-up”); muscle hypertrophy; NO weakness

Periodic Paralysis Comparison

Feature	Hypokalemic PP	Hyperkalemic PP
K+ during attack	Low (<3.5)	High or normal
Triggers	Carbs, rest after exercise, stress	Fasting, rest after exercise, cold, K+
Myotonia	Absent	Often present
Attack duration	Hours to days	Minutes to hours
Treatment	K+ replacement; acetazolamide prophylaxis	Carbs, inhaled β-agonist; acetazolamide or dichlorphenamide prophylaxis

💎 Board Pearl

HypoKPP: no myotonia. HyperKPP: myotonia common. Both worsen with rest after exercise. Acetazolamide works for both (metabolic acidosis → reduced attack frequency). Malignant hyperthermia = RYR1 mutation; treat with dantrolene.

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📊 Summary Tables & Quick Reference

Nerve Fiber Types Quick Reference

Fiber	Function	Lost First In
Aα (large, myelinated)	Motor, proprioception	Compression, ischemia
Aβ (large, myelinated)	Touch, pressure	Compression, ischemia
Aδ (small, myelinated)	Fast pain, temperature	Metabolic (later)
C (small, unmyelinated)	Slow pain, autonomic	Metabolic (diabetes), toxic

NMJ Disorders – RNS Patterns

Disorder	Low-Frequency RNS (2-3 Hz)	Post-Exercise/High-Frequency
Myasthenia Gravis	Decrement >10%	Repair of decrement (post-activation facilitation)
Lambert-Eaton	Low baseline; may decrement	Increment >100%
Botulism	Low baseline; may decrement	Small increment (20-40%)

Nerve Injury – Timing of EDX Findings

Finding	Timing After Injury
Reduced recruitment	Immediately
Reduced SNAP/CMAP distal to lesion	7-10 days (Wallerian degeneration complete)
Fibrillations in proximal muscles	2-3 weeks
Fibrillations in distal muscles	4-5 weeks
Nascent MUPs (reinnervation)	2-4 months (depends on distance)

Key Clinical Pearls

🔍 High-Yield Points

Velocity ≈ 6 × diameter: Large fibers conduct faster
Demyelinating = slow velocity, conduction block, temporal dispersion
Axonal = low amplitude, fibrillations on EMG
MG fatigues; LEMS facilitates
Neurapraxia: Best prognosis, conduction block, no fibs
Regeneration rate: 1 mm/day (1 inch/month)
F-waves: Test proximal nerve; prolonged in GBS
Always screen LEMS for small cell lung cancer

Red Flags

⚠️ Urgent Situations

Respiratory muscle weakness in MG: Check FVC; <15-20 mL/kg = intubate
Rapidly progressive weakness with areflexia: GBS – monitor respiratory function
Descending paralysis with autonomic symptoms: Botulism – antitoxin urgently
New-onset LEMS: Search for SCLC (may precede cancer by years)
Malignant hyperthermia: Stop anesthesia, give dantrolene, cool patient

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Nerves & Neuromuscular Junction

🔬 Nerve Structure & Organization

Peripheral Nerve Anatomy

Myelination

Nodes of Ranvier

⚡ Nerve Fiber Classification

Erlanger-Gasser Classification (Sensory & Motor)

Clinical Correlates of Fiber Type Involvement

🔗 Neuromuscular Junction

NMJ Anatomy

Neuromuscular Transmission Steps

Safety Factor

⚠️ Neuromuscular Junction Disorders

Comparison of Major NMJ Disorders

🔧 Nerve Injury & Regeneration

Seddon Classification

Sunderland Classification (More Detailed)

Wallerian Degeneration

Nerve Regeneration

📊 Electrodiagnostic Correlates

Nerve Conduction Study (NCS) Basics

Demyelinating vs Axonal Patterns

Key EDX Findings

F-Wave

H-Reflex

⚡ Channelopathies

Sodium Channelopathies

Calcium Channelopathies

Chloride Channelopathies

Periodic Paralysis Comparison

📊 Summary Tables & Quick Reference

Nerve Fiber Types Quick Reference

NMJ Disorders – RNS Patterns

Nerve Injury – Timing of EDX Findings

Key Clinical Pearls

Red Flags

📚 Related Resources