Signals at the Edge: An In-Depth Guide to the PNS
A clinician-friendly, student-ready reference covering anatomy, physiology, diagnostics (EMG/NCS & autonomic testing), common neuropathies, entrapments, and evidence-based management — designed in Audia’s aurora glass style.
Overview
The peripheral nervous system (PNS) comprises all neural structures outside the brain and spinal cord: cranial nerves (III–XII mostly mixed), spinal nerve roots, plexuses, peripheral nerves, neuromuscular junctions, and the autonomic (sympathetic, parasympathetic, enteric) networks. It relays sensation to the CNS and executes motor and autonomic output, maintaining posture, movement, and visceral homeostasis.
- Fiber classes
- Aα (motor/proprioception), Aβ (touch/vibration), Aδ (fast pain/cold), C (slow pain/warmth, autonomic).
- Myelin
- Schwann cells myelinate single internodes (PNS), enabling saltatory conduction; demyelination slows/blocks impulse.
- Motor unit
- α-motoneuron + all innervated muscle fibers; unit size & firing rate govern force; reinnervation enlarges MUAPs.
Quick Map
| Component | Key Notes |
|---|---|
| Roots & Rami | Ventral roots (motor), dorsal roots (sensory); lesions cause dermatomal deficits. |
| Plexuses | Cervical (C1–4), brachial (C5–T1), lumbar (L1–4), sacral (L4–S4): mixing creates peripheral field maps. |
| Peripheral Nerves | Named mixed nerves with fascicles, perineurium, epineurium; vasa nervorum vulnerable in vasculitis. |
| NMJ | ACh release → nicotinic receptors; blocked in myasthenia gravis, botulism, and by non‑depolarizing agents. |
| Autonomic | Sympathetic (T1–L2), parasympathetic (III, VII, IX, X, S2–4); dual-innervation of viscera; enteric plexuses. |
Plexuses & Major Nerves
| Region | Highlights |
|---|---|
| Cervical | Phrenic (C3–5) to diaphragm; ansa cervicalis to infrahyoids. |
| Brachial | Roots→trunks→divisions→cords→branches: musculocutaneous, axillary, radial, median, ulnar. |
| Lumbar | Femoral (knee extension, anterior thigh sensation); obturator (adduction, medial thigh). |
| Sacral | Sciatic → tibial & common fibular; pudendal; gluteal nerves to hip abductors/extensors. |
Dermatomes vs Cutaneous Nerves
Dermatomes map root-level innervation; cutaneous nerves reflect distal branching. Radiculopathy respects dermatomes and often includes back/neck pain; mononeuropathies respect peripheral nerve territories and may show Tinel/Phalen signs or entrapment tenderness.
Fast field cues
- C6 thumb; C7 middle finger; C8 little finger.
- L5 dorsum foot great toe; S1 lateral foot/little toe.
- Median palmar digits 1–3; ulnar 4–5; radial dorsal thumb web.
Conduction & Injury Patterns
| Type | Features |
|---|---|
| Demyelinating | Slowed conduction velocity, prolonged distal latencies, conduction block; preserved CMAP amplitudes early. |
| Axonal | Reduced CMAP/SNAP amplitudes; relative velocity preservation; length-dependent “stocking-glove” deficits. |
| Wallerian Degeneration | Distal axon/ myelin breakdown post-injury; regeneration guided by Schwann cells (1–3 mm/day). |
| Small Fiber | Burning pain, allodynia, normal NCS; diagnose with skin biopsy (IENFD) or QSART. |
Sympathetic • Parasympathetic • Enteric
- Sympathetic: thoracolumbar outflow; α/β adrenergic effects (↑HR, vasoconstriction, bronchodilation).
- Parasympathetic: craniosacral; muscarinic effects (↓HR, ↑GI motility, secretions).
- Enteric: myenteric & submucosal plexuses; semi-autonomous control of GI motility/secretion.
Autonomic testing
- Heart rate variability & deep breathing, Valsalva ratio.
- QSART/thermoregulatory sweat test.
- Tilt-table for orthostatic hypotension/POTS.
Common Peripheral Neuropathies
| Category | Examples & Notes |
|---|---|
| Demyelinating | GBS (AIDP) — ascending weakness, areflexia; albuminocytologic dissociation. CIDP — ≥8 weeks, proximal & distal weakness; responds to IVIG, steroids, plasmapheresis. |
| Axonal | Diabetic distal symmetric polyneuropathy (glove‑stocking sensory loss, burning pain); toxic (chemo: vincristine, cisplatin), nutritional (B12, thiamine). |
| Small Fiber | Burning dysesthesia, hyperalgesia, normal NCS; etiologies include diabetes, Sjögren, sarcoid, amyloidosis, idiopathic. |
| Mononeuropathies | Entrapments: median (carpal tunnel), ulnar (cubital tunnel/Guyon), radial palsy (“Saturday night”), peroneal at fibular neck, lateral femoral cutaneous (meralgia paresthetica). |
| Mononeuritis Multiplex | Asymmetric painful deficits from vasculitis (PAN, GPA, EGPA), diabetes; urgent rheum workup. |
| Hereditary | CMT1 (PMP22 duplication, demyelinating), CMT2 (axonal); pes cavus, distal wasting, reduced velocities (CMT1). |
| Immune NMJ | Myasthenia gravis (AChR/MuSK antibodies; fatigable weakness, ptosis, bulbar); Lambert‑Eaton (paraneoplastic; proximal weakness improves with activity). |
Red Flags
- Rapidly progressive weakness, respiratory or bulbar involvement.
- Marked asymmetry, vasculitic pain, systemic features (fever, weight loss).
- Severe autonomic failure (syncope, anhidrosis) or acute urinary retention.
- Subacute neuropathy with cancer risk or toxin exposure.
When to admit
Suspected GBS, myasthenic crisis, severe dysautonomia, or unclear rapidly progressive neuropathy.
Bedside & Laboratory
- Pattern recognition: length‑dependent vs focal vs multifocal; motor vs sensory vs autonomic.
- Strength (MRC), tone, DTRs, sensory modalities (light touch, pin, vibration, proprioception).
- Labs: A1c, B12/methylmalonic acid, TSH, SPEP/UPEP, autoimmune serologies (ANA/SSA), hepatitis, HIV.
CSF & biopsy
CSF protein elevation with normal cells in GBS/CIDP; nerve/skin biopsy for vasculitis, amyloid, small‑fiber neuropathy.
Electrodiagnostics
| Test | Interpretation |
|---|---|
| Nerve Conduction Studies (NCS) | Motor (CMAP) & sensory (SNAP) amplitudes, distal latency, velocity; demyelination → slowed CV/blocks; axonal → low amplitudes. |
| EMG | Spontaneous activity (fibs/PSWs), MUAP morphology; reinnervation with large MUAPs; myopathy with small, early‑recruiting MUAPs. |
| Repetitive Stimulation | Decrement >10% suggests NMJ transmission failure (MG); post‑exercise increment suggests LEMS. |
| Autonomic Tests | QSART, HR variability, tilt‑table for orthostatic intolerance and POTS. |
Principles of Treatment
- Address cause (glycemic control, stop toxins, treat infection/autoimmunity).
- Immune therapies: IVIG, corticosteroids, plasma exchange (GBS/CIDP/MG contexts as indicated).
- Neuropathic pain: gabapentinoids, SNRIs/TCAs, topical lidocaine/capsaicin.
- Rehabilitation: PT/OT for balance, strength, orthoses; fall prevention.
- Autonomic support: fluids, salt, compression, midodrine/fludrocortisone for OH; bowel/bladder programs.
Entrapment care
Activity modification, splints (e.g., neutral wrist for CTS), NSAIDs; consider steroid injection or decompression if severe/axonal loss.
Prognosis & Follow‑up
- Axonal degeneration lengthens recovery; demyelination may respond rapidly to immunomodulation.
- Track function (6‑minute walk, grip strength) and pain/sensory scales.
- Re‑evaluate diagnosis if atypical features emerge.
View the original document
Open the full PDF in a new tab for reference.
Open PNS.pdf