Nerve Fibre Classification & Conduction

Peripheral nerve fibres are classified by diameter, myelination and conduction velocity into the Erlanger–Gasser (A, B, C) and the sensory-specific Lloyd–Hunt (I–IV) systems. This is a high-frequency, low-difficulty NEET PG topic where the ordering of fibres for conduction velocity, susceptibility to pressure/hypoxia and to local anaesthetic block is repeatedly tested.

Why this topic matters

A single nerve trunk is a mixed cable carrying motor, sensory and autonomic axons of widely different sizes. The behaviour of each fibre — how fast it conducts, how easily it is silenced by pressure, anoxia or a local anaesthetic — is governed almost entirely by two physical variables: axon diameter and degree of myelination. Master these two levers and almost every "which fibre is blocked first/last" question becomes deducible rather than memorised.

Two parallel classifications

There are two systems you must keep separate. Erlanger–Gasser (A, B, C) applies to all peripheral fibres (motor + sensory + autonomic). The numerical Lloyd–Hunt system (Ia, Ib, II, III, IV) applies only to sensory (afferent) fibres and is favoured by physiologists describing muscle and cutaneous afferents.

Erlanger–Gasser (A, B, C) system

Class	Subtype	Myelin	Diameter (µm)	Velocity (m/s)	Function
A	α (alpha)	Heavy	12–20	70–120	Motor to extrafusal muscle (α-motor), proprioception (Ia, Ib)
A	β (beta)	Heavy	5–12	30–70	Touch, pressure (cutaneous mechanoreceptors)
A	γ (gamma)	Moderate	3–6	15–30	Motor to intrafusal muscle spindle fibres
A	δ (delta)	Light	2–5	12–30	Fast/sharp pain, cold, touch
B	—	Light (thin)	<3	3–15	Preganglionic autonomic
C	sC (sympathetic)	None	0.3–1.3	0.5–2	Postganglionic sympathetic
C	drC (dorsal root)	None	0.4–1.2	0.5–2	Slow/dull pain, warmth, itch

High-yield: Fastest → slowest conduction velocity = A-α > A-β > A-γ > A-δ > B > C. The single largest, most heavily myelinated fibre (A-α) is the fastest; the unmyelinated C fibre is the slowest.

High-yield: B fibres are the only myelinated autonomic fibres (preganglionic). Postganglionic autonomic fibres are unmyelinated C fibres. Do not confuse B with the dull-pain C fibre.

Lloyd–Hunt (numerical) system — sensory only

Number	Erlanger equivalent	Receptor / source	Function
Ia	A-α	Muscle spindle — annulospiral / primary ending	Dynamic length & velocity of stretch
Ib	A-α	Golgi tendon organ	Muscle tension (force)
II	A-β	Spindle secondary (flower-spray); cutaneous touch	Static length; touch/pressure
III	A-δ	Free nerve endings, deep pressure	Pain, cold, crude touch
IV	C (dorsal root)	Unmyelinated free nerve endings	Dull pain, temperature, itch

High-yield: Ia = primary spindle ending (annulospiral), Ib = Golgi tendon organ. A common trap swaps these. Mnemonic: "a" in Ia → annulospiral; b → between bone and muscle (tendon).

Determinants of conduction velocity

Conduction velocity (CV) is set by how quickly the local circuit current can depolarise the next excitable patch of membrane to threshold.

Diameter → ↓internal resistance → faster CV. A larger axon has a bigger cross-sectional area, lowering longitudinal (axoplasmic) resistance so local currents spread further and faster. For unmyelinated fibres CV is roughly proportional to √(diameter); for myelinated fibres CV is linearly proportional to diameter (empirically CV in m/s ≈ 6 × diameter in µm).

Myelination → saltatory conduction → faster CV. Myelin is a high-resistance, low-capacitance insulator wrapped by Schwann cells (PNS) or oligodendrocytes (CNS). It forces the action potential to "jump" between nodes of Ranvier.

Temperature. Warming increases CV (faster gating kinetics) up to a point; cooling slows it — the basis of cold-induced conduction block and of nerve conduction study standardisation.

Saltatory conduction — the node of Ranvier story

At nodes of Ranvier the axolemma is bare and densely studded with voltage-gated Na⁺ channels (≈ up to ~1000–2000/µm² at nodes vs sparse internodally). Between nodes the myelin sheath has very low capacitance, so the membrane there cannot easily store charge and the impulse effectively leaps node-to-node.

Stepwise flow:

1. Na⁺ influx at one node depolarises it fully (the active node).
2. Local circuit current flows down the low-resistance axoplasm, bypassing the insulated internode.
3. Current exits and depolarises the next node to threshold.
4. That node fires; the cycle repeats — the impulse "jumps" (Latin saltare, to leap).

High-yield: Saltatory conduction is faster and metabolically economical — Na⁺/K⁺ flux occurs only at nodes, so less Na⁺ enters per impulse and the Na⁺/K⁺-ATPase has less work to restore gradients.

Internodal distance is ≈ 1–2 mm and scales with fibre diameter; longer internodes (bigger fibres) mean fewer "jumps" per unit length and therefore faster conduction.

Pain fibre subtypes — A-δ versus C

Pain ("nociception") is carried by two distinct populations, producing the classic double/dual pain sensation after an acute injury.

Feature	A-δ (fast pain)	C (slow pain)
Myelin	Thinly myelinated	Unmyelinated
Velocity	12–30 m/s	0.5–2 m/s
Pain quality	Sharp, pricking, well localised	Dull, burning, aching, poorly localised
Latency	First pain (immediate)	Second pain (delayed ~1 s)
Neurotransmitter	Glutamate	Glutamate + substance P, CGRP
Dorsal horn lamina	I & V	I & II (substantia gelatinosa)
Tract	Neospinothalamic	Paleospinothalamic / spinoreticular

High-yield: First (fast) pain = A-δ, sharp and localised; second (slow) pain = C, dull, burning and diffuse, mediated partly by substance P. Capsaicin acts on TRPV1 on C fibres.

Susceptibility to block — three different orderings

This is the most examined nugget. The same fibres rank differently depending on the insult, so keep the three lists distinct.

1. Pressure (mechanical compression) and ischaemia/hypoxia — large fibres first. Order of failure: A (large myelinated) → B → C. Large myelinated fibres have high metabolic demand and are most vulnerable to compression and anoxia. This is why "Saturday night palsy" and tourniquet/crutch palsies first lose motor power and proprioception while crude pain persists.

High-yield: Pressure/hypoxia → large myelinated fibres fail first; pain (C) and autonomic function are relatively preserved. (Sensation order lost: proprioception/vibration → touch → temperature → pain last.)

2. Local anaesthetics — small fibres (and B) first. Order of block: B ≈ C (small) and A-δ first → larger A fibres last. Clinically the observed sequence is:

Autonomic (B) → pain & temperature (A-δ, C) → touch & pressure (A-β) → motor & proprioception (A-α) → vibration last.

High-yield: With local anaesthetics, B fibres (preganglionic autonomic) are blocked first — explaining early vasodilatation/hypotension in spinal anaesthesia. Smallest myelinated fibres are most sensitive; large A-α motor fibres are most resistant and recover first.

The apparent paradox (small fibres go first with drugs but large fibres go first with pressure) is a favourite MCQ. The mechanistic reason LA blocks small/B fibres preferentially: shorter internodal distances mean fewer nodes need be silenced to interrupt conduction, plus higher firing frequency (use-dependent block) of nociceptive fibres.

3. Hypoxia (revisited for clarity): mirrors pressure — A > B > C, i.e. large myelinated lost first; C fibres are most resistant to anoxia.

Comparative susceptibility table

Insult	Most sensitive (first lost)	Most resistant (last)
Pressure / mechanical	A-α (large myelinated)	C (unmyelinated)
Hypoxia / ischaemia	A (large myelinated)	C
Local anaesthetic	B, then small C/A-δ	A-α (large motor)

Mnemonic for LA block susceptibility order: "Before Conduction Aborts" → B → C → A, with autonomic lost first and motor last. Or recall the clinical sequence: pain → temperature → touch → pressure → motor.

Differential fibre block — clinical correlation

In spinal/epidural anaesthesia, the differing sensitivities produce a differential block with distinct dermatomal levels:

• Autonomic (sympathetic) block extends about 2 segments higher than the sensory level.
• Sensory block is roughly 2 segments higher than the motor block.
• Hence: Sympathetic level > Sensory level > Motor level.

High-yield: In subarachnoid block the sympathetic block is ~2 dermatomes above the sensory block, and sensory ~2 above motor. This explains hypotension/bradycardia occurring before the patient loses movement.

Compound action potential (CAP)

Stimulating a whole mixed nerve and recording at a distance gives a compound action potential with several peaks because faster fibres arrive first and slower fibres later — the volley disperses with distance.

• Recorded sequence of peaks: A (α, β, γ, δ) → B → C, increasing latency.
• The CAP is graded (its amplitude rises with stimulus strength as more fibres are recruited) — unlike the single-axon all-or-none action potential.

High-yield: A single axon obeys all-or-none; the compound action potential of a whole nerve is graded and shows multiple humps reflecting the fibre-type velocity spectrum.

Eponyms, cut-offs and named facts

• Erlanger & Gasser — Nobel Prize 1944 for fibre classification using the oscilloscope-recorded CAP.
• CV ≈ 6 × diameter (µm) for large myelinated fibres (rule of thumb).
• A-α: 70–120 m/s; C: 0.5–2 m/s — memorise the extremes.
• Node of Ranvier Na⁺ channel density vastly exceeds internodal density.
• Substance P / CGRP — C-fibre (slow pain) neuropeptides.
• TRPV1 receptor — capsaicin/heat on C fibres; TRPM8 — menthol/cold on A-δ/C.

Recently asked / exam angle

• "Which fibre is blocked first by local anaesthetic?" → B (preganglionic autonomic) / small fibres. Trap option: A-α.
• "Which fibre is most resistant to local anaesthetic?" → A-α (large motor).
• "Which is most susceptible to hypoxia/pressure?" → large myelinated A fibres; C fibres most resistant.
• "Fastest conducting fibre?" → A-α (Ia/Ib afferents and α-motor).
• "Golgi tendon organ afferent?" → Ib. "Primary spindle ending?" → Ia (A-α).
• "First pain vs second pain?" → A-δ (fast/sharp) then C (slow/dull).
• "Only myelinated autonomic fibre?" → B (preganglionic).
• "Type of conduction in myelinated nerve?" → saltatory, energy-efficient, node-to-node.
• "Order of sensory loss in nerve compression?" → proprioception/vibration → touch → temperature → pain last.
• Anaesthesia/PSM crossovers frequently ask the differential block hierarchy (sympathetic > sensory > motor levels).

Rapid revision

1. Velocity order: A-α > A-β > A-γ > A-δ > B > C.
2. A-α: motor + Ia/Ib proprioception, 70–120 m/s, fastest.
3. A-γ: supplies intrafusal (muscle spindle) fibres.
4. A-δ = fast sharp pain; C (dorsal root) = slow dull pain + substance P.
5. B = only myelinated autonomic = preganglionic; postganglionic = unmyelinated C.
6. Ia = annulospiral (spindle primary); Ib = Golgi tendon organ; II = spindle secondary; IV = C dull pain.
7. CV depends on diameter + myelination; CV ≈ 6 × diameter for large myelinated fibres.
8. Myelinated nerve = saltatory conduction, faster + metabolically economical.
9. Pressure/hypoxia → large (A) fibres fail first; C most resistant.
10. Local anaesthetic → B/small fibres blocked first; A-α motor most resistant & recovers first.
11. Spinal block hierarchy: sympathetic (~2 above) > sensory (~2 above) > motor.
12. Compound action potential is graded with multiple humps; single axon is all-or-none.