Types of nerve fibers !
- Table to know
- Exam priority: medium
Nerves carry fibers of different thickness: thick fast ones for movement and touch, thin slow ones for burning pain and temperature.
Updated 2026-11-01
Also called: Erlanger–Gasser classification, A, B and C fibers
See it in 3D: Sciatic nerve · Median nerve · Ulnar nerve
What it is
Peripheral nerve fibers are classified by diameter, myelination and conduction velocity. The letter system (A, B, C) is used for all fibers; a Roman-numeral system (Ia, Ib, II, III, IV) is used for sensory fibers from muscle and skin. The class explains why sharp pain arrives before burning pain and why local anesthetics block sensations in a predictable order.
Key facts
- Velocity in myelinated fibers rises roughly in proportion to diameter (about 6 m/s per µm).
- Aγ fibers are motor axons to the intrafusal fibers of muscle spindles and set spindle sensitivity.
Nerve fiber classes (approximate values)
| Class | Myelin | Diameter | Velocity | Main function |
|---|---|---|---|---|
| Aα (Ia, Ib) | Thick | ≈ 12–20 µm | ≈ 70–120 m/s | Skeletal motor neurons; spindle and tendon organ afferents |
| Aβ (II) | Yes | ≈ 6–12 µm | ≈ 35–75 m/s | Touch, pressure, vibration; spindle secondary endings |
| Aγ | Yes | ≈ 3–6 µm | ≈ 15–30 m/s | Motor to muscle spindle (intrafusal fibers) |
| Aδ (III) | Thin | ≈ 1–5 µm | ≈ 5–30 m/s | Sharp, fast pain; cold |
| B | Thin | < 3 µm | ≈ 3–15 m/s | Preganglionic autonomic |
| C (IV) | None | ≈ 0.2–1.5 µm | ≈ 0.5–2 m/s | Burning pain, warmth, itch; postganglionic sympathetic |
Clinical notes
- Small-fiber neuropathy (for example in diabetes) affects Aδ and C fibers first: burning feet and loss of pain and temperature with normal strength and reflexes.
- During a regional block, patients often lose temperature and pain sensation before touch and movement, and may feel warmth from sympathetic block (vasodilation) first.
Test yourself
Think of the answer, then tap the question to see it.
Why does a stubbed toe produce a sharp pain followed a second or so later by a dull, burning pain?
Sharp first pain travels in myelinated Aδ fibers (≈ 5–30 m/s); burning second pain travels in unmyelinated C fibers (≈ 0.5–2 m/s) and arrives later.
In what order do sensations usually disappear during a local anesthetic nerve block, and why?
Autonomic (B) and pain/temperature (Aδ, C) first, then touch (Aβ), motor (Aα) last. Thin fibers and short internodes need fewer blocked Na⁺ channels to stop conduction.
Which fiber type carries the afferent limb of the knee-jerk reflex?
Ia afferents (Aα class) from the muscle spindle primary endings, the fastest sensory fibers, which synapse directly on α motor neurons.
A diabetic patient has burning feet and cannot feel a pinprick, yet ankle reflexes and vibration are normal. Which fibers are affected?
Small fibers: Aδ and C. Reflexes and vibration travel in large Aα/Aβ fibers, which are spared in a pure small-fiber neuropathy.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Impulse conduction and myelin
- Pain and temperature: pathways and modulation
- Muscle spindle and Golgi tendon organ
- Sensory receptors, coding and neuronal circuits
Related conditions
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 12.2 Nervous Tissue (OpenStax (Rice University), 2022); 12.4 The Action Potential (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 47). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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