The unmyelinated peripheral nerve fiber (C fiber) transmits impulse by which mechanism, and how does its conduction velocity compare to the smallest myelinated fibers?

• A Continuous (non-saltatory) conduction at 0.5–2 m/s; slower than even the smallest myelinated (Aδ) fibers at 5–30 m/s ✓
• B Saltatory conduction using Schwann cell nodes of Ranvier at 5–10 m/s
• C Continuous conduction at 20–70 m/s; faster than myelinated fibers
• D Non-propagated graded potentials at the axon terminal only

Explanation

Unmyelinated C fibers (group IV afferents) are embedded in Schwann cell cytoplasm but lack a myelin sheath. Their action potentials conduct continuously (not saltatorially) along the entire axon membrane, making conduction very slow: 0.5–2 m/s. The smallest myelinated fibers (Aδ, group III) conduct at 5–30 m/s via saltatory conduction between nodes of Ranvier. C fibers transmit slow/burning pain, temperature, itch, and postganglionic sympathetic signals. The inverse relationship between fiber diameter and speed — and the role of myelination — is fundamental in nerve fiber histophysiology.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.