La d Which of the following has the least conduction velocity?

Correct Answer: B. Postganglionic sympathetic fibers

Postganglionic sympathetic fibers have the slowest conduction velocity among all nerve fiber types because they are unmyelinated C fibers with diameters of 0.5–1.5 μm. According to the Hodgkin relationship, conduction velocity is directly proportional to axonal diameter and myelination status: velocity = diameter × 6 (for myelinated) or diameter × 1 (for unmyelinated). Postganglionic sympathetic fibers conduct at only 0.5–2 m/s, making them the slowest. This slow conduction is functionally appropriate because sympathetic effects (like pupil dilation, sweating, or heart rate changes) do not require rapid transmission—they are modulatory and sustained rather than immediate. In contrast, preganglionic fibers are myelinated B fibers (1–3 μm, _{3–15 m/s), touch/pressure fibers are myelinated Aβ fibers (5–12 μm,} 30–70 m/s), and somatic motor fibers are large myelinated Aα fibers (12–20 μm, ~70–120 m/s). The unmyelinated nature of postganglionic sympathetic fibers is the key discriminator—they lack the myelin sheath that allows saltatory conduction in myelinated fibers.

Why the other options are wrong

A. Preganglionic autonomic fibers — Preganglionic fibers (both parasympathetic and sympathetic) are myelinated B fibers with diameters of 1–3 μm and conduct at 3–15 m/s, which is 5–30 times faster than postganglionic sympathetic fibers. The myelination allows saltatory conduction, significantly increasing velocity. This is a common trap because students may confuse preganglionic with postganglionic autonomic fibers. C. Touch and pressure fibers — Touch and pressure are transmitted by myelinated Aβ fibers (5–12 μm diameter) with conduction velocities of 30–70 m/s. These are much faster than postganglionic sympathetic fibers because of their larger diameter and myelination. NBE may pair this option to test whether students confuse sensory fiber classification with autonomic fiber classification. D. Somatic motor fibers — Somatic motor fibers are large myelinated Aα fibers (12–20 μm) with the fastest conduction velocities of 70–120 m/s. These are the largest and most heavily myelinated fibers in the nervous system, designed for rapid voluntary movement control. This option represents the opposite extreme from the correct answer.

High-Yield Facts

Mnemonics

ABCD Fiber Classification (Erlanger-Gasser) Aα (motor, 70–120 m/s) > Aβ (touch, 30–70 m/s) > B (preganglionic, 3–15 m/s) > C (postganglionic, 0.5–2 m/s). Largest to smallest diameter and fastest to slowest. "Postganglionic = Puny & Slow" Postganglionic fibers are unmyelinated (C fibers), tiny (0.5–1.5 μm), and slowest (0.5–2 m/s). Preganglionic are myelinated (B), bigger (1–3 μm), and faster (3–15 m/s).

NBE Trap

NBE commonly pairs preganglionic and postganglionic autonomic fibers as options to test whether students understand that preganglionic fibers are myelinated (faster) while postganglionic are unmyelinated (slower)—a critical distinction often confused in rapid exam conditions.

Clinical Pearl

In Indian clinical practice, the slow postganglionic conduction velocity explains why sympathetic responses (like the delayed onset of sweating in heat stress or gradual pupil dilation in dim light) are slower than somatic reflexes (like the immediate patellar reflex). This physiological lag is clinically relevant in autonomic dysfunction assessment and in understanding why anticholinergic drugs (blocking postganglionic parasympathetic) have delayed onset compared to neuromuscular blocking agents.

_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 45 (Autonomic Nervous System); Ganong's Review of Medical Physiology, Ch. 4 (Nerve Fibers & Conduction Velocity)_