Question

Slow conduction velocity is seen with which one of the following nerve fibers?

• A. Preganglionic autonomic nerve fibers
• B. Postganglionic autonomic nerve fibers
• C. Motor nerves
• D. Sympathetic nerve fibers

Hint:

Slowest = thin unmyelinated C fibers - which autonomic fibers fall in that class?

Answer 1

The Correct Option is B

Step 1: In the Erlanger and Gasser classification, nerve fibers are graded by diameter and myelination, which together set conduction velocity. Larger and myelinated fibers conduct fast; thin and unmyelinated fibers conduct slowest.
Step 2: C fibers have a diameter of only about 0.2 to 1.0 micrometre and are unmyelinated, giving the slowest conduction of roughly 0.2 to 2 m/s.
Step 3: Functionally, C fibers carry slow (second) pain, temperature and pressure, and importantly they include the postganglionic autonomic fibers. So slow conduction maps to postganglionic autonomic fibers.
Step 4: Preganglionic autonomic fibers are type B (thinly myelinated, faster), motor nerves are large myelinated type A-alpha (fast), and the broad term sympathetic fibers includes faster preganglionic B fibers. Hence the slowest are the postganglionic autonomic (C) fibers.