Question

A 35-year-old female was watching TV for long hours with her hands under her head. She complains of a tingling sensation over her arm. Which type of nerve fibers is most likely to be affected?

• A. A fibers
• B. B fibers
• C. C fibers
• D. Sympathetic nerve fibers

Hint:

Pressure block hits the large myelinated fibers first; local anaesthetic hits the small ones first.

Answer 1

The Correct Option is A

Step 1: Identify the mechanism. Sustained pressure on a limb (here, the arm compressed under the head) causes a pressure block of nerve conduction, the familiar "limb falling asleep" with tingling (paraesthesia).
Step 2: Susceptibility to pressure. Nerve fibers differ in vulnerability. Large, heavily myelinated A fibers are the most susceptible to compression (pressure) block, whereas the small unmyelinated C fibers are the most resistant to pressure (but most sensitive to local anaesthetic). So the answer is the A fibers, option (a).
Step 3: Eliminate the rest. B fibers are preganglionic autonomic and C fibers are unmyelinated; both are comparatively pressure-resistant. Sympathetic fibers (B and C type) are not the first to drop out with mechanical compression.
Ref: Arvind Arora Review Book of Physiology (p. 58-59).