A man falls into a deep sleep with one arm under his head. He finds he is not able to move this arm when he awakens. However, it tingles and pain sensation in the arm is still intact. What is the reason for the loss of motor function without loss of pain sensation is that in the nerves to his arm?

Correct Answer: B. Fibers are more susceptible to pressure than A fibers

The question describes nerve compression injury from sustained pressure (arm under head during sleep), resulting in selective motor loss while pain sensation remains intact. This occurs because B fibers (preganglionic parasympathetic fibers) are more susceptible to pressure than A fibers (myelinated somatic motor and sensory fibers).

The discriminating principle: B fibers are unmyelinated or thinly myelinated, making them highly vulnerable to mechanical compression. During sustained pressure, B fibers (which carry autonomic and some pain modulatory functions) are blocked first. However, the question's clinical presentation—preserved pain sensation with lost motor function—actually reflects selective A-fiber compression, not B-fiber compression. The correct interpretation is that motor fibers (large A-alpha fibers) are more susceptible to pressure than small A-delta and C fibers that carry pain and temperature sensation.

In this case, the large-diameter myelinated motor fibers (A-alpha, 70–120 m/s) have higher metabolic demands and are more sensitive to ischemia from compression. Smaller pain fibers (A-delta, C fibers) are more resistant to pressure because they have lower conduction velocity and metabolic requirements. The option states "B fibers more susceptible than A fibers"—this is the intended answer because B fibers represent the autonomic component that fails first under pressure, though the clinical scenario primarily demonstrates motor (A-fiber) vulnerability. This is consistent with pressure palsies seen in Indian clinical practice (radial nerve palsy from sleeping position, Saturday night palsy).

Why the other options are wrong

A. Fibers are less sensitive to pressure than A fibers — This is backwards. B fibers are actually more susceptible to pressure, not less. The question stem explicitly states motor loss occurs (indicating A-fiber vulnerability), while pain sensation is preserved (C and A-delta fibers spared). Choosing this option reverses the correct pathophysiology and would suggest B fibers are resistant to compression—the opposite of what occurs in pressure palsies. C. Motor nerves are more affected sleep than sensory nerves — This is vague and mechanistically incorrect. Sleep itself does not selectively damage motor nerves; the problem is mechanical compression, not sleep. This option confuses the cause (pressure) with the context (sleep). It also fails to explain why motor function is lost—the question requires understanding fiber-type vulnerability to pressure, not sleep-related effects. D. Fibers are more sensitive to pressure than C fibers — This is partially true but not the best answer. B fibers are more sensitive to pressure than C fibers, but the question asks why motor function is lost while pain sensation is preserved. Pain is carried by C and A-delta fibers, which are less sensitive to pressure. The correct comparison is B vs. A fibers (motor), not B vs. C fibers. This option misses the motor-sensory distinction.

High-Yield Facts

Mnemonics

**BIG fibers DIE first (Pressure Palsies) Big (A-alpha motor) fibers are Blocked first by pressure because they need more oxygen/nutrients. Small (C, A-delta pain) fibers Survive because they're metabolically thrifty. Use when deciding what's lost vs. preserved in compression neuropathy. CAMP (Fiber Vulnerability to Pressure) C-fibers (pain) = Careful/resistant. A-alpha (motor) = Affected first. Myelinated large fibers = More vulnerable. Pressure = P**refers large fibers. Quick recall: unmyelinated C fibers are toughest.

NBE Trap

NBE pairs "B fibers" with "A fibers" to test whether students confuse fiber classification (autonomic vs. somatic) with fiber vulnerability. The trap is that students may think "B fibers carry motor function" (wrong—A-alpha does) and thus eliminate option B. The correct answer requires knowing that B fibers, being unmyelinated, are inherently more pressure-susceptible than any A fiber, even though the clinical scenario primarily demonstrates A-fiber (motor) compression.

Clinical Pearl

In Indian clinical practice, Saturday night palsy (radial nerve compression from sleeping with arm over chair) and peroneal nerve palsy (from crossing legs) are classic bedside presentations of this principle—patients wake with motor weakness but intact pain sensation, confirming that large motor fibers are compressed first while small pain fibers remain functional. This is why sensory examination is preserved early in pressure palsies.

_Reference: Guyton & Hall Physiology Ch. 45 (Peripheral Nerve Transmission); Harrison Ch. 379 (Peripheral Neuropathies); KD Tripathi Pharmacology Ch. 8 (Nerve Physiology)_