According to the strength-duration curve, which order of nerve fiber recruitment occurs as amplitude increases?

• A. Motor nerve fibers, sensory nerve fibers, pain nerve fibers
• B. Pain nerve fibers, motor nerve fibers, sensory nerve fibers
• C. Sensory nerve fibers, motor nerve fibers, pain nerve fibers
• D. Motor nerve fibers, pain nerve fibers, sensory nerve fibers

Answer: (C)

The strength-duration curve shows how nerve fibers are recruited as stimulus amplitude rises, and this depends on fiber size and excitability. Larger-diameter fibers have lower thresholds, so they respond to smaller currents first. Among peripheral nerves, large-diameter sensory fibers (carrying touch and proprioception) are recruited with lower amplitudes. As you increase the amplitude further, motor fibers (also large-diameter) are activated next, producing muscle contraction. The smallest-diameter nociceptive fibers (pain signals) require the highest currents, so they’re recruited last.

So, as amplitude increases, the sequence is sensory fibers first, then motor fibers, then pain fibers. This explains why low-current stimulation tends to feel like a tingling or sensory sensation, while higher currents can cause muscle contraction and, at even higher currents, pain.