Why are very long pulse durations required for denervated muscles?

• A. To directly depolarize muscle fibers
• B. To reduce cost
• C. To improve skin sensation
• D. To activate sensory nerves

Answer: (A)

When a muscle is denervated, the motor nerve can no longer reliably carry a signal to the muscle to trigger a contraction. To produce a contraction, stimulation must directly depolarize the muscle cell membrane itself. Denervated muscle membranes have changed excitability: their chronaxie is much longer and they require more time and charge to reach the threshold for depolarization. Because of this, long pulse durations—often tens to hundreds of milliseconds—are needed to directly activate the muscle fibers. Shorter pulses that effectively stimulate innervated nerves won’t reliably evoke a contraction in denervated muscle. The other ideas (cost, skin sensation, or activating sensory nerves) don’t address the fundamental need to overcome the muscle membrane’s higher threshold with direct fiber stimulation.