What happens when an external electrical current enters through the skin?

• A. It blocks neural activity.
• B. It reduces neural conduction.
• C. It has no effect on nerves.
• D. It may initiate an action potential once depolarization exceeds the threshold.

Answer: (D)

External electrical current can change the membrane potential of nerves. Nerves are excitable cells with a resting inside that is negatively charged relative to the outside. When current flows through the skin into the underlying tissue, it can depolarize the nerve membrane. If this depolarization reaches the threshold, voltage-gated sodium channels open, the membrane potential rapidly reverses, and an action potential is generated and travels along the nerve. Subthreshold depolarizations don’t trigger an action potential, while stronger or longer-lasting currents can push the membrane past the threshold. So, the external current may initiate an action potential once depolarization exceeds the threshold.