The correct answer is: d. The electrochemical potential difference between the two metals. This potential difference generates an electric current that depolarizes the frog nerve/muscle membrane and triggers action potentials, causing the leg to jerk.​

Introduction

In the late 1700s, Luigi Galvani observed that a frog’s leg twitched when its nerve–muscle preparation was simultaneously contacted by two different metals, such as copper and iron, completing a circuit through the moist tissue. The key reason for this movement is the electrochemical potential difference between the metals, which generates a current that depolarizes excitable membranes and fires action potentials.​

Why option (d) is correct

d. The electrochemical potential difference between the two metals

• Two dissimilar metals (e.g., copper and iron) in contact form a tiny galvanic cell because each metal has a different work function and electrochemical potential.​

• When these metals are bridged by a conductive path (the moist frog tissue), electrons flow, creating a voltage across the nerve membrane; if this depolarization reaches threshold, it opens voltage-gated channels, generates an action potential, and the muscle contracts, making the leg jerk.​

Thus, the twitch is due to the voltage/current created by the metal–metal electrochemical potential difference, not because metals “remove charge” or mimic neurotransmitters.

Why the other options are wrong

a. Metals conduct charge away from cell membranes

• Metals do conduct electricity, but in Galvani’s setup the crucial phenomenon is that the two different metals create a potential difference and drive current, not merely “conduct charge away” from the membrane.​

• If there were no electrochemical potential difference between the metals, simple conduction alone would not generate the necessary transmembrane depolarization to reach threshold and fire action potentials.​

• Therefore, this option is incomplete and misleading; conduction is incidental, while the driving electrochemical potential difference is the real cause.

b. Soluble metal ions mimic neurotransmitters

• Neurotransmitters are specific signaling molecules (like acetylcholine, glutamate, GABA) that bind to receptors and open ion channels in synapses, not just any metal ion in solution.​

• In Galvani’s classic experiment, the effect occurs immediately upon metal contact and circuit closure; it is explained by an electrical current, not by slow diffusion and receptor binding of soluble metal ions.​

• Copper or iron ions do not act as physiological neurotransmitters at the frog neuromuscular junction, so this option is incorrect.

c. The temperature difference between metals and tissues

• The twitch occurs because of electrical stimulation, not heating; the temperature difference between a room-temperature metal and tissue is small and does not produce rapid, threshold-level depolarization.​

• Heating could, in extreme conditions, damage or inactivate tissue rather than reproducibly generate precise action potentials, and Galvani’s observed twitches were reliably tied to electrical contact, not to warming or cooling.​

• Hence, temperature difference is not the relevant property here.

Key Concept: Metals as a Bioelectric Stimulus

• Galvani’s frog-leg experiments were an early demonstration of bioelectricity: muscles and nerves respond to electrical potentials generated externally or internally.​

• Volta later showed that the bimetallic contact itself can generate electricity, leading to the invention of the voltaic pile and clarifying that dissimilar metals plus an electrolyte (frog tissue) form a primitive galvanic cell.​

In exam terms, always connect Galvani’s frog-leg twitch with “electrochemical potential difference between two metals” → current through tissue → depolarization → action potential → muscle contraction, which corresponds to option (d).