Rheobase is defined as the minimum current required to elicit a response with a stimulus of

• A. Infinite duration
• B. Very long duration
• C. Short duration
• D. Zero duration

Answer: (A)

Rheobase is the smallest current that will trigger a response if the stimulus could last for an infinitely long time. In the strength–duration relationship, as you make the stimulus duration longer, the required current drops and approaches a lower limit. Only when the duration is limitless does that minimum current become well defined, which is why the description uses infinite duration. Short or zero duration would require much higher currents or may not evoke a response at all, and “very long duration” is not precise enough—the concept is defined by infinite duration. Relatedly, chronaxie describes the minimum time needed for a current equal to twice the rheobase to produce a response.