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Special purpose real-time analog computers are used to measure and control nerve membrane potential or current in a squid axon or a single frog node. Under current control, the membrane potential has a region of discontinuity and an "action potential" rather similar to that observed in normal impulse propagation. With potential control, the current pattern is a continuous function of the potential, and a negative resistance is found in the region of potential discontinuity for the current-controlled membrane. The membrane's electrical characteristics may therefore be compared with some two-terminal transistor switching circuits.