Simple integrated circuit model of propagation along an excitable axon

We present a simple integrated circuit model of an excitable nerve axon based on a rotating switch. The heart of the circuit consists of two integrated circuits-a dual rotating switch and a quad comparator. Both the passive and active response of a segment of axon membrane are modeled. A single segment exhibits passive cable behavior up to a threshold voltage. Once threshold is reached, the segment ''fires,'' i.e., currents representative of sodium and potassium depolarizing and repolarizing currents flow sequentially. Furthermore, successive segments have been connected to model an extended section of excitable axon. A linear array of light emitting diodes (LEDs) connected to each segment allows the voltage on that segment to be visibly monitored. When mounted as a two-dimensional LED array, this results in a ''spatial oscilloscope'' which shows the action potential propagating along the axon. The circuit could serve as a teaching tool for both the biophysics of nerve signal propagation and for electronics. It has the virtue of being relatively simple to understand as it is based on a rotating switch. (C) 1996 American Association of Physics Teachers.