SIMPLE INTEGRATED-CIRCUIT MODEL OF PROPAGATION ALONG AN EXCITABLE AXON

We present a simple integrated circuit model of an excitable nerve axo n based on a rotating switch. The heart of the circuit consists of two integrated circuits-a dual rotating switch and a quad comparator. Bot h the passive and active response of a segment of axon membrane are mo deled. A single segment exhibits passive cable behavior up to a thresh old voltage. Once threshold is reached, the segment ''fires,'' i.e., c urrents representative of sodium and potassium depolarizing and repola rizing currents flow sequentially. Furthermore, successive segments ha ve been connected to model an extended section of excitable axon. A li near array of light emitting diodes (LEDs) connected to each segment a llows the voltage on that segment to be visibly monitored. When mounte d as a two-dimensional LED array, this results in a ''spatial oscillos cope'' which shows the action potential propagating along the axon. Th e circuit could serve as a teaching tool for both the biophysics of ne rve 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.