THE SPIKE GENERATION ZONE OF THE AMPULLARY ELECTRORECEPTOR .1. STIMULUS-RESPONSE CHARACTERISTICS OF A RELAXATION-OSCILLATOR CIRCUIT MODEL

Sensory input to the central nervous system begins with a transduction step, specialized to the sensory modality involved, resulting in the production of postsynaptic electrical input to the outermost branches of a dendritic tree. Spatiotemporal summation of this 'slow' input as it converges upon the axon then initiates the production of or modulat es the rate of ongoing production of 'fast' neural spikes destined for the central nervous system. We present a novel circuit design consist ing of an operational amplifier, a tunnel diode and linear passive com ponents, intended to model the spike generation zone at which the tran sformation of neural input from slow to fast format takes place. Our c ircuit is shown to be a relaxation oscillator of the van der Pol type. Simulated postsynaptic current modulates the frequency of spike produ ction by the relaxation oscillator model, producing a stimulus-respons e characteristic which can be compared with those observed in vivo. St imulus-response data for our model match data available in the literat ure for the ampullary electroreceptor of elasmobranch fish.