Electrokinetic considerations of mechanoelectrical transduction.

Abstract

Summary1. An electrokinetic mechanoelectrical transducer analog has been subject to detailed analysis. The model consists of a two‐compartment model of a pressure sensitive “cell” bounded by a membrane, which can be represented as a microporous, elastic structure containing fixed charge groups. This ion and water‐permeable membrane is supposed to be polarized by a more or less constant electric current. The model is metastable in the sense that it can be stimulated both with changes of the electric current and/or by changes of a pressure gradient. Some modifications have now been introduced into an earlier, simpler version of this “membrane oscillator” in order to simulate certain physiological conditions. The result presented have been obtained by analog computation.2. Stimuli with electrical current pulses and externally applied pressure pulses evoke similar “action potential” responses, demonstrating the intimate coupling between the driving forces in this electrokinetic model (i.e. the chemical potential gradient, the electrical potential gradient and the pressure gradient).3. It is shown that the artificial transducer elicits a static response to maintained, constant pressure stimuli in the form of repetitive “action potentials.” The relation impulse frequency vs. stimulus strength is largely linear and analogous to many of those reported in the physiological literature on pressoreceptors, i.e. external pressure produces a “frequency modulation” of the evoked action potentials.4. The frequency response to pressure stimuli of increasing repetition rate shows that the artificial transducer is insensitive to high‐frequnecy vibrations, that it shows “pararesonance” behavior at intermediate frequencies and a dynamic type response at low repetition rates (in the latter case the response, in the main, occurred at high rates of change of the increasing pressure stimuli; on the other hand, during the decline of the stimuli a “postexcitatory depression” could be observed).5. With the background of existing concepts in receptor physiology a hypothetical “1‐step process” (direct coupling between pressure‐effect and electrical response) and a “2‐step process” (involving a primary “generator potential”) was discussed. The artificial transducer model demonstrated that the generator potential type of response and the oscillatory response type can belong to one and the same system, the difference only arising from a different degree of damping. A method is outlined which can discriminate between the 1‐step and the 2‐step process, based on the pattern of the firing level and the undershoot level of the action potentials.6. The role of the polarizing, electric “resting current” is considered as being a manifestation of the existence of a biochemically driven “fuel cell.”