DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

7STIMULATION OF EXCITABLE TISSUESAn electrically excitable cell in its resting state is essentially a charged capacitor. The cellmembrane is the dielectric, the ionic solutions on either side of the membrane constitutethe plates, and differences in the concentrations of ions on each side generate a potentialdifference of about 70 to 90 mV (measured inside the cell against a reference in theextracellular fluid). To generate an action potential, the membrane capacitance must bedischarged by about 15 mV in a small region. This results in a brief sequence of openingsand closings of sodium and potassium channels in the membrane, which results in the flowof the action current. The action current depolarizes and then repolarizes adjacent regionsof the cell membrane, giving rise to the action potential.Excitable cells can be activated by a variety of stimuli, which include burning, mechanicaltrauma, electrical currents, and very intense variable magnetic fields. If sufficientlystrong, any of these stimuli can depolarize the membrane of the excitable cells to a thresholdvoltage level at which the regenerative mechanisms of the action potential take over.However, the most common method of stimulating excitable tissue artificially is to pass anelectrical current through the target tissue.Hodgkin and Huxley’s classical experiments on excitable cells were carried out by placingelectrodes inside the cells under study. They wisely chose a huge cell membrane (atleast as far as cells go), the giant squid axon, to make it easier to manipulate the electrodeswithout destroying cells. To analyze the nonlinear properties of ion conductances underlyingaction potentials, Hodgkin and Huxley [1952] used the voltage-clamp technique 1developed by Kenneth Cole. As shown in Figure 7.1, space-clamp experiments usuallyinvolve inserting two electrode wires into the axon, one for recording the transmembranevoltage and the other for passing current into the axon. In the voltage clamp, the same1Cell electrophysiology is outside the scope of this book. However, if you are interested in the subject, we wouldlike to refer you to what we consider is the most no-nonsense source of information on cell electrophysiologyand biophysics techniques: Axon Instruments Inc. publishes The Axon Guide for Electrophysiology & Biophysics,which is a practical laboratory guide covering a broad range of topics, from the biological basis of bioelectricityand a description of the basic experimental setup (including how to make pipette microelectrodes) to the principlesof operation of advanced electrophysiology lab hardware and software. Best of all, you can download thecomplete guide free from Axon’s Web site at www.axon.com/MR_Axon_Guide.html.Design and Development of Medical Electronic Instrumentation By David Prutchi and Michael NorrisISBN 0-471-67623-3 Copyright © 2005 John Wiley & Sons, Inc.305