Considerable advances in understanding the ionic currents in nerve and muscle cells followed rapidly after the introduction of glass capillary microelectrodes by Ling and Gerard (1949). These electrodes made it possible to record routinely the membrane potentials within living cells. In fact, two microelectrodes are often inserted into a cell, the second electrode being used to control either the current or the voltage across the cell membrane (Moore and Cole, 1963). These two possibilities are illustrated in Fig. 5.1. On the left-hand side of this figure is shown the arrangement for passing constant currents of variable magnitudes into a cell. The magnitude is controlled schematically by a battery and a variable resistor which limits the current. The second microelectrode measures membrane potential. The cell body is assumed to be small enough that it is isopotential (i.e., there are no significant voltage gradients within the cell body). This assumption can be checked theoretically (Clark and Plonsey, 1966) or experimentally by measuring the voltage at different points. The spread of voltage in cells which are not isopotential will be considered in Chapter 6. When small currents are passed through the membrane, the voltage changes smoothly to a new value. From the steady state values measured in this way, a current-voltage curve such as shown in Fig. 4.3 or 4.4 can be measured. However, with currents that exceed a certain threshold voltage, a characteristic sequence of voltage changes, known as an action potential, takes place. The current-voltage curve can then no longer be measured with steady currents.
KeywordsIonic Current Cardiac Muscle Threshold Voltage Outward Current Potassium Current
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