Impale Cell \
FIGURE 2 Intracellular recording of the resting potential. (A) One input to a voltmeter is connected to a microelectrode, and the second input is connected to a reference electrode in the extracellular medium. No potential difference is recorded when the tip of the microelectrode is outside the cell. (B) When the tip of the microelectrode penetrates the cell, a resting potential of —60 mV is recorded. (Modified from Kandel, ER. The cellular basis of behavior. San Francisco: Freeman, 1976.)
a glass microelectrode that is connected to a suitable voltage recording device, such as a voltmeter, a pen recorder, or an oscilloscope. A glass microelectrode is nothing more than a piece of thin capillary tubing that is stretched under heat to produce a very fine tip having a diameter less than 1 mm. The electrode is then filled with an electrolyte solution such as KCl to conduct current. Initially, with the microelectrode in the extracellular medium, no potential difference is recorded, simply because the extracellular medium (the extracellular fluid) is isopotential. If, however, the microelectrode penetrates the cell membrane so that the tip of the microelectrode is inside the cell, a sharp deflection is obtained on the recording device (Fig. 2B). The potential suddenly shifts from its initial value of 0 mV to a new value of —60 mV. The inside of the cell is negative with respect to the outside. The potential difference that is recorded when a living cell is impaled with a microelectrode is known as the resting potential. The resting potential remains constant for indefinite periods of time as long as the cell is not stimulated or no damage occurs to the cell with impalement. The resting potential varies somewhat from nerve cell to nerve cell (—40 to —90 mV), but a typical value is about 60 mV.
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