. Biophysical research methods. Biophysics -- Research. VIII. BIOELECTRIC MEASUREMENTS 241 aritlim of the K+ concentration outside the cell as a function of the membrane potential, a straight line should be obtained. Figure 2 shows the results of a set of measurements taken on the giant nerve cell of the squid. Here the membrane potential is plotted against the logarithm of the K+ concentration outside the cell. When the K+ concentrations inside and outside are equal, there is no membrane potential. However, as the K+ concentration outside the cell is reduced, the negative potential of the cyt
. Biophysical research methods. Biophysics -- Research. VIII. BIOELECTRIC MEASUREMENTS 241 aritlim of the K+ concentration outside the cell as a function of the membrane potential, a straight line should be obtained. Figure 2 shows the results of a set of measurements taken on the giant nerve cell of the squid. Here the membrane potential is plotted against the logarithm of the K+ concentration outside the cell. When the K+ concentrations inside and outside are equal, there is no membrane potential. However, as the K+ concentration outside the cell is reduced, the negative potential of the cytoplasm of the cell increases in a straight line until cjuite a low value of K+ concentration is reached, and then this relationship breaks down. Why the Nernst -20 Fig. 2. Membrane potential from the squid giant axon, meas- ured between one electrode in- side the cell and another out- side. The potential is measured as a function of the relative K "*" concentration outside the cell compared to the normal K"*" concentration of sea water (12).. + 60. RELATIVE POTASSIUM 10 100 CONCENTRATION relation is not followed beyond this point is not known, but presum- ably some of the postulates are no longer valid. Experiments such as these give us considerable confidence that the above explanation for cell potentials is correct in a general way. 2. Electrodes In making potential measurements in biological systems it is al- ways necessary to make an electrical connection between the meas- uring device and the electrolytic solution of the biological system. This in\()lves a metal-to-liquid junction. Ideally one could think of finding a metal that would show no potential difference between it- self and an aqueous solution. This appears to be impossible and the pi'oblem has resolved itself into one of finding an electrode that will give constant and repi()ducil)le potentials when placed in contact with any salt- solution. When a metal is placed in water there is a
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