Exercise 6.8 - Measurement of Transmembrane Potential




  1. Using the recording device from Exercise 6.7, measure the voltage across the membrane with 0.1 M NaCl on both sides of the membrane.

  2. Add 1.0 ml of 1 M NaCl to the external chamber and mix gently. Immediately record the changes in voltage. 9

  3. When the change begins to stabilize, introduce 1.0 ml of KCl into the teflon cup and continue to monitor the voltage.

  4. Graph the change in voltage across the membrane with time and the addition of first NaCl and then KCl.

  5. Compare your results with the action potential of a typical vertebrate neuron.


The same apparatus as that used for measurement of resistance can be used to measure a potential, provided there is a difference in the salt concentration in the teflon cup and outside of the cup. This is best accomplished by the slight addition of a salt solution to the teflon cup, although rather elaborate procedures exist for complete substitution of the solutions (extreme care must be taken not to rupture the membrane).

In vertebrate nerve, the potential that is generated is from an influx of Na^+ followed by an efflux of K^+. These fluxes are mimiced by the addition of NaCl to the outer chamber, followed by an addition of KCl to the inner chamber.

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Cell Biology Laboratory Manual
Dr. William H. Heidcamp, Biology Department, Gustavus Adolphus College,
St. Peter, MN 56082 -- cellab@gac.edu