Question

Studies on the squid giant axon were instrumental to our current understanding of how action potentials are generated. You decide to do some experiments on the squid giant axon yourself.

A. You remove the cytoplasm in an axon and replace it with an artificial cytoplasm that contains twice the normal concentration of K+ by adding KOAc, where OAc– is an anion to which the membrane is impermeable. In this way, you double the internal concentration of K+ while maintaining the bulk electrical balance of the cytoplasmic solution. Will this make the resting potential of the membrane more or less negative?

B. You add NaCl to the extracellular fluid and effectively double the amount of extracellular Na+ ions. How does this affect the action potential?

C. You replace half of the NaCl in the extracellular fluid with choline chloride. (Choline is a monovalent cation much larger than Na+. Note that the presence of choline will not impede the flow of Na+ through its channels.) How will this affect the action potential?

Answer 1

A. ANS: Increasing the concentration of potassium in the cytoplasm of the squid axon will make the membrane potential more negative. Doubling the amount of potassium increases the driving force for potassium to move out of the cell, leaving the inside of the cell more negative and thus decreasing the membrane potential. (Remember, from the Nernst equation, the driving force for an ion across a membrane is proportional to the ratio of the concentration of the ion on the outside to the concentration of the ion inside.)

B. ANS: Doubling the amount of sodium in the extracellular fluid will increase the height of the peak of the action potential. Again, this is because now the driving force for sodium to enter the cell is greater than it was before. Thus, when sodium channels open, the flux of sodium ions is now greater. (Remember that flux is the number of ions entering per second.)

C. ANS: The action potential will reach a height that is less than that normally achieved. (Choline is added in this case to maintain bulk electrical neutrality. Because sodium channels aren't permeable to choline, they don't contribute to electrochemical gradient.) You have now halved the concentration of sodium and thus decreased the driving force for sodium to enter the cell.