Question

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

Altering the extracellular fluid with choline chloride may affect action potential amplitude and propagation rate due to changes in the ionic composition and the electrochemical gradient that governs depolarization.

Step-by-step explanation:

Replacing half of the NaCl in the extracellular fluid with choline chloride in a squid giant axon experiment would affect the action potential of the neuron. Since choline is a larger monovalent cation that does not impede the flow of Na+ through its channels, the primary ions responsible for generating an action potential, namely Na+ and K+, will still be able to move through their respective channels.

However, the overall ionic composition of the extracellular fluid will be altered, which might affect the amplitude or the rate at which action potentials are generated. This is due to potential changes in the electrochemical gradient for Na+, which drives the depolarization phase of the action potential. Depolarization occurs when voltage-gated sodium channels open, allowing Na+ ions to rush into the cell, making the interior less negative.

Subsequently, voltage-gated potassium channels open to allow K+ to exit, repolarizing the membrane. If choline affects the extracellular Na+ concentration and thus the gradient, the steepness of depolarization and the speed of action potential propagation could be affected.