Question

A group of scientists are investigating the properties of the action potential in the squid giant axon. Specifically, they decide to place the axon in an extracellular solution that has a 10% higher concentration of K+ than normal.

They then measure an action potential at one end of the axon and note that it has a different shape. How would you expect the shape of this action potential to change given the extracellular manipulation? (Select all that apply). There are two correct answers
1. The rising phase is faster. 2. The rising phase is slower. 3. The falling phase is faster.
4. The falling phase is slower.

Answer 1

An increased extracellular K+ concentration would lead to a slower falling phase of the action potential, as K+ would leave the neuron more slowly due to a reduced concentration gradient. This manipulation would not directly affect the rising phase, which is driven by Na+ inflow.

Step-by-step explanation:

When considering how the action potential of a squid giant axon might change in an extracellular solution with a 10% higher concentration of K+ ions, we need to look at the effect of this concentration change on the diffusion of K+ ions during the repolarization phase of the action potential. In a normal setting, the concentration gradient for K+ across the membrane is the driving force for its efflux, leading to the repolarization of the neuron after the peak of the action potential.

If the external concentration of K+ is increased, the concentration gradient of K+ across the membrane is reduced, which would decrease the driving force for K+ to exit the neuron. Consequently, the -falling phase of the action potential would be slower because K+ would exit the neuron at a reduced rate, delaying the repolarization process (so Option 4 is correct). However, this change in K+ concentration would not directly affect the rising phase of the action potential, which is primarily driven by Na+ inflow. Therefore, there would be no expected acceleration in the rising phase (so Options 1 and 2 are incorrect). As the falling phase is slower, the return to resting membrane potential would also be delayed, making it less likely that hyperpolarization would be any faster. Therefore, Option 3 is incorrect as well.