Hodgkin-Huxley Fast Subsystem

1. Explain why replacing the extracellular Na⁺ with choline has little effect on the resting potential of an axon. Calculate the new resting potential with 90% of the extracellular Na+ removed. Why is the same not true if K⁺ is replaced? (Assume the conductances are constant.)

2. Show how the Hodgkin-Huxley fast subsystem depends on the slow variables: i.e., show how the v nullcline moves as n and h are changed, and demonstrate the saddle-node bifurcation in which v_e and v_s disappear.