Experiments from the assignment
Part 1 default: neuron at rest (−70 mV), a 15 mV shock at t = 0 fires an action potential; a second 15 µA pulse at 10 ms falls in the refractory period and fails.
Stimulus & ion parameters
vhold is recomputed from the chord-conductance rule of Part 2: vhold = 0.0945 ENa + 0.9055 EK (so changing EK shifts the resting potential exactly as the assignment has you calculate). Gates start at their steady-state values for vhold, then the voltage jumps by Δv at t = 0.
Gate story (Part 1 essay, in one paragraph)
Order of events: the m gates open first (fast Na⁺ activation) → Na⁺ rushes in and v shoots toward ENa = +45 mV. The h gates then close (Na⁺ inactivation) ending the upstroke, while the slow n gates open and K⁺ flows out, repolarizing the membrane and briefly overshooting to a hyperpolarized after-potential (v approaches EK). A second shock at 10 ms fails because h is still low (Na⁺ channels inactivated) and n is still high (K⁺ conductance elevated) — the absolute refractory period. Later, a stronger-than-threshold stimulus can fire again (relative refractory period). In hyperkalemia the depolarized rest potential fires once then blocks (inactivated Na⁺ channels) — why IV KCl is lethal; in hypokalemia the hyperpolarized rest slows the approach to threshold. With a sustained current, spike frequency codes stimulus intensity until adaptation silences the train.