Action potential

Experiment. The current pulse acts for 0 ≤ t < 2 ms, then switches off. At 6.3 °C, try 0 µA/cm² for rest, 2 for a weak response and 15 for a spike. Voltage and current share one time axis; the current strip has its own labelled scale. The maximum is the highest voltage so far, even when no spike occurs.

Controls. Start/Resume runs, Pause stops, One step advances one 0.001-ms step, and Reset restores the stationary initial state using the current inputs. The final step may be shorter. Physical edits invalidate the run; playback and view changes preserve time. Hidden tools pause both clocks. With reduced motion, a fresh Start prepares a paused state; Resume explicitly animates. Input drafts and view settings are saved, trajectories are not.

HH model. This homogeneous membrane uses inside-minus-outside V in mV, time in ms, current density in µA/cm², conductances in mS/cm² and capacitance in µF/cm². Positive current depolarizes. C=1, gNa=120, gK=36, gL=0.3; ENa=50, EK=−77, EL=−54.4 mV. C·dV/dt=I−INa−IK−IL, with INa=gNa·m³·h·(V−ENa), IK=gK·n⁴·(V−EK), IL=gL·(V−EL). Gates obey dq/dt=α(1−q)−βq: m is sodium activation, h sodium inactivation, n potassium activation. All six rates scale by φ=3^((T−6.3)/10); equilibrium gates stay unchanged and time constants divide by φ. Other parameters stay fixed. The stationary start is approximately −64.999722 mV.

Limits. Shared RK4 uses Δt=0.001 ms, stable rate limits at −40/−55 mV, exact pulse/end boundaries and 0.02-ms plot samples. Peaks are captured at integration steps. Nonfinite states or gates outside [0,1] stop without clipping. Inputs: 0–80 µA/cm², 0–30 °C, 5–100 ms, playback 0.5–20 model ms/second. A 512-step/0.05-second frame budget can slow playback. The passive 3D sketch colours one compartment uniformly: no spatial propagation or conduction speed is computed. This classical squid-membrane model is not a universal neuron model.

Sources: NEURON HH rates and Hodgkin–Huxley, modern-convention supplement. This tool retains EL=−54.4 mV rather than NEURON’s slightly different default.