Biology / electrophysiology
A neuron fires and recovers in milliseconds. A ventricular heart cell holds a plateau. Explore the currents behind that difference, then test when each cell can fire again.
Beeler-Reuter ventricular cell · Hodgkin-Huxley neuron
Membrane potential
One cell's membrane voltage.
No tissue propagation or contraction model.
Pulse markers show the exact scheduled stimulus onsets.
Outward current is positive. Inward current is negative.
Preparing the baseline experiment.
| Cell | Peak | APD50 | APD90 | Second pulse |
|---|---|---|---|---|
| Run an experiment | ||||
APD is measured from maximum intrinsic upstroke to 50% or 90% repolarization toward the pre-stimulus voltage. A later stimulus can interrupt this measurement.
Recovery of excitability
Filled circles: second full response. Crosses: no full response. Hollow circles: indeterminate. The x axis uses milliseconds on a logarithmic scale. These are tested intervals, not an absolute refractory-period measurement.
| Cell | Interval (ms) | S2 / threshold | S2 (µA/cm²) | Outcome |
|---|
01 / Understand the plateau
Fast sodium entry drives the upstroke. In the ventricular model, a slower inward current, primarily carried by calcium, helps sustain depolarization while outward potassium currents oppose it. Repolarization follows as this balance changes.
The plateau and long recovery emerge from the equations. They are not drawn into the animation.
02 / Test a mechanism
Choose “Compare calcium conductances” to run 100%, 75%, 50%, and 25% of the baseline slow inward conductance. Compare the plateau and APD90, then run paired pulses at the chosen conductance.
Each condition recalibrates its resting excitation threshold. Exported methods include that threshold and the actual stimulus amplitudes.
03 / Interpret recovery
A cell may fail to generate a second full action potential at one stimulus strength and respond at another. Sweep intervals, then map second-pulse strength against interval to investigate recovery.
“No full response” applies to our stated response criteria and tested pulse. It does not prove that no stimulus could excite the cell.
This is a comparison of two published model systems: a mammalian ventricular model and the original squid-axon Hodgkin-Huxley kinetics. Their temperature assumptions differ. The results explain model behavior and are not a controlled comparison of human nerve and heart tissue.
Each heart trial begins with 1,000 ms of unstimulated conditioning; each neuron trial uses 100 ms. S1 begins at 10 ms. S2 begins after the selected onset-to-onset interval. Pulses cannot overlap. Each trial restarts from the same conditioned state.
Resting threshold is bracketed by doubling from 1 to at most 100 µA/cm², then estimated by 15 bisections at the chosen pulse duration. A full response must rearm below −40 mV, cross −20 mV with intrinsic upstroke above 5 mV/ms, and reach 0 mV. It must begin within 30 ms of the stimulus. This deliberately excludes passive plateau bumps and may exclude partial regenerative responses.
Both models use RK4 with a 0.01 ms maximum step, split at stimulus boundaries and output times. Samples are recorded every 0.05 ms. Playback reveals these computed samples without changing the solver. All sweeps use a fixed, documented grid; the first successful tested interval is not an exact boundary.
Beeler and Reuter, 1977 · Pinned Physiome reference and attribution (CC BY 3.0) · Hodgkin and Huxley, 1952 · Validation and reproduction notes · Explore the neuron