FitzHugh–Nagumo/Noble-style cardiac toy: fast upstroke, plateau-like repolarization, refractory recovery gate, and S1-S2 premature-stimulus capture vs block.
This is a teaching model in the FitzHugh–Nagumo/Noble spirit, not a diagnostic ionic model. Use S2 interval and cell state to see refractory block, prolonged APD, and rapid re-excitation.
Live graphs
About this model
This simulator uses a compact cardiac excitable-cell toy model in the FitzHugh–Nagumo / Noble tradition. The dimensionless voltage u has a fast regenerative inward current h u²(1−u)/τ_in and a slower outward leak u/τ_out; the gate h represents availability of inward current and recovers slowly at rest. Displayed voltage is mapped to a ventricular-like scale V ≈ −85 + 125u mV. The first graph shows the action potential: rapid upstroke, plateau-like delayed repolarization, and return to rest. The second graph shows the recovery gate and stimulus train. An S1-S2 protocol paces the cell several times and then applies a premature extra stimulus; if the recovery gate is still low, S2 is blocked by refractoriness, while shorter refractory settings permit rapid re-excitation. The Long-QT preset prolongs APD and refractory time, connecting delayed repolarization to arrhythmia vulnerability.
Who it's for: Students in physiology, biophysics, biomedical engineering, or nonlinear dynamics learning cardiac excitability and refractory-period concepts before full ionic models.
Key terms
Cardiac action potential
Refractory period
FitzHugh-Nagumo
Noble model
APD90
S1-S2 protocol
Long QT
Arrhythmia
How it works
A compact cardiac action-potential simulator: fast excitation, plateau-like repolarization, slow recovery, and S1-S2 refractory testing.
Key equations
du/dt = I_stim + h u²(1−u)/τ_in − u/τ_out, dh/dt = (1−h)/τ_open at rest, dh/dt = −h/τ_close during excitation. Voltage display: V ≈ −85 + 125u mV.
Frequently asked questions
Is this a quantitative ventricular ionic model?
No. It is a two-variable teaching model designed to expose the geometry of excitation and recovery. Full cardiac models include many currents, calcium handling, cell-type differences, and tissue propagation.
What does APD90 mean here?
APD90 is the time from upstroke crossing to roughly 90% repolarization. In this toy it is measured on the dimensionless voltage and reported in milliseconds, so it should be read qualitatively: longer APD means a longer refractory window.
How does this relate to arrhythmias?
Arrhythmias often depend on timing: a premature beat may be blocked if tissue is refractory, or may propagate when some regions have recovered and others have not. The simulator shows the single-cell timing ingredient behind that tissue-level problem.