Action Potential Propagation Simulator

Axon Model

Stimulus strength 1.4×

Excitability (channel density) 1.0×

Refractory period 4 ms

Probe V_m

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Conduction speed

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Spikes fired

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Time since spike

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How it works

A resting axon sits near V_m = −70 mV. A stimulus above threshold (≈ −55 mV) opens voltage-gated Na⁺ channels, driving rapid depolarization toward +40 mV; K⁺ channels then open and Na⁺ channels inactivate, producing repolarization back past rest (a brief undershoot) before recovery. Each patch of membrane is modelled as a node on a discrete cable: current from a firing node depolarizes its neighbours through the axoplasm, so once threshold is crossed anywhere the spike regenerates itself down the whole length — a travelling wave, not a signal that decays with distance.

dV/dt ≈ D·∇²V − I_ion(V) + I_stim
node fires when V ≥ V_threshold
after firing: node is refractory for T_refr

Stimulus strength — current injected at the left end; below threshold nothing propagates (subthreshold, decays locally), above it a full spike launches.
Excitability — scales channel density / threshold; higher values fire more readily and propagate faster.
Refractory period — how long a node stays unresponsive after firing, which sets the minimum spacing between spikes and enforces one-way propagation.
Myelinated — jumps the wave between widely-spaced "nodes of Ranvier" instead of continuous cable conduction (saltatory conduction), the reason myelinated axons conduct far faster than bare ones.

Real-world relevance: this all-or-nothing regenerative propagation is how every nerve impulse in the body travels — from a stubbed toe reaching the spinal cord to a motor command reaching a muscle — and demyelinating disease (e.g. multiple sclerosis) works by disabling exactly the saltatory speed-up modelled here.

Depolarized (peak) Resting Refractory