This page needs JavaScript to run the live model. In words: a resting neuron is about 70 mV negative inside. A push past threshold opens voltage gated sodium channels, sodium rushes in and the inside flips positive to about +30 to +40 mV, then potassium channels open, potassium flows out and the voltage falls, dipping below rest before settling. Myelinated nerve fibres carry this signal at up to about 120 m/s, bare ones at about 2 m/s.