The action potential of the squid giant axon is formed by just two voltage-dependent conductances in the cell membrane, ... H. Phase plane trajectories of the muscle spike potential. Biophys.

An action potential occurs when the cell reaches this threshold, at which point the neuron is said to "fire" or "spike" as an electrical impulse ripples through its extremity.

An action potential occurs when the cell reaches this threshold, at which point the neuron is said to "fire" or "spike" as an electrical impulse ripples through its extremity.