Fig. 4.

Extracellular field created by an activation wave-front. At the exact region of the wave-front, axial current flows from the activated tissue to the resting tissue, creating a system representative of a propagating dipole. Transmembrane current then flows out of the membrane ahead of the wave, to return at the wave-back via newly activated sodium channels, thus forming a current loop. Consequently, the extracellular field is positive ahead of the wave-front and negative behind it