This example models a singles element cube with two different cell types, each with their own CellML model. Diagonally through the cube are lines of Hodkin-Huxley type cells (Purkinje-like cells) and the remainder of the tissue is modelled using the Noble 98 ventricular cell model. The image below shows the simulation results at 50 ms, with an iso-surface in the potential field drawn at Vm = 0 mV. You can imagine the Hodgkin-Huxley cells running through the middle each of the red arms of the iso-surface.

The Hodgkin-Huxley "Purkinje fibres" are stimulated with a 0.5 ms pulse at time 0 ms and the activation wave propagates out through the tissue.