Bipolar and Tripolar cuff electrodes.

A bipolar nerve cuff electrode has two contacts for current flow. One contact, the 'cathode', depolarizes the membrane (towards more positive potentials), where the Action Potential(AP) is generated. The other, an 'anode' injects current and hyperpolarizes the axon membrane potential (towards more negative potentials) and can arrest AP propagation.
Because current flows in all directions, pathways at the open end of the nerve cuff form 'virtual' contacts that act on the axons. A 'virtual cathode' outside the 'anodic' end of the electrode, and a 'virtual anode' outside the 'cathodic' end. APs may be generated at this 'virtual cathode' when the current flow at this location causes threshold depolarization.
In a tripolar cuff electrode, a central 'cathode' is flanked by a pair of 'anodes'. The total current to the 'cathode' is divided between these two 'anodic' contacts. Since there is current flowing to both anodes, the potential at each end is similar and there is a reduced tendency for current to flow outside the cuff. This ensures that most of the current is contained within the insulating walls of the electrode, limiting the formation of 'virtual' cathodes outside the two 'anodic' ends of the cuff. By distributing the currents unequally to the two anodes, one of them can arrest the 'cathode' initiated APs coming to it and the other can suppress the tendency for a virtual cathode to develop outside the arresting end yet allow the evoled AP to 'escape' and propagate at the other end, called the escape end.