Monopolar Nerve Cuff

A monopolar nerve electrode requires less hardware than a tripolar electrode and is simpler to implement. Tripolar cuff electrode (above in Figure) uses the two flanking contacts as 'anodes' and ,can create 'virtual cathodes' that may initiate action potentials (AP)
The monopolar electrode configuration simplifies design by having a single contact that acts as the 'cathode' and creates 'virtual anodes'. Relative current flow at the two ends of the electrode can be determined by the position of the contact within the insulating electrode sheath. The relative amplitudes of the stimulus pulse at the 'virtual anodes' allow arrest or escape of the propagating AP.

With monopolar cathode near the distal end of the electrode (open circles), maximum muscle force was generated below 1 mA current amplitude and AP propagation was arrested distally at higher amplitudes. With monopolar cathode near the proximal end of the electrode (asterisks), maximum muscle force was generated until 3 mA current amplitude.
Muscle force (normalized to the maximum force) in the cat Gastrocnemius during stimulation of the Sciatic nerve with a monopolar nerve cuff is shown in the figure. A 24 mm long cylindrical electrode was placed on the Sciatic nerve and isometric Gastrocnemius forces were recorded.
Open circles - monopolar contact was 3 mm from distal end of electrode and 21 mm from proximal end (schematic on left). Asterisks - monopolar contact was 21 mm from distal end of electrode and 3 mm from proximal end (schematic on right). Red ovals represent the distal Gastrocnemius muscle. Stimulus pulse were of 320 µsec plateau phase and falling phase time constant of 600 µsec.
The 'block window' is defined as the range of current amplitudes between complete arrest of propagation by a 'distal cathode' (point A on graph) to the onset of arrest by a 'proximal cathode' (point B on graph). Beyond point C on the graph, the 'virtual anodes arrested the APs at both ends of the electrode.