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.
Propagation and arrest
with Monopolar Electrodes.
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.