Selective Activation – Animal Experiments

Example of Fascicular Selectivity: Cathodic Steering Current


Shown here is an example where cathodic steering was used to activate selectively the medial gastrocnemius branch of the sciatic nerve. At the time of the experiment, the only thing known is that the contacts are ~90˚ apart on the sciatic nerve. When stimuli are applied to the contacts the torque pathway can be compare to the torque recorded when stimuli are applied to electrodes placed on each of the four motor branches. The location of the contacts on the sciatic nerve, relative to the fascicles, can be imagined by knowing there are four fascicles contained in the cuff and that the contacts are spaced ~90˚ apart around the nerve. Verification can be made at the termination of the experiment when the sciatic nerve and cuff electrode are excised and sectioned.
Stimuli applied to the 90˚ contact produced the torque pathway shown here. An educated guess would suggest that stimuli applied to this contact activated the tibial branch. Stimuli applied to the 180˚ contact followed a pathway in the direction produced by the two heads of the gastrocnemius and soleus. This pathway exceeded the torque produced by the lateral gastric and likely included the medial gastric branch. Adding the MG torque to the LG torque produced a result that is consistent with this idea.
In this case, selective activation of the medial gastroc was obtained by cathodic steering, that is drawing, the excitatory field toward the medial gastroc, away from the lateral gastroc, with a cathodic stimulus applied to the 90˚ contact.

Tarler, M.D. and J.T. Mortimer, “Selective and Independent Activation of Four Motor Fascicles Using a Four Contact Nerve Cuff Electrode”, IEEE Trans. on Rehabilitation Engineering, Vol 12, pp251-257. 2004.