EMG Response

Asymmetric current injection, with more current flowing in the distal 'blocking anode' could arrest APs at lower amplitudes, resulting in decrease in the rectified, integrated EMG response.
The rectified, integrated EMG responses (excluding any anodic break response) are shown in the figure as a function of the stimulus amplitude, from two different experiments, on the left and right.
The proximal 'anode' was 8 mm from the 'cathode', and the distal 'anode' was 6 mm away. Rectangular pulses of 8 mS duration were applied. The maximum response was determined with 100µsec square pulses. The balance refers to the fraction of the total applied current that is flowing through the proximal 'anode'. The EMG was recorded from the medial Gastrocnemius with bipolar intramuscular electrodes, digitized and processed.
On the left, at zero balance( solid line, no current through the distal, 'blocking anode') APs were not arrested. With increasing current in the distal ‘anode’, EMG diminished and then disappeared. On the right, the direct response returns at higher amplitudes from the 'virtual cathode' initiated APs. The experimental results, shown in the figure, indicate that the current required to arrest APs increases as a larger percent of the current is diverted to the escape anode. These results also show that window between when arrest occurs and the virtual cathode become sufficient to cause excitation increases as a greater proportion of the anodic current is delivered to the escape anode.

Monophasic (above) and balanced biphasic (bottom) blocking current waveforms applied to tripolar nerve cuff electrode. Total current divided unequally between proximal and distal contacts ('anodes'). The central contact (the 'cathode') receives the opposite current phase, negative stimulus pulse followed by positive reversal current (if any). In the bottom figure, the reversal current I_r shown exaggerated for clarity.

Force developed in the Soleus muscle (Fs) by spinal reflex from skin afferents on the other hind limb could be blocked by pulse trains on the Soleus motor nerve(left side of figure). In a decerebrate preparation, cutaneous stimulation of the contra-lateral hind limb developed a background force in the Soleus (S) and Medial Gastrocnemius (MG) muscles. Application of a 2 second train of blocking pulses (3.0 mA, 0.5 mS plateau phase, 0.7 mS decay time constant) to the nerve to the Soleus, during this procedure, resulted in a 80% force reduction at 20 Hz. (right top) and a 90% reduction at 30 Hz. (right bottom)
Decerebration was done at the level of the superior colliculi. All nerve branches of the Sciatic beyond the poplitial fossa, other than branches to the Soleus and Medial gastrocnemius were severed. A blocking electrode was placed on the Soleus nerve branch. Isometric forces were recorded from the two muscles.

van den Honert, C. and J. T. Mortimer (1981). “A technique for collision block of peripheral nerve: frequency dependence.” IEEE Trans Biomed Eng. 28(5): 379-82.