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In the "normal" direction of the stimulating current, + charges are delivered to the inside of the nerve membrane, and excitation occurs just outside the end of the tube, where current moves outward through the membrane. When the direction of current is reversed, excitation still occurs where current exits the axon, but that region is now inside the tube, further away from the recording tube.

Furthermore, in the region just outside the tube end, the current is directed inward, + charges pile up on the outside of the membrane; that region becomes hyperpolarized and requires more charge to be displaced if the impulse is to pass that zone. For these reasons the delay of the AP is increased and the AP is usually of less amplitude than the response evoked by the "normal" direction of the stimulus.

 
 
Figure 25. Effect of direction of stimulus current.

 
 
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