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When an impulse invades the freshly cut end, the advancing negative "sink" at the center of the impulse draws leading-edge current; that provides the voltage drop seen as the initial upward deflection of the action potential. As the depolarized zone penetrates further toward the end of the nerve in the recording tube, it encounters the region from which current is already flowing toward the cut end. Note the injury currents are directed opposite to the leading-edge currents. The leading-edge currents, drawn from a region that is only partially depolarized, are nevertheless of respectable magnitude. In the region of lowered membrane potential, the impulse becomes smaller and draws less current, so as it terminates, the longitudinal gradient is finally determined wholly by the existing injury potential; there is no reversal of current direction and the AP is monophasic. In effect, the reversal current is already present as injury current, and the advancing impulse can contribute no more.

 
 
Figure 18. Healed and cut end records.

 
 
To help yourself understand the currents at the nerve end, consider Figure 18A.  
 
Figure 18A. Healed-cut end challenge.

 
 
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