In the spinal cord A-delta fibers synapse with motor neurons in the ventral motor pathway, so firing generates voltage directly from the A-delta fibers and sub-threshold voltage from the motor fibers. In minutes a nurse can test all the major nerves and their branches in a region - 18 (9 bilateral) in the cervical and 14 (7 bilateral) in the lumbar region. The nerve(s) requiring the highest voltage to fire identifies the injured nerve(s). Once the injured nerve is identified, testing proximal and distal to a suspected site of injury easily verifies the location.
For example; testing above and below both right and left medical elbows detects cubital tunnel entrapment. The non-symptomatic side acts as a control. Comparing median nerve branches in the fingers with the radial nerve (back of the hand) allows differentiation between carpal tunnel entrapment and nerve root pathology since the median and radial nerves originate from the same nerve roots, C6-7. The palmar branches of the median and ulnar nerve pass over the wrist, not through the carpal tunnel or Guyon's canal respectively, so palmar sites differentiate between proximal and wrist entrapments. The same differentiation is at works in the lower extremity where, like the cervical study, all the lumbosacral sites are proximal to sites of entrapment in the ankle. Any branch of a cutaneous nerve can be tested to map the dysfunctional area.