Fig. 2. (  A ) Criteria for antidromic stimulation. Antidromic action potential occurs at a constant latency following the stimulus artifact. The dot marks the stimulus artifact. The second row shows antidromic action potentials following a high frequency train of 250 Hz and an antidromic action potential collided with an orthodromic stimulus elicited by stimulating the receptive field. (  B ) Cross-section of the first cervical segment depicting the contralateral stimulation sites for antidromic activation of lumbar dorsal horn neurons. (  C ) Cross-section of the spinal cord at the ipsilateral side of the fifth lumbar segment depicting the depths for recordings of each dorsal horn neuron. Each filled circle represents one stimulation or recording site for one neuron.