Fig. 3.

Time course of the excitability changes. In the top panel are extracellular records from a midlumbar interneuron with corresponding cord dorsum records. The top traces show that conditioning stimuli to the TA-EDL nerve at 5T delivered 30 msec before a 6 μA stimulus to the motor nuclei increased terminal excitability to give consistent antidromic activation (firing index = 1). Conditioning stimuli to the same nerve at 2.5T did not increase the firing index above 0. The graph below summarizes the time course and stimulus intensity dependence of this effect. The firing indices for stimuli delivered at different conditioning–test intervals are plotted for three different strengths of conditioning stimulation (5T, 3T, and 2T) as well as in the absence of conditioning stimulation (broken line). Firing indices were calculated from the frequency of antidromic activation in batches of at least 20 stimulus presentations at each strength and at each time point.