Article Figures & Data
Figures
- Fig. 1.
Changes in threshold for antidromic activation after conditioning afferent stimulation. The recordings shown are intra-axonal records (three traces superimposed) from a midlumbar interneuron. In the top traces, stimuli of 11 μA delivered to the lower lumbar motor nuclei (MN) were subthreshold for antidromic activation. The same stimuli evoke antidromic spikes when preceded, 35 msec earlier, by a single stimulus to the quadriceps nerve at 5× threshold (Q 5T). This conditioning stimulus generates one to two orthodromic spikes at short latency. The inset shows the antidromic spikes on a faster timebase, where they can be distinguished from the stimulus artifact.
- Fig. 2.
Intracellular recordings from a midlumbar interneuron. Four sets of records each show three superimposed sweeps. In each record, the top trace is an intracellular recording, and the bottom trace a recording from the cord dorsum at the L6–L7 junction. A 9 μA test stimulus was delivered to the motor nucleus in all traces. Antidromic spikes were evoked when the interval between the conditioning stimulus to the quadriceps nerve (5T) preceded the test stimulus by 10 and 15 msec but not 5 msec. Arrowsindicate the onset of the conditioning stimulus.
- 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.
- Fig. 4.
Excitability increases brought about by intraspinal stimuli at intensities subthreshold for spike generation. The left panel shows the initial stimuli of a train delivered to the motor nucleus (23 μA). The plots show for three different cells the firing index for each successive stimulus in a train (5 msec intervals). In each case, the firing index increases with successive stimuli from close to 0 to close to 1.
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