Figure 5. Changes in psychophysical (i.e., subjective) and electrophysiologic (i.e., nociceptive reflex) pain thresholds to single and repeated (five stimuli at 2 Hz) electrical stimulation of the sural nerve after injection of the epidural solution (mean and SD of all measurements), expressed as percent of basal values. Means (SD) of all basal values of single and repeated stimulation were 13.9 (16.1) mA and 7.3 (3.0) mA, respectively. Clonidine significantly increased subjective and nociceptive reflex thresholds to single and repeated (temporal summation) electrical stimulation of the sural nerve. The threshold to nociceptive reflex was increased to a significantly larger extent than the subjective threshold. *P < 0.0001, compared with placebo. **P <0.05, compared with subjective threshold.