Fig. 2. Differences in the sensitivity to tetrodotoxin (TTX) of action potentials in unmyelinated human nerve fibers induced by stimulation with or without a ramp current. (  A ) Stimulus waveforms and representative superimposed examples of compound action potentials. Two stimulus conditions (no ramp, ramp; see  fig. 1A; stimulus interval 2 s) were tested in the control solution and during application of tetrodotoxin. (  B ) Continuous recording of peak height evoked by the two stimulus conditions (no ramp, ramp) before, during, and after application of tetrodotoxin (90 nm). Also illustrated is the effect of tetrodotoxin on the latency (lat.) to 50% of the action potential produced by the 1-ms current pulse (no ramp). Note that action potentials at the end of the current ramp are more sensitive to the blocking effect of tetrodotoxin. Asterisks (*) indicate the short pulse in the stimulus condition and the corresponding stimulus artifact in the recordings of compound action potentials.