Fig. 7. Response of ascending interneurone (asc int) VE-1 to bath application of serotonin (5-HT). (A) Bath application of 1 mmol l^-1 serotonin for 3 min (indicated by thick bar) elicited no significant changes in the membrane potential of VE-1, although the tonically occurring spikes of the reductor motor neurone (red mn) were completely suppressed for long periods. (B) The excitatory response of VE-1 to sensory stimulation at 20 Hz of 11 stimuli (indicated by arrows) was increased after serotonin perfusion, although the excitatory level of VE-1 returned to its initial level after approximately 20 min of washing. The time indicated on each trace (Bi-vi) shows the elapsed time after serotonin application. The dashed line indicates the resting membrane potential level.