Fig. 6. Application of the 5-HT/OA mixture (50 µmol l^–1 for 30-min) or 5-HT (50 µmol l^–1 for 30-min) changed the activity profile of fibers in the descending connectives (Conn) and altered the way in which this activity was recruited by brain command-like cells. (A) As is the case for cell Tr1 (Cellucci et al., 2000), intracellular depolarization of cell Tr2 triggered an increase in activity descending through a single hemiconnective (ca. 2800 fibers between M1 and M2) (Wilkinson and Coggeshall, 1975). This elevated activity long outlasted (by several seconds) the train of action potentials in cell Tr2, and appeared to represent the recruitment of other descending fibers in the brain that may also be involved in mediating the decision to swim or not to swim. (B) During treatment with the amine mixture, the basal activity in the hemiconnective increased. In addition, the delayed response of units in the connective to a spike train in cell Tr2 was no longer present. (C) A representative 1-s recording of descending activity from the brain in normal saline (left) and 1-s sampling of activity after 3 min of a 50 µmol l^–1 5-HT exposure. In all preparations examined (N=5), there was a recruitment of larger amplitude fibers during 5-HT application.