Fig. 5. Amine modulation of command-like neurons known to activate or inhibit swimming. Bath application of a mixture of 5-HT and OA (50 µmol l^–1) caused the inhibition of cell Tr1 (Brodfuehrer and Friesen, 1986b) and SIN1 (Brodfuehrer and Burns, 1995). (A) During perfusion of a saline baseline (left), swim-trigger neuron Tr1 fired a train of action potentials just before a swim episode (see swim motor neuron bursts in the DP nerve extracellular recording). After a 30-min application of the 5-HT/OA mixture (middle), Tr1 became hyperpolarized (5 mV) and fired no action potentials. Furthermore, Tr1 appeared to receive less synaptic activity (i.e. fewer small and rapid fluctuations in membrane potential) following mixture application. During washout of the mixture (with saline, right), Tr1 partially repolarized and synaptic inputs to Tr1 resumed. Tr1 spiked once preceding the onset of a swim bout. (B) During perfusion of a saline baseline (left), swim-inhibitory neuron SIN1 fired action potentials at a constant rate. After a 30-min application of the mixture (middle), the membrane potential of SIN1 became inhibited (18 mV), and large, rhythmic inhibitory inputs were observed. During washout of the mixture (right), SIN1 partially repolarized and rhythmic membrane potential fluctuations were phase-locked with swim-motor bursts in the DP nerve.