Fig. 2. Serotonergic neurons of the head brain were activated by swim-initiating stimuli and remained active throughout swimming. An electrical stimulus (100 ms, 3–5 V pulses delivered for 1 s) delivered to a posterior DP nerve has previously been shown to initiate swimming (Hashemzadeh-Gargari and Friesen, 1989). (A) Electrical shock (black bar) delivered to a DP(16) nerve excited the serotonergic LL cells of the first neuromere of the SEG (left). In addition, LL received rhythmic synaptic input correlating with individual swim-motor bursts during the expression of a spontaneous swim episode (right). (B) SEG Retzius cell (Rz) is excited by a swim-initiating electrical shock to a DP(16) nerve (left), and was persistently depolarized throughout the duration of the swim episode. During a spontaneous bout of swimming, the RZ fired above 1 Hz at the onset of swimming and remained active at a relatively high firing frequency (5 Hz or greater) (right). These data are consistent with 5-HT being secreted in the brain during both swim-initiation and swim episode maintenance via the somatic release of 5-HT (De-Miguel and Trueta, 2005).