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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).
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