The nudibranch Melibe leonina swims by rhythmically flexing its body from side to
side at a frequency of 1 cycle every 2-5 sec. Melibe swim spontaneously, when they
are dislodged from the substrate, or when they come in contact with predatory seastars,
such as Pycnopodia helianthoides. Intracellular recordings obtained from semi-intact
swimming Melibe reveal a population of similar to15 swim motoneurons (SMNs) in each
pedal ganglion. In general, SMNs in one pedal ganglion fire out-of-phase with SMNs
in the opposite pedal ganglion, resulting in rhythmic side-to-side bending movements.
In isolated brains, recordings from SMNs yield similar results, indicating the existence
of a swim central pattern generator (CPG). There is no evidence for synaptic interactions
between SMNs and either inhibiting or exciting SMNs has no impact on the swim pattern.
The SMNs are driven by a CPG consisting of 4 interneurons; 2 in the cerebropleural
ganglia and 1 in each pedal ganglion. Appropriate bursting activity in the swim interneurons
is necessary for swimming to occur. Either hyperpolarization or depolarization of
any of the 4 CPG interneurons disrupts the normal swim pattern. Swimming behavior,
and the fictive swim motor program expressed by the isolated brain, are inhibited
by light and nitric oxide donors. NADPH-diaphorase staining and nitric oxide synthase
(NOS) immunocytochemistry of Melibe brains suggests the source of nitric oxide might
be a pair of bilaterally symmetrical cells located in the cerebropleural ganglia.
