Fig. 1. (A) Drawing of the brain of Melibe leonina. The two fused ganglia that constitute most of the brain are the cerebral (C) and pleural (P) ganglia (also referred to jointly as the cerebropleural ganglia). Near the large commissure that connects the two halves of the brain are the paired tentacular lobes (T). The left and right pedal ganglia (PD) are lateral to the cerebropleural ganglia. Interneurons of the swim central pattern generator (CPG) are indicated on the right half of this drawing (although the CPG is actually bilateral). Swim interneuron I (SiI) is located in the cerebropleural ganglion. It projects to the ipsilateral pedal ganglion, where it synapses on swim interneuron II (SiII) and swim motoneurons (indicated as filled circles in the left pedal ganglion). SiII synapses with motoneurons in the ipsilateral pedal ganglion and with SiII and motoneurons in the contralateral pedal ganglion via the pedal connective (PC), which encircles the esophagus in vivo. S, statocyst. (B) Typical recording from swim motoneurons in an isolated brain. Contralateral motoneurons fire in anti-phase, producing the alternating rhythmic bursting pattern characteristic of fictive swimming. lSMN, left swim motoneuron; rSMN, right swim motoneuron.
