Central pattern generator for escape swimming in the Notaspid sea slug Pleurobranchaea californica

Escape swimming in the notaspid opisthobranch Pleurobranchaea is an episode of alternating dorsal and ventral body flexions that overrides all other b ehaviors. We have explored the structure of the central pattern generator ( CPG) in the cerebropleural ganglion as part of a study of neural network in teractions underlying decision making in normal behavior. The CPG comprises at least eight bilaterally paired interneurons, each of which contributes and is phase-locked to the swim rhythm. Dorsal flexion is mediated by hemig anglion ensembles of four serotonin-immunoreactive neurons, the As1, As2, A s3, and As4, and an electrically coupled pair, the A1 and A10 cells. When s timulated, A10 commands fictive swimming in the isolated CNS and actual swi mming behavior in whole animals. As1-4 provide prolonged, neuromodulatory e xcitation enhancing dorsal flexion bursts and swim cycle number. Ventral fl exion is mediated by the A3 cell and a ventral swim interneuron, I-VS, the soma of which is yet unlocated. Initiation of a swim episode begins with pe rsistent firing in A10, followed by recruitment of As1-4 and Al into dorsal flexion. Recurrent excitation within the As1-4 ensemble and with A1/A10 ma y reinforce coactivity. Synchrony among swim interneuron partners and bilat eral coordination is promoted by electrical coupling among the A1/A10 and A s4 pairs, and among unilateral As2-4, and reciprocal chemical excitation be tween contralateral As1-4 groups. The switch from dorsal to ventral flexion coincides with delayed recruitment of A3, which is coupled electrically to Al, and with recurrent inhibition from A3/I-VS to A1/A10. The alternating phase relation may be reinforced by reciprocal inhibition between As1-4 and I-VS. Pleurobranchaea's swim resembles that of the nudibranch Tritonia; we find that the CPGs are similar in many details, suggesting that the behavi or and network are primitive characters derived from a common pleurobranchi d ancestor.