Swimming In The Pteropod Mollusc, Clione limacina II. Physiology*

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Abstract
The central pattern generator (CPG) for swimming inClione limacina was localized in cutting experiments. A separate pattern generator for each wing islocated in the ipsilateral pedal ganglion. The CPGs are tightly coupled butcan be isolated by severing the pedal commissure. Removal of the cerebralganglia results in a decrease in swim frequency and regularity suggesting descending modulation of the CPGs. Two classes of pedal neurones display firing patterns in phase with swimming movements. One class, swim motor neurones, are further divided intodepressor and elevator groups. Motor neurone recordings show complex subthreshold activity consisting of alternate depolarizations and hyperpolarizations. The complex activity is in antiphase in antagonistic motorneurones. Significant motor neurone-motor neurone interactions do notoccur centrally as neither electrical coupling nor chemical synaptic interactionscould be demonstrated. Injected currents do not alter the motorneurone firing rhythm or the swimming rhythm. Motor neurone cell bodies are located in the lateral region of the ipsilateralpedal ganglion, near the emergence of the wing nerve. Each motor neuroneprovides a single axon to the wing nerve which branches repeatedly in thewing. Each motor neurone has an extremely large innervation field, somecovering up to half of the wing area. The second class of pedal neurones that display firing patterns in phasewith either wing upswing or downswing are pedal-pedal inter neurones. Eachswim interneurone provides axon branches in both pedal ganglia and the axonruns in the pedal commissure. Interneurone axon branches occur in thelateral neuropile of each pedal ganglion, in the region of motor neurone branching.The swim interneurones presumably play a major role in bilateralcoordination of the wings and are involved in pattern generation since injectedcurrents were found to accelerate or slow the firing rhythms of interneuronesand motor neurones, and wing movements. Note: A significant portion of this work was conducted at Friday Harbor Laboratories, Friday Harbor, Washington, U.S.A.