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

ABSTRACT The central pattern generator (CPG) for swimming in Clione limacina was localized in cutting experiments. A separate pattern generator for each wing is located in the ipsilateral pedal ganglion. The CPGs are tightly coupled but can be isolated by severing the pedal commissure. Removal of the cerebral ganglia 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 into depressor and elevator groups. Motor neurone recordings show complex subthreshold activity consisting of alternate depolarizations and hyperpolarizations. The complex activity is in antiphase in antagonistic motor neurones. Significant motor neurone-motor neurone interactions do not occur centrally as neither electrical coupling nor chemical synaptic interactions could be demonstrated. Injected currents do not alter the motor neurone firing rhythm or the swimming rhythm. Motor neurone cell bodies are located in the lateral region of the ipsilateral pedal ganglion, near the emergence of the wing nerve. Each motor neurone provides a single axon to the wing nerve which branches repeatedly in the wing. Each motor neurone has an extremely large innervation field, some covering up to half of the wing area. The second class of pedal neurones that display firing patterns in phase with either wing upswing or downswing are pedal-pedal inter neurones. Each swim interneurone provides axon branches in both pedal ganglia and the axon runs in the pedal commissure. Interneurone axon branches occur in the lateral neuropile of each pedal ganglion, in the region of motor neurone branching. The swim interneurones presumably play a major role in bilateral coordination of the wings and are involved in pattern generation since injected currents were found to accelerate or slow the firing rhythms of interneurones and motor neurones, and wing movements.