Control of locomotion in marine mollusc Clione limacina. II. Rhythmic neurons of pedal ganglia.

Activity from neurons in isolated pedal ganglia of Clione limacina was recorded intracellularly during generation of rhythmic swimming. To map the distribution of cells in a ganglion, one of two microelectrodes was used to monitor activity of the identified neuron (1A or 2A), while the second electrode was used to penetrate successively all the visible neurons within a definite area of the ganglion. In addition, pairs of neurons of various types were recorded in different combinations with each other. Intracellular staining of neurons was also performed. Each ganglion contained about 400 neurons, of which about 60 neurons exhibited rhythmic activity related to a swim cycle. These rhythmic neurons were divided into 9 groups (types) according to axonal projections, electrical properties and the phase of activity in a swim cycle. Three types of interneurons and six types of efferent neurons were distinguished. Type 7 and 8 interneurons generated only one spike of long (50-150 ms) duration per swim cycle. Type 7 interneurons discharged in the phase of the cycle that corresponded (in actual swimming) to the dorsal movement of wings (D-phase). Type 8 interneurons discharged in the opposite phase corresponding to the ventral movement of wings (V-phase). With excitation of type 7 interneurons, an IPSP appeared in the type 8 interneurons, and vice versa. Neuropilar branching of these neurons was observed in the ipsilateral ganglion. In addition, they sent an axon to the contralateral ganglion across the pedal commissure. Efferent neurons (i.e. the cells sending axons into the wing nerve) generated spikes of 1-5 ms duration. Type 1 and 3 neurons were excited in the D-phase of a swim cycle and were inhibited in the V-phase. Type 2 and 4 neurons were excited in the V-phase and inhibited in the D-phase. Type 10 neurons received only an excitatory input in the V-phase, while type 6 neurons received only an inhibitory input in the D-phase. Type 12 interneurons were non-spiking cells, they generated a stable depolarization ("plateau") throughout most of the V-phase. Neurons of the same type from one ganglion (except for type 6) were electrically coupled to each other. There were also electrical connections between most neurons firing in the same phase of the cycle, i.e. between types 3 and 7, as well as between types 2, 4 and 8. Type 7 interneurons from the left and right ganglia were electrically coupled, the same was true for type 8 interneurons.