Neural control of swimming in Aplysia brasiliana. II. Organization of pedal motoneurons and parapodial motor fields.

1. We have examined the locations and functional properties of a large number of motoneurons in the pedal ganglia of Aplysia brasiliana. These neurons control movement of the parapodia and body during swimming. We have grouped the motoneurons into classes based on several criteria, including the topology of the cells and their axons, the properties of their peripheral motor fields, and their phasic activity during an induced swim motor program. 2. A total of 410 motoneurons were analyzed. There are at least 16 distinguishable motor fields in the parapodia, based on the region affected, direction of contraction, and phase of neuronal activity during fictive swimming. 3. Motoneurons for each motor field tend to appear in the same region of the ganglion in different preparations. 4. Most motoneurons have only ipsilateral effects. About 1% cause contralateral contraction, and they project directly to the contralateral parapodium. 5. Three types of motoneuron are described that cause parapodial expansion. 6. Two other groups of motoneurons were found that innervate either the columellar muscle or longitudinal foot muscles. 7. Almost all motoneurons fired rhythmically during fictive swimming, including those controlling foot and columellar muscle.