Motor program for pedal waves during Aplysia locomotion is generated in the pedal ganglia

Abstract

The activity in the nerves innervating the foot of Aplysia was examined during pedal wave generation. Cyclic patterned discharges (i.e. bursting) in the pedal nerves was associated with the pedal wave. Individual units exhibited bursting during pedal wave generation, but fired tonicly when the pedal wave was absent. Bursting in the nerves persisted after the foot was deafferented, confirming previous behavioral results that pedal waves were the result of a centrally generated motor program. Deafferentation caused increased burst durations and decreased spike frequencies within each burst. This suggests that sensory input from the foot excites the oscillator which underlies pedal wave generation and serves to increase the amplitude of the oscillations. Bursting in the nerves persisted after surgically isolating the pedal ganglia, showing that the neural circuitry necessary for pedal wave generation resides in the pedal ganglia. After isolating the pedal ganglia, bursts in the nerves were longer, less vigorous and less frequent. This suggests that input from the other central ganglia affects both the amplitude and the period of the oscillator. The firing of units in homologous pedal nerve branches, showed similar, but not identical patterns with the pedal commissure intact. Individual homologous units differed in both absolute timing and frequency of firing. After cutting the commissure, synchronization of activity in the homologous nerves was lost. This indicates that bilateral coordination of the pedal wave motor programs generated in each pedal ganglion is maintained via the pedal commissure. The possible mechanisms involved are considered.