Motorneuronal control of locomotion in Aplysia

We have carried out a combined behavioral and cellular analysis of escape locomotion in Aplysia. Using videotape recording we obtained a detailed description of the coordinated movements of the different regions of the foot and body during locomotion. Alternating waves of extension and longitudinal contraction begin at the head and propagate caudally through each pedal segment at a constant rate. Cobalt backfill of pedal nerves indicated that certain regions of the pedal ganglia were likely to contain motor neurons for the foot and body wall musculature. We examined these areas using intracellular techniques and identified three unique cells and three regional classes of neurons having clear motor effects on the foot and body wall. We also found that locomotion is driven by a central program. The basic locomotor pattern of the identified motor neurons and regional classes of motor neurons persists even after the circumesophageal ganglia have been isolated from the periphery. The motor neurons are not synaptically interconnected; patterned bursting during locomotor activity is produced by cyclic synaptic input. Because the locomotor system has large neurons favorable for cellular analysis and because locomotion is characterized by features of both stereotypy and flexibility, Aplysia promises to be useful for investigating the mechanisms underlying both the generation and modulation of a central program.