Head waving in Aplysia californica. III. Interganglionic pathways underlying the coordination and control of searching movements.

The central nervous system of the sea hare Aplysia californica is divided into four pairs of ganglia and a fused abdominal ganglion. Work in several laboratories has established that the neural circuitry for producing locomotor rhythms is contained within the pair of pedal ganglia. The cerebropedal connective carries command-like information from the cerebral ganglia, and neurones projecting in the pedal commissure act to synchronize the retrograde pedal wave during locomotion. During head waving, the longitudinal muscles on the two sides of the body work in antiphase, in contrast to the bilateral synchrony of the locomotor rhythm, so we have investigated the roles of these two interganglionic pathways in the production and coordination of head-waving movements. Unilateral transection of the cerebropedal connective prevents animals from making normal horizontal bends towards the side of the lesion: specifically, animals could no longer keep the transverse axis of the head parallel to the substratum. We hypothesize that the increase in twisting about the long axis of the animal is due to decreased activation of longitudinal muscles on the lesioned side, whilst the control of the circular and helical muscles is maintained by the pedal commissure. Transection of the pedal commissure had no effect on individual head-waving movements, but the erect posture of the animal during head waving was disrupted, possibly because of an overall decrease in circular muscle activity. Double lesions (transection of one cerebropedal connective and the pedal commissure) caused a significant reduction in the animal's ability to extend its body during a bout of head waving, particularly on the side of the lesion. This suggests that, in addition to the activation of specific excitatory circuits for posture and movement, the interganglionic pathways are necessary for tonic inhibition of the motor circuits in the pedal ganglia.