Control of pedal and parapodial movements in Aplysia. I. Proprioceptive and tactile reflexes

Abstract

The foot of A. californica is primarily innervated by nerves from the pedal ganglia. Three pedal nerves and their branches supply the anterior, middle and posterior portions of the foot. The posterior midline is innervated by a nerve from the parapedal commissure. The parapodia are innervated by nerves from both the pedal and pleural ganglia. Each parapodium is supplied by branches of 3 ipsilateral pedal nerves and branches of a pleural-pedal anastomosis. The left parapodium is innervated by a nerve which is unique to the left pleural ganglion. The hemisected foot of Aplysia and the isolated parapodia both exhibited spontaneous waves of contraction. Most but not all of the contractions in the latter were due to respiratory pumping and were abolished by cutting the pleuroabdominal connectives. The contractions in both halves of the hemisected foot and the isolated parapodia remained closely coordinated. Bilateral coordination is probably controlled by the CNS and does not require a peripheral nerve net. The activity of units in the parapedal commissure nerve was correlated with pedal contractions and provided an accurate index of foot activity. The motor program for generating these waves originates in the CNS. Pedal and parapodial contractions could be elicited by proprioceptive stimuli. Stretching a hemifoot, a portion of a segmented foot, or an isolated parapodium all caused pedal contractions both bilaterally and in other foot segments. Similarly stretching a parapodium or a hemifoot caused both parapodia to contract. For both foot and parapodia, direct and crossed reflexes are present and reciprocal pedal and parapodial reflexes are present. These are mediated by the CNS and do not require a peripheral nerve net for their coordination or spread. Pedal and parapodial contractions could be elicited by tactile stimuli. Touching the anterior tentacles caused bilateral contractions of the foot and parapodia. These contractions could be obtained by touching either the foot or parapodia. Parapodial contractions could also be elicited by stimulating the siphon. The latter were abolished by cutting the pleuroabdominal connectives. Multiple command inputs are probably present for the pedal and parapodial motor systems. Proprioceptive reflexes may contribute to pedal and parapodial coordination during locomotion. Tactile reflexes appear to be part of the general defensive withdrawal behavior exhibited by Aplysia.