Passive hinge forces in the feeding apparatus of Aplysia aid retraction during biting but not during swallowing

Swallowing and biting responses in the marine mollusk Aplysia are both mediated by a cyclical alternation of protraction and retraction movements of the grasping structure, the radula and underlying odontophore, within the feeding apparatus of the animal, the buccal mass. In vivo observations demonstrate that Aplysia biting is associated with strong protractions and rapid initial retractions, whereas Aplysia swallowing is associated with weaker protractions and slower initial retractions. During biting, the musculature joining the radula/odontophore to the buccal mass (termed the "hinge") is stretched more than in swallowing. To test the hypothesis that stretch of the hinge might contribute to rapid retractions observed in biting, we analyzed the hinge's passive properties. During biting, the hinge is stretched sufficiently to assist retraction. In contrast, during swallowing, the hinge is not stretched sufficiently for its passive forces to assist retraction, because the odontophore's anterior movement is smaller than during biting. A quantitative model demonstrated that steady-state passive forces were sufficient to generate the retraction movements observed during biting. Experimental measures of the relative magnitude of the hinge's active and passive forces at the protraction displacements of biting suggest that passive forces are at least a third of the total force.