| The feeding behaviors performed by the buccal mass of Aplysia californica provide an appropriate subject for the study of the neural control of adaptive behavior. However, in order to understand the neural control of such a mechanically complex and flexible muscular structure, it is necessary to understand its mechanical properties. For this purpose we have developed an MRI interface and imaging sequence allowing realtime imaging of freely moving subjects. The interface acquires interleaved orthogonal images at a rate of 6 frames per second, allowing localization on freely moving subjects and a means to confirm slice position and orientation relative to the subject. The resulting realtime images were analyzed, and measurements of the kinematics of the buccal mass throughout the swallowing cycle were made. In addition, a kinematic model of the buccal mass was constructed whose input parameters were measured from the realtime MR images. The model includes a 3-dimensional reconstruction of the internal radula/odontophore which uses kinematic relationships derived from video of moving isolated odontophores to extract information about the 3-dimensional shape from parameters that can be measured in the mid-sagittal MR images. The model was verified by comparing the coronal MR images which were interleaved with the mid-sagittal images to coronal slices through the 3dimensional model and by comparing the lengths of the 13 and 12 muscles, previously measured in-vivo, to the lengths predicted by the model. Once verified, the model was used, together with the kinematic measurements made directly on the MR images, to characterize the 3-dimensional kinematics of the structure throughout the movement cycle, as well as to develop hypotheses about the mechanisms of the observed movements. The observed and modeled kinematics suggest that some of the internal structures of the buccal mass, including the 17 muscle, the radular stalk, and the 15 muscle, may have a context dependent function, i.e. they perform different behavioral functions depending on their mechanical context relative to their surrounding structures. These features add to the functional flexibility of the buccal mass, and also make a thorough understanding of its mechanical properties all the more critical for relating neural activity to behavior. |
