PROPRIOCEPTIVE INPUT TO FEEDING MOTOR PROGRAMS IN APLYSIA

Although central pattern generators (CPGs) can produce rhythmic activi ty in isolation, it is now generally accepted that under physiological conditions information from the external and internal environment is incorporated into CPG-induced motor programs. Experimentally advantage ous invertebrate preparations may be particularly useful for studies t hat seek to characterize the cellular mechanisms that make this possib le. In these experiments, we study sensorimotor integration in the fee ding circuitry of the mollusc Aplysia. We show that a premotor neuron with plateau properties, B51, is important for generating the radula c losing/retraction phase of ingestive motor programs. When B51 is depol arized in semi-intact preparations, radula closing/retractions are enh anced, When B51 is hyperpolarized, radula closing/retractions are redu ced in size. In addition to being important as a premotor interneuron, B51 is also a sensory neuron that is activated when the feeding appar atus, the radula, rotates backward. The number of centripetal spikes i n B51 is increased if the resistance to backward relation is increased . Thus, B51 is a proprioceptor that is likely to be part of a feedback loop that insures that food will be moved into the buccal cavity when difficulty is encountered. Our data suggest, therefore, that Aplysia are able to adjust feeding motor programs to accommodate the specific qualities of the food ingested because at least one of the neurons tha t generates the basic ingestive motor program also serves as an on-lin e monitor of the success of radula movements.