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.