© 1974 by The Society for Integrative and Comparative Biology
Neuronal Substrates of Behavioral Hierarchies and Associative Learning in Pleurobranchaea
The Thimann Laboratories, University of California at Santa Cruz Santa Cruz, California 95064
The carnivorous marine mollusc Pleurobranchaea californica exhibits two forms of behavioral plasticity: behavioral choice and associative learning. Behavioral choice is the selection of one behavioral act in preference to another, as dictated by abehavioral hierarchy. In Pleurobranchaea, feeding behavior occupies a relatively dominant position in the behavioral hierarchy, but feeding is hormonally suppressed during egg-laying. Associative learning has been demonstrated in Pleurobranchaea by means of classical and avoidance conditioning of the feeding behavior.
As a step toward understanding the neural substrates of choice and learning, we have studied the neurophysiological organization of the feeding system. The feeding rhythm can be driven in the isolated nervous system by tonic electrical stimulation of certain peripheral nerves. Using this preparation, we have shown that feeding is accomplished by a network of identified neurons distributed in the brain and buccal ganglion. This network includes: (i) motoneurons in the brain and buccal ganglion; (ii) ascending interneurons that originate in the buccal ganglion, furnish excitatory and inhibitory postsynaptic potentials to feeding motoneurons, and send coordinating information to the brain; and (iii) descending interneurons that originate in the brain and are believed to exercise a "command" function in the feeding network. It is speculated that the putative feeding command interneurons play a crucial role in mediating behavioral hierarchies and associative learning.