An identified interneuron contributes to aspects of six different behaviors in Aplysia

Previous results have indicated that the bilateral cerebral interneuron CC5 mediates the pedal artery shortening that is a component of defensive withdrawal responses involving the head. Current studies suggest that CC5 contributes to aspects of at least six different behaviors: locomotion, head turning, defensive head withdrawal, local tentacular withdrawal, rhythmic feeding, and head lifting. In addition to receiving input from mechanoreceptors in the head, CC5 receives synaptic input during fictive locomotor and feeding programs. Firing of CC5 produces widespread monosynaptic or polysynaptic actions in all ganglia in the animal. CC5 excites presumptive motor neurons for the neck, and its activity can contract neck muscles. The pedal artery shortener motor neuron (PAS), a key excitatory follower cell of CC5, fires during ipsilateral head turning, head withdrawal, tentacle withdrawal, feeding, and locomotion. For all behaviors, except locomotion and biting, responses of PAS were eliminated by cutting the ipsilateral-pleural connective, which interrupts the only direct connection of CC5 to the ipsilateral PAS. The data suggest that CC5 is a multifunctional interneuron that plays different roles during different behaviors. The neuron appears to be involved in producing coordinated movements of the head, involving both somatic and visceral muscles. For some behaviors, or for certain aspects of behaviors, CC5 appears to act as an individual command-like neuron; for other behaviors, CC5 appears to act more as an element of a distributed circuit and is neither necessary nor sufficient for any aspects of the behavior.