A PAIR OF IDENTIFIED INTERNEURONS IN APLYSIA THAT ARE INVOLVED IN MULTIPLE BEHAVIORS ARE NECESSARY AND SUFFICIENT FOR THE ARTERIAL-SHORTENING COMPONENT OF A LOCAL WITHDRAWAL REFLEX

A bilateral pair of cerebral interneurons, called CC5, contribute to t he generation of a number of different behaviors involving head moveme nts. Each cell sends its axon to the ipsilateral and contralateral ped al and pleural ganglia. A weak tactile stimulus to the head excites th e ipsilateral CC5; a strong stimulus excites both the ipsilateral and contralateral cells. Firing of CC5 produces powerful shortening of the ipsilateral pedal artery (PA) by means of monosynaptic excitation of the pedal artery shortener (PAS) neuron, the single motor neuron for t he artery. A weak touch to a tentacle excites the ipsilateral PAS and evokes a local withdrawal response accompanied by shortening of the ip silateral PA. In vivo recording of the pedal artery nerve (PAn) showed that PAS was activated bilaterally during defensive head withdrawal e licited by a strong stimulus and was activated unilaterally by a weak stimulus. The responses were eliminated by cutting the ipsilateral cer ebral-pleural connective (C-PLC). Electrical stimulation of the cerebr al-pleural connective provided evidence that all of the excitatory inp ut to PAS via this connective is provided by CC5. A variety of experim ental results indicates that during a local withdrawal reflex of the t entacle, CC5 is necessary and sufficient for the unilateral PA-shorten ing component of the response and therefore functions as a command neu ron for a component of the behavior. The data suggest that during defe nsive head withdrawal, the two CC5 neurons may act conjointly as a two -neuron command system that is necessary and sufficient for the bilate ral arterial-shortening component of the behavior.