DIFFERENT ROLES OF NEURONS B63 AND B34 THAT ARE ACTIVE DURING THE PROTRACTION PHASE OF BUCCAL MOTOR PROGRAMS IN APLYSIA-CALIFORNICA

The buccal ganglion of Aplysia contains a central pattern generator (C PG) that organizes sequences of radula protraction and retraction duri ng food ingestion and egestion. Neurons B63 and B34 have access to, or are elements of, the CPG. Both neurons are depolarized along with B31 /B32 during the protraction phase of buccal motor programs. Both cells excite the contralateral B31/B32 neurons and inhibit B64 and other ne urons active during the retraction phase. B63 and B34 also both have a n axon exiting the buccal ganglia via the contralateral cerebrobuccal connective. Despite their similarities, B63 and B34 differ in a number of properties, which reflects their different functions. B63 fires du ring both ingestion and egestion-like buccal motor programs, whereas B 34 fires only during egestion-like programs. The bilateral B63 neurons , along with the bilateral B31 and B32 neurons, act as a single functi onal unit. Sufficient depolarization of any of these neurons activates them all and initiates a buccal motor program. B63 is electrically co upled to both the ipsilateral and the contralateral B31/B32 neurons bu t monosynaptically excites the contralateral neurons with a mixed elec trical and chemical excitatory postsynaptic potential (EPSP). Positive feedback caused by electrical and chemical EPSPs between B63 and B31/ B32 contributes to the sustained depolarization in B31/B32 and the fir ing of B63 during the protraction phase of a buccal motor program. B34 is excited during the protraction phase of all buccal motor programs, but, unlike B63, it does not always reach firing threshold. The neuro n fires in response to current injection only after it is depolarized for 1-2 s or after preceding buccal motor programs in which it is depo larized. Firing of B34 produces facilitating EPSPs in the contralatera l B31/B32 and B63 neurons and can initiate a buccal motor program. Fir ing in B34 is strongly correlated with firing in the B61/B62 motor neu rons, which innervate the muscle (I2) responsible for much of protract ion. B34 monosynaptically excites these motor neurons. B34 firing is a lso correlated with firing in motor neuron B8 during the protraction p hase of a buccal motor program. B8 innervates the 14 radula closer mus cle, which in egestion movements is active during protraction and in i ngestion movements is active during retraction. B34 has a mixed, but p redominantly excitatory, effect on B8 via a slow conductance-decrease EPSP. Thus firing in B34 leads to amplification of radula protraction that is coupled with radula closing, a pattern characteristic of egest ion.