The Feeding CPG in Aplysia Directly Controls Two Distinct Outputs of a Compartmentalized Interneuron that Functions as a CPG Element

In the context of motor program generation in Aplysia, we characterize several functional aspects of intraneuronal compartmentalization in an interganglionic interneuron, CBI-5/6. CBI-5/6 was shown previously to have a cerebral compartment (CC) that includes a soma that does not generate full-size action potentials, and a buccal compartment (BC) that does. We find that the synaptic connections made by the BC of CBI-5/6 in the buccal ganglion counter the activity of protraction-phase neurons and reinforce the activity of retraction-phase neurons. In buccal motor programs, the BC of CBI-5/6 fires phasically and its premature activation can phase advance protraction termination and retraction initiation. Thus, the BC of CBI-5/6 can act as an element of the central pattern generator (CPG). During protraction, the CC of CBI-5/6 receives direct excitatory inputs from the CPG elements, B34 and B63 and during retraction it receives antidromically propagating action potentials that originate in the BC of CBI-5/6. Consequently, in its CC, CBI-5/6 receives depolarizing inputs during both protraction and retraction and these depolarizations can be transmitted via electrical coupling to other neurons. In contrast, in its BC, CBI-5/6 uses spike-dependent synaptic transmission. Thus, the CPG directly and differentially controls the program phases in which the two compartments of CBI-5/6 may transmit information to its targets.