Neural mechanisms of motor program switching in the mollusc Pleurobranchaea. III. Role of the paracerebral neurons and other identified brain neurons

Identified neurons in the cerebropleural ganglion (brain) of the mollusc Pleurobranchaea were stimulated and recorded from intracellularly while recording the identified motor program from buccal muscles (reduced preparation) or nerves (isolated central nervous system). Neurons studied included the metacerebral giant neurons (MCGs), phasic paracerebral neurons (PC(p)'s), polysynaptic excitors of the PC(p)'s (PSEs), type II electrotonic neurons (ET(II)'s), type I electrotonic (ET(I)'s) and several other identified neurons or neuronal classes. Intracellular stimulation of the above identified neurons generally elicited the ingestion motor program of its characteristic components, but never the egestion motor program and seldom its characteristic components. Intracellular recordings from these neurons in the isolated central nervous system preparation while eliciting the ingestion and egestion motor program generally showed cyclic membrane potential oscillations in phase with both motor programs, indicating that these neurons receive synaptic feedback from the ingestion and egestion central pattern generator(s). This study is therefore consistent with the view that an interrelated cluster of brain neurons is specialized to command the ingestion motor program. A neural model of motor programs switching in the buccal motor system is formulated, comprising separate command pathways for ingestion and egestation that converge on a common central pattern generator(s).