Action-potential broadening and endogenously sustained bursting are substrates of command ability in a feeding neuron of Pleurobranchaea

The ventral white cells (VWCs) of the buccal ganglion of Pleurobranchaea, so named for their position and color, are a bilateral pair of neuron somata. Each sends a single axon out its contralateral stomatogastric nerve and has a dendritic field originating close to the soma. The VWCs exhibit spontaneous episodes of prolonged depolarization (duration 1-4 min) accompanied by repetitive action-potential activity and separated by regular intervals of 3-30 min. Such prolonged burst episodes can be triggered by short pulses of depolarizing current. During the repetitive activity of the spontaneous bursts or that driven by imposed depolarization, the action potentials progressively broaden to 5-16 times their initial duration. During spontaneous bursting or activity driven by imposed depolarization, the cyclic motor output of the feeding network is initiated or accelerated with a latency corresponding with the development of appreciable VWC spike broadening. When broadening of antidromic VWC spikes is suppressed by imposed hyperpolarization of the soma, the frequency of feeding cycles is significantly lower than when broadened spikes are allowed to develop. When trains of spikes are driven by depolarizing current, the motor output of the feeding network is not initiated until the VWC spikes have broadened to a repeatable 'threshold' duration, regardless of the intensity of the depolarizing current. The endogenous production of prolonged burst episodes, triggered by depolarizing current pulses, and progressive spike broadening can be demonstrated in the surgically isolated VWC soma. The paired VWCs are strongly electrically coupled and display highly synchronous activity. They receive synaptic inputs from many previously identified interneurons of the feeding network and are thus reciprocally coupled within the network. These results demonstrate that the capacity of this neuron to generate broadened action potentials during repetitive activity confers the ability to command coordinated motor-network output. The appropriate repetitive activity can be produced endogenously in the form of prolonged bursts of spikes.