Identification and initial characterization of a cluster of command and pattern-generating neurons underlying respiratory pumping in Aplysia californica.

1. Respiratory pumping is a relatively stereotyped behavior that occurs both spontaneously and in response to tactile stimuli, anoxia, food presentation, and changes in illumination (10, 20, 37, 39, 42, 43). While considerable progress has been made in elucidating the motor elements underlying this behavior, little is known regarding the cells responsible for its initiation or patterning. This paper describes the identification and initial characterization of a cluster of command and pattern-generating neurons that subserve this behavior. 2. The command and pattern-generating neurons consist of a cluster of at least seven mutually excitatory cells (the L25 cells) located on the left ventral surface of the abdominal ganglion. These cells fire spontaneous bursts of spikes synchronously with the activity of other interneurons and motor neurons activated during spontaneous respiratory pumping. 3. Artificially firing action potentials in the L25 cells to mimic a spontaneous burst produces the same synaptic effects in motor neurons and interneurons activated during respiratory pumping, while hyperpolarizing a single L25 cell can phase shift a subsequently occurring spontaneous burst. 4. The L25 cells are mutually excitatory; spikes in one L25 cell lead to excitatory postsynaptic potentials (EPSPs) and spikes in other L25 cells. At least some of the mutually excitatory effects are mediated via electrical synapses. 5. The L25 cells make mutually inhibitory connections with other interneurons in the abdominal ganglion. They produce slow and long-lasting inhibitory postsynaptic potentials (IPSPs) in the cardiac command neuron LlO and in interneuron L24. LlO and L24 in turn produce fast IPSPs in the L25 cells. 6. Respiratory pumping activity can be elicited by stimulation of siphon skin, and this activity is due in part to activation of the L25 cells. At least part of the activity in L25 cells produced by tactile stimulation of the skin or electrical stimulation of nerves appears to be due to regenerative feedback among the L25 cells.