There is increasing evidence that an essential component of the rhythm generating circuitry is located within a discrete region of the ventrolateral medulla termed the preB6tzinger complex. Nevertheless, an understanding of the preB6tzinger contribution to breathing awaits a comprehensive description of the pertinent properties and network interactions of the constituent neurons. The goal of this project to provide a reasonably complete description of the respiratory neuron types present in the preB6tzinger complex, including their discharge patterns, response to activation of selected afferent inputs, neurotransmitter (GABA, glutamate, glycine) content and pattern of synaptic connections formed with other preB6tzinger neurons. Given the small size of the preB6tzinger complex (in the adult rat it is approximately 0.6 mm long and about 1.5 mm in diameter, including dendrites), it is within our means to provide this analysis. Three Specific Aims will be undertaken. In Aiml, intracellular or extracellular recording will be used to classify neurons with respect to discharge pattern and their response to stimulation of vagus and superior laryngeal nerve afferents. The recorded neurons will then be injected with dye. Subsequent immunohistochemical analysis at the light and ultrastructural levels will identify the neurotransmitter content (GABA, glycine, glutamate) and their axonal projection patterns. In Aim 2, synaptic interactions between preB6tzinger neurons will be identified with complementary electrophysiological and anatomical approaches. In the electrophysiological approach, either spike triggered averaging or cross-correlation approaches will be used with paired neuronal recordings. In anatomical experiments intra- or juxtacellular labeling will be used to dye-label cells in 2 different functional groups for subsequent light and ultrastructural analysis of their synaptic interactions. In Aim 3, we will develop a detailed computational model of the preB6tzinger respiratory network using the identified neuronal properties and connectivity. The hypothesis to be addressed is that the neuronal types and synaptic interactions within the preB6tzinger complex are sufficient for respiratory rhythm generation in vivo.