Abstract
Two respiration-related rhythm generators have been identified in the medulla of rodents to produce intrinsic periodic bursts: One is the pre-Bötzinger complex (PBC), producing inspiratory (Insp) neuron bursts, and the other is the para-facial respiratory group (pFRG), predominantly producing pre-inspiratory (Pre-I) neuron bursts. Although both generators can be independently active under specific conditions, the pFRG-Pre-I rhythm generator interacts with PBC-Insp rhythm generator as a coupled oscillator system to regulate the rhythm of the intact system. Previous studies reported existence of excitatory and inhibitory synaptic connections from Pre-I to Insp neurons and inhibitory synaptic connection from Insp to Pre-I neurons. Several lines of evidence support that the pFRG-Pre-I rhythm generator functions as a primary respiratory rhythm generator to determine frequency of inspiratory burst at least in the brainstem-spinal cord preparation of newborn rats. Since these two rhythm generators possess different sensitivity to various neuromodulators, we hypothesized that the dominance of these rhythm generators to determine basic respiratory rhythm depends on the background stimulation level of the system. These rhythm generators may function also in vivo in which the background stimulation level may differ depending on conditions such as arousal level, anesthesia and afferent inputs. Thus, consideration of rhythm generator dominance in the determination of output frequency may be important in understanding the neuronal mechanisms of respiratory rhythm generation under various in vivo as well as in vitro conditions. [J Physiol Sci. 2007;57 Suppl:S22]
