Organization of the brain stem neural mechanisms for generation of respiratory rhythm--current problems.

The brain stem neural mechanism for central regulation of breathing is regarded as a complex neuronal mechanism consisting of several functional subsystems subserving different functions. One of its functions is the generation of the respiratory rhythm. Evidence indicates with certainty that the subsystem for respiratory rhythm-generating mechanisms is located in the medullary structure outside the DRG and VRG regions which have been postulated for many years as the hypothetical site generating respiratory rhythm. DRG and VRG are thought to be premotor neuron pools. Rhythmic activity originating in the medulla is dominant in terms of the spontaneity over other rhythmic activity in the pontine mechanisms as well as those in the spinal cord. Evidences for heterogeneity of functional properties of respiratory neurons in the brain stem are demonstrated. Possible functional differentiation among respiratory neurons is suggested. Neuronal mechanisms involving respiratory neurons identified as members of primary respiratory neuron populations or neuronal networks consisting of various types of respiratory neurons located in the lateral region of the bulbar reticular formation may play important roles in generation of respiratory rhythms. Precise neural processes within the neuronal mechanisms for respiratory rhythm generation are rather equivocal and remain to be determined by further investigation.