Neurophysiology of the motor output pattern generator for breathing.

In cats, there is a central pattern generator (CPG) capable of producing the motor output to drive breathing without any phasic sensory input. The phrenic activity driven by the CPG is, on each cycle, a long inspiratory discharge with continuously rising intensity, an abrupt turn-off, and a pause. The CPG is normally overridden by a reflex termination of inspiration. A sufficient lung inflation will terminate inspiration early, but has no other effect on the motor output. This is the Hering-Breuer inspiration inhibiting reflex. The lung volume necessary to terminate inspiration decreases during the course of an inspiration. The slowly adapting bronchial stretch receptors running in the vagus are the responsible afferents. These vagal afferents drive cells in the ventrolateral nucleus of the solitary tract. These cells also receive a rising excitation from the CPG. They sum these two inputs and thus show a falling threshold to vagal input. They probably generate the inspiration terminating trigger for the Hering-Breuer reflex. In deeply anesthetized breathing, the only active expiratory cells are in the nucleus retroambigualis. These do not make synapses with inspiratory cells. Thus breathing models based on reciprocal inhibition between inspiratory and expiratory cells cannot be correct. Inspiratory cells must themselves be capable of generating the rhythm. Invertebrates present many examples of repetitive burst generation by a single synergistic population. This usually involves cells that, in the absence of synaptic input, either fire continuous trains or rhythmical bursts of spikes. The cardiac ganglion of crustaceans may be a useful model for burst generation similar to breathing. In deeply anesthetized breathing there are two populations of medullary respiratory cells, those in the nucleus of the solitary tract (NST) and those in the nucleus retroambigualis (NRA). The NST has axons ramifying in the NRA and thus could drive it, but the reverse connections do not exist. Thus the current "best guess" is that the cells of the NST are the central pattern generator for breathing.