Ventral respiratory group
The (VRG) is a column of neurons located in the ventrolateral region of the medulla, extending from the caudal facial nucleus to -400μm obex. The four cell groups of the VRG are the rostral nucleus retrofacialis, caudal nucleus retroambiguus, nucleus para-ambiguus, and the pre-Bötzinger complex.
Respiratory effects 
The VRG contains both inspiratory and expiratory neurons. The VRG is secondarily responsible for initiation of inspiratory activity, after the dorsal respiratory group. The nucleus para-ambiguus is active during inspiration, while the nucleus retrofacialis and the nucleus retroambiguus are active during exhalation.
The VRG is responsible for motor control of inspiratory and expiratory muscles during exercise.
The pre-Bötzinger complex is the hypothesized location of central respiratory rhythm pattern generation circuitry. The pre-botzinger complex also contains pacemaker cells which initiate spontaneous breathing. It is currently unclear how this system regulates its output to effect motoneuron bursting, which in turn is responsible for inspiratory muscle innervation. Research into the function of the pre-Bötzinger complex is currently being studied by making use of the in vitro slice preparation.
See also 
Smith J, Ellenberger H, Ballanyi K, Richter D, Feldman J (1991). "Pre-Bötzinger complex: a brainstem region that may generate respiratory rhythm in mammals". Science 254 (5032): 726–9. doi:10.1126/science.1683005. PMID 1683005.
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