The respiratory center is composed of several groups of neurons located bilaterally in the medulla oblongata and pons varolii of the brain stem. It is divided into three major collections of neurons: (i) a dorsal respiratory group, located in the dorsal portion of the medulla, which mainly causes inspiration; (ii) a ventral respiratory group, located in the ventrolateral part of the medulla, which mainly causes expiration; and (iii) the pneumotaxic center, located dorsally in the superior portion of the pons, which mainly controls rate and depth of breathing.