Respiratory centers are a collection of brainstem nuclei located in both the medulla and pons. They receive afferent information on the partial pressures of arterial oxygen, arterial carbon dioxide, and blood pH. This information is then used to regulate breathing rate, so that homeostasis can be maintained. The respiratory centers can be organized into four divisions: the inspiratory, pneumotaxic, apneustic, and expiratory centers, but only the inspiratory and expiratory centers are found within the medulla. Depression in activity of the respiratory center can occur from sedative use, brain trauma, brain tumors, or brainstem damage. Stimulation can occur from use of amphetamines.