medulla oblongata

The medulla oblongata is the lowest part of the brainstem, where the brain narrows down to meet the spinal cord. For breathing it is the engine room: this is where the basic in-and-out rhythm is born before anything else fine-tunes it.

It contains two key neuron groups. The dorsal respiratory group is mainly active during inspiration, while the ventral respiratory group contributes to forceful breathing and expiration. A small cluster near them, often called the pre-Bötzinger complex, is widely regarded as the principal pacemaker that generates the inspiratory rhythm. The medulla also holds the central chemoreceptors that sense carbon dioxide and pH.

Because the medulla also controls heart rate and blood pressure, injury here is especially dangerous: a stroke or compression in this region can stop breathing and circulation together. This is why brainstem function is central to clinical assessments of severe coma.