…is the title of a really fascinating paper that describes two cases of pathological yawning that turned out to be a sign of brain stem injury. Yawning is an action of “accessory muscles,” which are muscles that can be called upon to assist during deep inspiration. What are accessory muscles, you ask?
Normally, your diaphragm can control quiet breathing at rest, and for deep breaths, you may use your external intercostals, which pull your ribs up and push your sternum out. For strenuous activities, like running away from a grizzly, your accessory muscles expand your thoracic volume even more. Accessory muscles include the the sternocleidomastoid (turn your head to the side. Feel that tendon sticking out of your neck? That’s the SCM), scalene, and sometimes even pectoralis major muscles.
When you yawn–taking a large, sharp breath inward–you are using accessory muscles. So what do these accessory muscles have to do with the brain stem?
It is postulated that the midbrain and reticular formation, part of the brain stem, exert some control over the cervical nerves that control accessory muscles (C2, C3, etc). In the cases described above, yawning was associated with ischemic brain lesions that resolved when the brain lesions were treated.