The diaphragm, a smooth skeletal muscle used to control respiration, is nominally an involuntary muscle, although limited voluntary control can be achieved. The diaphragm operates using the phrenic and lower intercostal nerves to communicate with the brainstem.
The diaphragm achieves respiration by contracting downward, which expands the abdominal cavity and causes the lungs to fill with air. Exhalation of the air comes during relaxation of the diaphragm muscle, which allows the elastic recoil of the lung tissue to shrink the abdominal cavity again.
Involuntary control of the diaphragm is lost with the onset of congenital central hypoventilation syndrome, commonly known as Ondine's curse. Treatment for this disorder generally requires the installation of mechanical respiration to prevent suffocation when the patient sleeps.