The diaphragm is a skeletal muscle that separates the thoracic and abdominopelvic cavities. It is innervated by the phrenic nerve. The diaphragm is the main muscle of respiration, with diaphragmatic contraction responsible for 75% of the air movement in a normal breath at rest. Respiratory control is both voluntary and involuntary. Because of the involuntary control, the diaphragm is commonly mistaken to be made up of smooth muscle, but this is incorrect. The brain's involuntary respiratory centers regulate the activity of the diaphragm and intercostal muscles in response to sensory information (very much like an involuntary reflex arc). These involuntary centers are housed in the brainstem. The cerebral cortex controls voluntary breathing, which also affects the diaphragm and intercostal muscles. For forced inhalation or exhalation, additional skeletal muscles (e.g., pectoralis minor, scalenes, sternocleidomastoid) may also be involved.