ABSTRACT
Normal respiration is characterized by quiet, active inspiration followed by passive expiration. Both phases are smooth and symmetrical1. The breath is generated by medullary neurons, but neural control of breathing also involves other brainstem centers, cervical vertebral segments, and the cerebral cortex. This allows involuntary rhythmic breathing, as well as involuntary nonrhythmic control (e.g. during swallowing) and voluntary control2. The diaphragm is the main inspiratory muscle. During contraction the diaphragmatic dome moves caudally, enlarging the thorax and creating the negative intrathoracic pressure that pulls air into the lungs3. The external intercostal muscles also help by moving the ribs cranially and outward during inspiration. Contraction of abductor muscles in the nares, pharynx, and larynx maximizes upper airway diameter to facilitate airflow during inspiration. During passive exhalation, elastic recoil of the lung and chest wall moves air out. Abdominal and internal intercostal muscles are also used for expiration; their contraction decreases thoracic size, thereby increasing intrathoracic pressure and forcing air out of the lungs3.
