Inspiration begins when cells of the solitary tract nucleus (along with cells of the ventral respiratory group) begin a steady increase in firing, leading to contraction of the respiratory muscles. When neurons in the DRG fire, impulses travel down the phrenic and intercostal nerves to stimulate the diaphragm and external intercostal muscles. The thoracic cavity expands and air rushes into the lungs due to the negative pressure, producing inspiration. When the cells stop firing, the inspiratory muscles relax, inspiration ceases, and exhalation begins.
The rhythm of the DRG produces a respiratory rate of 12-16 breaths per minute in humans. Inspiration usually lasts approximately 2 seconds, and expiration lasts about 3 seconds. The normal inspiration rate and rhythm is called eupnea, and difficult respiration is termed dyspnea.
^Kolesnikova, EE (2011). "Role of glutamate and GABA in mechanisms underlying respiratory control". Neurophysiology (Bogomolets Institute of Physiology, Kiev, Ukraine) 42 (4): 294–304.|accessdate= requires |url= (help)
Berne, Robert M.; Levy, Matthew N.;Koeppen, Bruce M.;Stanton, Bruce A. (2004). Physiology, 5th edition. St. Louis, MO: Elsevier. p. 513. ISBN0-323-03390-3.