Cough. Illustration showing exhalation during coughing. Irritation of the respiratory passages initiates the cough reflex. Nerve impulses pass from the irritated area through the vagus nerves (yellow) to the medulla (brainstem). Here the respiratory centres (coloured red, green & mauve) send nerve signals to the respiratory muscles causing about 2.5 litres of air to be inhaled. The epiglottis & vocal cords close trapping the air in the lungs. The abdominal & internal intercostal muscles then contract forcefully, & the epiglottis & vocal cords suddenly open so that the air is forced out of the lungs under pressure (blue arrows), often carrying foreign matter with it.