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Cough Reflex

The bronchi and trachea are so sensitive to light touch
that very slight amounts of foreign matter or other
causes of irritation initiate the cough reflex. The larynx
and carina (the point where the trachea divides into the
bronchi) are especially sensitive, and the terminal bronchioles
and even the alveoli are sensitive to corrosive
chemical stimuli such as sulfur dioxide gas or chlorine
gas. Afferent nerve impulses pass from the respiratory
passages mainly through the vagus nerves to the
medulla of the brain. There, an automatic sequence of
events is triggered by the neuronal circuits of the
medulla, causing the following effect.
First, up to 2.5 liters of air are rapidly inspired.
Second, the epiglottis closes, and the vocal cords shut
tightly to entrap the air within the lungs. Third, the
abdominal muscles contract forcefully, pushing against
the diaphragm while other expiratory muscles, such as
the internal intercostals, also contract forcefully. Consequently,
the pressure in the lungs rises rapidly to as
much as 100 mm Hg or more. Fourth, the vocal cords
and the epiglottis suddenly open widely, so that air
under this high pressure in the lungs explodes outward.
Indeed, sometimes this air is expelled at velocities
ranging from 75 to 100 miles per hour. Importantly, the
strong compression of the lungs collapses the bronchi
and trachea by causing their noncartilaginous parts to
invaginate inward, so that the exploding air actually
passes through bronchial and tracheal slits. The rapidly
moving air usually carries with it any foreign matter that
is present in the bronchi or trachea.