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.