Coughing is a reflex mechanism by which the body rids the lower respiratory tract of any irritant that enters through the air and less frequently any fluids (drinks) and solids (food) that may spill into the respiratory tract. Coughing can be initiated voluntarily but lacks the force and pressure as when triggered by the cough reflex.
The lower respiratory tract (trachea, bronchi and bronchioles) is lined with epithelium that contains sensory receptors known as pulmonary irritant receptors. It is highly sensitive to chemical and mechanical stimuli and even very small particles can irritate the lining. Inflammation and mucus hypersecretion within the tracheobronchial tree can also trigger coughing.
A cough can also be triggered by any irritation of the upper respiratory tract, particularly the larynx and pharynx. This area is less sensitive to chemical stimuli compared to the lower respiratory tract. It may result in an expiratory cough, sometimes referred to as a shallow cough, where residual air in the lungs is forced out without the preceding deep inhalation as described below.
Process of Coughing
When the pulmonary irritant receptors are stimulated, nerves impulses are sent via the vagus nerve to the medulla of the brain. This triggers the following sequence of events :
- Deep and sudden inhalation fills the lungs with up to 2.5 liters of air.
- The epiglottis closes and the vocal cords contract to close the larynx fully.
- The abdominal muscles contract with force and pushes against the diaphragm.
- There is simultaneous forceful contraction of the other muscles of expiration.
- These muscle contractions increase the pressure in the lungs.
- When the pressure if sufficiently high, the vocal cords relax and the epiglottis suddenly opens.
- The high air pressure within the lungs rushes out almost in an explosive manner.
The pressure also causes the bronchi and parts of the trachea to collapse to form slits through which the rushing air can sweep out any irritants. The speed of the air rushing out respiratory tract can reach speeds of 100 mph (miles per hour).
Ear Cough Reflex
Irritation within the ear canal can also trigger a cough.
The auricular branch of the vagus nerve may stimulate the sequence of events that occur in the medulla to initiate the cough mechanism. This is also know as the Arnold’s reflex, named after the auricular branch of the vagus nerve which is also known as Arnold’s nerve.
The esophagus and certain abdominal organs which are also innervated by the vagus nerve may elicit a cough as well.