The cough reflex has both sensory (afferent) and motor (efferent) pathways. The internal laryngeal nerve carries the sensory information away from the area above the glottis in the trachea to the CNS via cranial nerve X (vagus). Stimulation of this area by dust or other foreign particles produces a cough, which is necessary to remove the foreign material from the respiratory tract before it reaches the lungs. [2] The mechanism of a cough is as follows:

• Diaphragm and external intercostal muscles contract, increasing the volume fo the lungs and making the pressure of air within the lungs lower than atmospheric pressure.
• Air rushes into the lungs in order to equalise the pressure.
• The glottis closes to prevent air escaping whilst the diaphragm relaxes and expiratory muscles contract.
• This reduces the volume of the lungs, therefore increasing pressure.
• The pressure of air within the lungs is now greater than atmospheric pressure and so air is trying to escape.
• Glottis opens, releasing air at over 100 mph.

Therefore, respiratory muscle weakness, tracheostomy and vocal cord pathology may all prevent effective clearing of the airways.