Cough reflex

Physiology of cough

The cough receptors or rapidly adapting irritant receptors are located mainly on the posterior wall of the trachea, larynx, and at the carina of trachea, the point where the trachea branches into the main bronchi. The receptors are less abundant in the distal airways and absent beyond the respiratory bronchioles. When triggered, impulses travel via the internal laryngeal nerve, a branch of the superior laryngeal nerve which stems from the vagus nerve (CN X) to the medulla of the brain. This is the afferent neural pathway. Unlike other areas responsible for involuntary actions like swallowing, there is no definitive area that has been identified as the cough center in the brain.

The efferent neural pathway then follows, with relevant signals transmitted back from the cerebral cortex and medulla via the vagus and superior laryngeal nerves to the glottis, external intercostals, diaphragm, and other major inspiratory and expiratory muscles. The mechanism of a cough is as follows:

• Diaphragm (innervated by phrenic nerve) and external intercostal muscles (innervated by segmental intercostal nerves) contract, creating a negative pressure around the lung.
• Air rushes into the lungs in order to equalise the pressure.
• The glottis closes (muscles innervated by recurrent laryngeal nerve) and the vocal cords contract to shut the larynx.
• The abdominal muscles contract to accentuate the action of the relaxing diaphragm; simultaneously, the other expiratory muscles contract. These actions increase the pressure of air within the lungs.
• The vocal cords relax and the glottis opens, releasing air at over 100 mph.
• The bronchi and non-cartilaginous portions of the trachea collapse to form slits through which the air is forced, which clears out any irritants attached to the respiratory lining.

Stimulation of the auricular branch of the vagus nerve supplying the ear may also elicit a cough. This ear-cough reflex is also known as Arnold's nerve reflex (ANR), linked to the auricular branch of vagus nerve. It is an example of vagal hypersensitivity. Weakness of the respiratory muscles, tracheostomy, or vocal cord pathology (including paralysis or anesthesia) may prevent effective clearing of the airways.