The cough reflex occurs when stimulation of cough receptors in the respiratory tract by dust or other foreign particles produces a cough, which causes rapidly moving air which usually remove the foreign material before it reaches the lungs. This typically clears particles from the bronchi and trachea, the tubes that feed air to lung tissue from the nose and mouth. The larynx (voice box) and carina (at the bottom of the trachea, as it splits into bronchi) are especially sensitive. Cough receptors in the surface cells (epithelium) of the respiratory tract are also sensitive to chemicals. Terminal bronchioles and even the alveoli are sensitive to chemicals such as sulfur dioxide gas or chlorine gas. Physiology of cough The cough receptors or rapidly adapting irritant receptors are located mainly on the posterior wall of the trachea, pharynx, 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