THE lining of the windpipe (trachea) and its branches is sensitive to light touch. This is particularly true of the larynx and the lower part of the trachea where it divides into a branch to each lung. From these sites nerve fibres pass into the vagus nerves and send information to the medulla of the brain. This information can then stimulate the cough reflex. Nerve impulses are sent down the spinal cord causing the diaphragm to relax, and the abdominal and chest wall muscles to contract. The larynx closes because of nerve impulses reaching it via the glossopharyngeal nerve. Pressure builds up in the lungs and air cannot escape through the larynx. The trachea, which has rings of cartilage at the front and sides but a soft membrane at the back, is compressed so that the D shaped cross-section becomes almost V-shaped. Then the larynx opens and an explosive expulsion of the air from the chest occurs, hopefully taking the irritant stimulating particle with it. I am told that the speed of the expired air can reach 100 kM/hour.
A similar type of reflex occurs when one sneezes. The lining of the nose and the resonating cavities of the head, (the sinuses), send impulses to the medulla, but by different nerves depending on the site stimulated. A similar explosive expulsion of air occurs.
Large dust particles are filtered out of respired air by, in the first instance, the hairs at the end of the nose. Those that get past are very likely to impinge on the wall of the inside of the nose because their momentum is greater than that of the air which swirls round the turbinate bones in the nose. Turbinate bones are so-called because of the turbidity produced in the air flow. Smaller particles are able to penetrate further down the respiratory tree but only the very smallest, measuring perhaps a micron in diameter, get past the divisions of the bronchi and into the air sacs of the lung. These are too small to contain bacteria but not too small to contain viruses.
The cells lining the airways have hairs on their outer membrane called cilia. These minute cilia wave rhythmically, their main power thrust being to push particulate matter towards the throat where it can then be spat out or swallowed. In addition other cells, called goblet cells, secrete mucus which keeps the lining of the air passages moist.
Various facts arise because of the anatomy and physiological reactions of the airways. Should the cough reflex be interfered with, such as when assisted ventilation on a ventilator occurs, either with a tube inserted through the larynx or into the trachea, the cough mechanism is mechanically impaired and it is difficult for the patient to clear the bronchi and will necessitate mechanical aspiration of the secretions. Some medicines, especially the ACE inhibitors used for high blood pressure, can produce a dry cough as a side effect. Not all dust substances are equally effective in producing cough or sneeze reflexes, some such as pepper or riot control gases, are well known stimulators of sneezing and coughing. It is not necessarily externally applied substances which irritate the bronchi. Cancers of the lung can cause almost continuous cough reflex actions. Continual coughing can cause the windpipe to remain permanently V-shaped in cross-section – a scabbard deformity. Coughing is often due to excess nasal secretion (catarrh), impinging on the back of the throat – a condition called chronic post nasal drip.
The explosive force of a cough projects potentially infected secretions out as an aerosol. The larger droplets fall to the floor within a few metres where they dry out and can become infected dust particles. Medium sized particles can be inhaled by a second person and impinge on the membranes of their nose, throat and bronchi, causing airborne spread of infection. The smaller the particle the further down the bronchial tree the infected material can get. It is not only bad manners to sneeze or cough without trying to prevent the expelled particles becoming an infective aerosol, it can be lethal. Tuberculosis of the lung is becoming resistant to antibiotics, is increasing in frequency and is preventable. Coughing is one part of the body’s defence against infection. Suppressing a cough by medication can be wrong, though it is sometimes necessary to relieve distress.