A recent article in the Journal of Allergy and Clinical Immunology discussed current research into why asthma patients cough.
The answer, as it seems to be to many complex questions, is multifactorial. The human brain is the source of a myriad of protective reflexes from blinking to coughing. Protecting the airway compels a complex and delicate neural network. Think about the high degree of neurologic coordination required for swallowing without aspirating, even though the esophagus and trachea lie against one another. There is no question that asthmatics have a more delicate (“hair trigger”) cough reflex. What makes their cough-nerves more sensitive?
It seems to be a function of both structural issues and functional issues. The main structural issue is airway narrowing, which is why asthmatics wheeze and have shortness of breath. The narrowing of the airway “tugs” on the cough-nerves and by stretching them makes them more sensitive. One example of this heightened sensitivity is coughing due to laughing. When we laugh, we move a greater volume of air in and out of our lungs and this in turn leads to very slight cooling of the airway. This slight cooling is perceived as an airway threat and hence the coughing.
The functional issue is inflammation. You can think of this as “burning” the cough-nerve fibers. Because protecting our airway is so important the cough-nerve fibers are unmyelinated; that is, they don’t have the protective myelin coating that most nerves in the human body have. This makes them very vulnerable just as a bare copper wire would more easily spark.
Treating the cough of asthma boils down to expanding the narrowed airways and reducing inflammation.