The term "bronchial hyperresponsiveness" seems to convey a clear message: the airways respond abnormally. The result of the airway challenge with an agonist is airway narrowing, often interpreted solely as bronchoconstriction and hence often called 'bronchospasm'; spasm is a 'sudden, violent, involuntary contraction of a muscle or a group of muscles ...' [1]. This term suggests a major role of airway smooth muscle contraction in bronchial hyperresponsiveness. However, it is currently believed that non-muscular components of the airway wall are important determinants of the degree of airway narrowing that results from smooth muscle stimulation. Non-muscular components of the airway and the lung parenchyma can affect the degree of smooth muscle shortening and airway narrowing that occurs in response to airway smooth muscle stimulation.
We shall focus on the mechanical and geometric factors which influence the magnitude of airway narrowing in response to airway challenge with an agonist. It will then be easier to understand that for the same smooth muscle stimulation there can be a large difference in the degree of airway narrowing for asthmatic and non-asthmatic subjects. It will also be easier to understand that smooth muscle relaxants are very helpful in acutely relieving airways obstruction in subjects with "hyperresponsiveness", but that they do not alter the underlying airway "hyperresponsiveness".