Anatomy of Airways Smooth Muscle

Abstract

The airways of mammals are endowed with smooth muscle. The main effects of the contractile activity of this musculature are: i. to alter the calibre of the airway, hence affecting air pressure and air flow; ii. to alter the rigidity of the airway wall. However, the physiological role of airways smooth muscle is still a matter of speculation [1]. Contractions are predominantly isotonic, i.e. they involve a reduction in length of the muscle and an increase in its thickness. On both accounts, muscle contraction reduces the calibre of the airway segment involved. An isometric component in the contraction is also present. This component is a modest one, on account of the small resistance offered by the content of the airways to compression; it is more substantial, however, when the musculature works against the elasticity of the surrounding tissues, including elastic fibres and cartilages. Airways smooth muscle produces tonic contractions, since it is hardly possible that there are phasic contractions in train with the respiratory cycle. However, the discharge of nerve impulses — not only in sensory fibres but also in efferent fibres to smooth muscle — can be synchronous with specific phases of the respiratory cycle [2].