Nasal physiology: neurochemical receptors, nasal cycle, and ciliary action.

The complex interaction of neurotransmitters, vascular supply, and mucociliary function constitute one of the main defense mechanisms of the respiratory tract, modulating airflow and response to noxious assault. One of the main controls of airflow relies upon the modulation of vasodilation and vasoconstriction via the autonomic control of a sinusoid system. In addition to noradrenaline and acetylcholine, an ever-increasing number of neurotransmitters are involved, including neuropeptide Y, vasoactive intestinal peptide, peptide histidine leucine, substance P, and calcitonin gene-related peptide. The existence of a reciprocating cycle of congestion and decongestion has been recognized for over a century, although its exact function is unknown. Recent studies using acoustic rhinometry have demonstrated that the cycle is present in some form in the majority of adults, in children as young as 3 years, and that it persists after cessation of nasal airflow. It may, however, be overridden or modulated by many environmental and pathological situations. By contrast, the defense function of mucociliary clearance is well-established, and while also subject to environmental influences, relies upon an innate and cyclical pattern of mucus flow from within the sinuses and nasal cavity into the oropharynx. The content of the mucus is fundamental to its protective function and the control of mucociliary clearance intimately related to autonomic activity.