The physiology of the nose.

The nose, as an organ initiating reflexes affecting itself and the rest of the body, and as a target organ of control, is highly complex. Its innervation includes parasympathetic, sympathetic, sensory/afferent, and somatic motor nerves, which combine in a variety of morphologic pathways. The vasculature of the nose contains capacitance vessels such as sinusoids and distensible venules, as well as arteriovenous anastomoses, arterioles, capillaries, and venules. The secretory tissue of the nose includes epithelial cells, submucosal glands, and relatively large anterior or lateral serous glands; in addition, some species have specialized secretory glands. The nose is the source of many powerful reflexes, including the diving response, sneeze and sniff reflexes, and reflexes affecting autonomic nervous function to the cardiovascular system, airways in the lungs, the larynx, and other organs. Axon reflex control of the nasal vasculature is also important. The nasal vasculature can be shown to be under parasympathetic and sympathetic control, but there is little precise information concerning the effect of nerves on different types of blood vessels. Pharmacologic experiments show that vascular resistance and vascular volume can be separately influenced by nerves and mediators, and that vascular resistance and airway resistance are not necessarily inversely related. Nasal secretion is also under the influence of both parasympathetic and sympathetic nerves, and can be induced by a wide range of neurotransmitters and mediators. In general, the concentrations needed to promote secretion are considerably higher than those that affect vascular resistance, at least with regard to the lateral nasal gland of the dog. In humans, nasal patency is affected by several nervous inputs, presumably acting via vascular beds. The nasal cycle is the alternation of resistances between the two sides of the nose, on which other changes are superimposed. In exercise and hyperpnea, nasal airflow resistance decreases, presumably with vascular decongestion. Recent studies of the crutch reflex (the ipsilateral nasal congestion caused by stimulation of the axilla) show that some of the reflex inputs to the nose can be unilateral.