The role of the nervous system in rhinitis.

The nose provides defensive and homeostatic functions requiring rapid responses to physical and chemical stimuli. As a result, it is armed with a complex nervous system that includes sensory, parasympathetic, and sympathetic nerves. Sensory nerves transmit signals from the mucosa, generating sensations, such as pruritus; motor reflexes, such as sneezing; and parasympathetic and sympathetic reflexes that affect the glandular and vascular nasal apparatuses. Reflexes directed to the nose are also generated by inputs from other body regions. Hence all symptoms that constitute the nosologic entity of rhinitis can be triggered through neural pathways. In addition, neural signals generated in the nose can influence distal physiology, such as that of the bronchial tree and the cardiovascular system. Neural function can be chronically upregulated in the presence of mucosal inflammation, acutely with an allergic reaction, or even in the absence of inflammation, as in cases of nonallergic rhinitis. Upregulation of the nasal nervous system can occur at various levels of the reflex pathways, resulting in exaggerated responses (neural hyperresponsiveness), as well as in increased capacity for generation of neurogenic inflammation, a phenomenon that depends on the release of neuropeptides on antidromic stimulation of nociceptive sensory nerves. The molecular mechanisms of hyperresponsiveness are not understood, but several inflammatory products appear to be playing a role. Neurotrophins, such as the nerve growth factor, are prime candidates as mediators of neural hyperresponsiveness. The many interactions between the nervous and immune systems contribute to nasal physiology but also to nasal disease.