Allergic Rhinitis 82 study and to define them separately quite difficult. By gross dividing, however, these physiological and morphological attributes can be used to identify at least three broad classes of afferent nerve fibers: rapidly adapting mechanoreceptors (RARs), slowly adapting mechanoreceptors (SARs) and unmyelinated C-fibers (C-fibers) (Mazzone et al., 2003). 2.1.1 Rapidly adapting receptors (RARs) While the anatomical arrangement of RARs termination is unknown, functional studies suggest that these receptors terminate within or beneath the epithelium and are localized to both intra-and extrapulmonary airways (Bergren & Sampson, 1982; Riccio et al., 1996c, Ho et al., 2001). RARs, as its name implies, is differentiated from the other airway afferent nerves by their rapid (1-2sec) adaptation to sustained lung inflations (