Role of laryngeal afferents in cough

The superior laryngeal nerve (SLN) is the main source of laryngeal afferent activity. A clear respiratory modulation can be noted when recording from the peripheral cut end of this nerve in several mammalian species. This modulation is due to three types of sensory endings: cold, pressure and 'drive' receptors. Although respiratory-modulated receptors play an important role in the function of the upper airway, they are not generally viewed as a primary factor in the elicitation of cough. Other more likely candidates for this role are thought to be the so-called 'irritant' endings. These are receptors that do not discharge in close association with the breathing cycle, but are usually silent or randomly active in control conditions. However, they are promptly recruited when the laryngeal mucosa is exposed to mechanical and/or chemical irritation. In fact, these receptors respond to well recognized tussigenic stimuli and are therefore thought to provide the triggering mechanisms for the cough reflex from the larynx. Endings with similar characteristics are also found in the most proximal areas of the tracheo-bronchial tree. On the basis of their response to irritants, these receptors are identified under the common denomination of 'irritant receptors'. However, within this category of endings we find a wide range of distinctive characteristics, be this in terms of responsiveness to water solutions of various osmolarity and composition or to particular responses to substances produced within the body (autacoids) or experimentally administered.