Afferent nerves in reflex bronchoconstriction.

Bronchomotor tone is controlled by reflex mechanisms that have afferent pathways originating from different sites and from receptors transducing different stimuli: mechanoreceptors, thermoreceptors, chemoreceptors and nociceptors. Mechanical and chemical irritations of the nasal cavity and the nasopharynx usually lead to a decrease in bronchomotor tone. However, in patients with asthma and chronic obstructive lung diseases, nasal stimulation causes bronchoconstriction. Some experimental data indicate a bronchoconstrictive effect of nasal cooling. At present, it is not possible to identify specific receptors as responsible; but specific cold receptors have been found in the nose. Mechanical and chemical irritations of the larynx, cooling of the larynx, and exposure to both hypo- and hypertonic water solutions cause bronchoconstriction. Of the several receptors described in the larynx the following could be involved: rapidly adapting irregularly firing irritant receptors with myelinated fibres, specific cold receptors and C-fibre receptors. Of possible relevance is the marked inhibitory effect that low temperatures have on laryngeal mechanoreceptors. All the four types of nervous receptors present within the tracheobronchial tree and lung parenchyma contribute to the regulation of bronchomotor tone. Rapidly-adapting irritant receptors, and bronchial and pulmonary C-fibre receptors, that respond to several mechanical and chemical challenges, cause a reflex bronchoconstriction. Slowly-adapting stretch receptors are stimulated by increases in broncho-motor tone and reflexly inhibit bronchoconstriction subserving a negative feedback mechanism.