Sensory neurophysiology of airways

Although not specifying the natural stimulus to the receptors, one term does adequately describe the basic criterion used for the identification of these receptors in neurophysiologic studies, and that is the rapidly adapting response to maintained hyperinflation and deflation of the lungs. The results of these studies provide strong evidence that histamine acts directly on intrapulmonary rapidly adapting receptors to stimulate them. Thus, marked constriction of airway smooth muscle (as with acetylcholine) does not necessarily excite these receptors, and a decrease in the bronchoconstrictor effect of histamine (as with isoproterenol) does not necessarily eliminate its excitatory effect on the receptors. It is also possible that histamine sensitizes these receptors to naturally occurring mechanical stimuli, as occurs with other receptors. The author does not interpret these findings to indicate that bronchoconstriction induced in dogs by histamine aerosol is entirely reflex, originating from activation of rapidly adapting receptors. As mentioned earlier, there is, as yet, no proof that stimulation of these vagal sensory receptors in the airways results in reflex bronchoconstriction. If, however, such a reflex does occur, then the direct excitatory effects of histamine, and possibly other substances, most notably those released during anaphylaxis, may have important implications with regard to control of bronchial tone. Hence, activation of intrapulmonary rapidly adapting receptors could either initiate acute bronchospasm or markedly intensify that produced directly by various substances.