The NANC system
The complex interplay among efferent and afferent autonomic nerves in
the regulation of many aspects of airway functions is beyond the scope
of this paper 10. Briefly, in addition to being
regulated by the classical cholinergic bronchoconstrictor and adrenergic
bronchodilator neural pathways, bronchial smooth muscle tone is also
modulated by NANC nerves, which can be either inhibitory (i-NANC) or
excitatory (e-NANC) (figure 1)10,11. Excitatory
NANC-mediated bronchoconstrictor responses are believed to be under the
control of a subpopulation of non-myelinated C-fibers, primary afferent
neurons which release neuropeptides such as Sub P, neurokinin A and B
and the calcitonin gene-related peptide. In addition to
bronchoconstriction, these neurotransmitters can cause mucus secretion,
bronchial artery dilatation and postcapillary venule leakiness,
processes called “neurogenic inflammation” 11-13.
The excitatory NANC system is thought to play a role in allergic and
nonallergic asthma, in allergen-induced bronchoconstriction and, as
follows below, in the pathogenesis of RSV infection in young children5,14.