Non-adrenergic non-cholinergic neural control of human airways.

In addition to classical cholinergic and adrenergic neural mechanisms, a third division of autonomic control has been recognised in human airways. Non-adrenergic inhibitory nerves are the dominant inhibitory neural pathway in human airway smooth muscle and there is increasing evidence that VIP and a related peptide, PHM, may be the neurotransmitters. These peptides are probably cotransmitters of acetylcholine in the airways and may modulate cholinergic effects. A defect in this system could occur in asthma because inflammation may more rapidly inactivate these neurotransmitter peptides. Non-cholinergic excitatory nerves have also been described in animal airways, although their existence in human airways is less certain. The neurotransmitter may be substance P or a related peptide neurokinin A, which could be released by axon reflex. Another peptide, calcitonin gene-related peptide, is colocalized with substance P and appears to be much more potent in human airways. Non-adrenergic non-cholinergic mechanisms may also regulate mucus secretion and the bronchial microvasculature. The role of this nervous system in health and disease is still uncertain as there are no specific blockers available.