Autonomic innervation of the human respiratory tract as revealed by histochemical and ultrastructural methods

The major findings and conclusions of this study are the following: Indirect evidence for the presence of the bronchoconstrictive transmitter acetylcholine in the human bronchial smooth muscle was obtained by demonstration of acetylcholinesterase-positive nerve fibers and nerve profiles of cholinergic type. Acetylcholinesterase-positive nerve fibers and nerve profiles of cholinergic type were found in human bronchial glands. A sparse adrenergic innervation of the human bronchial smooth muscle and glands was found by using catecholamine histofluorescence. This observation was supported by finding ultrastructurally adrenergic-like nerve profiles close to smooth muscle cells. Direct evidence for the presence of a new possible bronchodilating transmitter VIP (vasoactive intestinal peptide) in the human bronchial smooth muscle was obtained both by light microscopical demonstration of VIP immunoreactive nerve fibers and by localization of VIP like immunoreactivity in granules in nerve profiles. Nerve fibers containing VIP-immunoreactivity were found in the human bronchial glands by light microscopy. Substance P-immunoreactive nerves were found in the lower respiratory tract of the rabbit but not of man by light microscopy. Intraepithelial nerves were demonstrated in man from trachea to segmental bronchi and their concentration was established. The intraepithelial nerves seem to have two predominant locations: either close to the airway lumen or near the basement membrane. Nerves near the lumen were found only in larger airways e.g. trachea and lobar bronchi.