Neuromuscular structure and function in the airways.

The smooth muscle and the neural pathways of the airways have many similarities to the muscle and innervation of the gastrointestinal tract, as expected from their common embryological origin. The smooth muscle and its innervation vary in structure and function from species to species and extrapolation from one species to another may not be valid. In the human trachea and main stem bronchi the smooth muscle has numerous cell-to-cell connections of the nexus or gap junction type. The principal nervous pathways in the human airways are excitatory, which is cholinergic, and inhibitory, which is nonadrenergic or purinergic. Adrenergic nerves have not been demonstrated in the smooth muscle, but may be present in the ganglions that are found around the trachea and bronchi. Possible defects in the innervation may lead to alterations in the intrinsic activity, of the smooth muscle and be responsible for certain reactive states of the smooth muscle.