ABSTRACT
We conclude that pulmonary visceral and vascular muscles in tetrapods are innervated by parasympathetic cholinergic, parasympathetic inhibitory (acting via NO with or without VIP), sympathetic adrenergic and primary afferent sensorimotor neurons. Elements of doubt remain, even about the mammalian lung. Little is known of the innervation of fish lungs or even bird lungs. The physiological role of the innervation of visceral muscle or, in birds and mammals, of vascular muscle remains obscure to us. In reptiles, amphibians and lungfish the parasympathetic cholinergic vasoconstrictor innervation is used to limit vascular pressures arriving at the lung capillaries and to produce gross matching of perfusion to ventilation. In swimbladders, parasympathetic cholinergic neurons probably stimulate lactate formation by the gas gland, inducing gas secretion into the swimbladder and increasing buoyancy. The vasculature and visceral musculature are innervated by sympathetic adrenergic neurons: their actions, regionally mediated by α- or β-adrenoceptors, lead to resorption of gas. The very different patterns of motor innervation of lungs and the swimbladders do not help in resolving the homologies of these structures.
