State of the Art Nerve Supply to the Lungs* JOHN B. RICHARDSON

Contents Introduction Dog General Description Airway Smooth Muscle Vasculature Sensory Receptors Morphologic Summary Physiologic Studies Cat General Description Airway Smooth Muscle Vasculature Sensory Receptors Morphologic Summary Chicken General Description Airway Smooth Muscle Sensory Receptors Physiologic Studies Mouse General Description Vasculature Sensory Receptors Rat General Description Vasculature Sensory Receptors Morphologic Summary Rabbit Airway Smooth Muscle and Vasculature Neuroepithelial Bodies Sensory Receptors 1 From the Department of Pathology, McGill University, 3775 University, Montreal, Quebec H3A 2B4, Canada. 2 Supported by Grant No. MA-4536 from the Medical Research Council of Canada.

Guinea Pig General Description Nonadrenergic Innervation Reptiles and Amphibians Nonhuman Primates General Description Airway Smooth Muscle Vasculature Physiologic Studies Humans Introduction Thomas Bartholinus gave a gross description of the nerves to the human lung three hundred years ago (1). In his observation of the anatomy of the lung, he noted that the "nervorum stomachicorum" were always located on the posterior part of the bronchial tubes and that small twigs or branches of the nerve crawled over the external membranous part (1). Several years later, Thomas Willis, in a description of "the offices and uses of the intercostal pair," stated "that many small fibers and shoots are spread into the sanguiferous vessels, as also into the coats of the trachea and the oesophagus" (2). He ascribed a function to these nerves: "their office is, that they may respectively draw together and spread abroad those channels of inspired and expired blood and air, according to the way and manner where with the pulse and breathing ought to be performed; whereby the motions of either might be the better retarded or accelerated, according to the necessities or requirings of the heart" (2). Willis also mentioned a possible role of these nerves in disease in his description of a case of asthma (3). These early observations have certainly been enlarged upon, but as will be seen in this review, our understanding of the

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nerves in the lung is little better than that of Thomas Willis. Studies in animals have provided most of our information on the nerves in the lung, and many of these studies have been done on dogs and rodents with few studies of human lungs. There is an obvious variability in the innervation of the lung among species of animals, and extrapolations from one species to another, with regard to either physiologic responses or anatomic distribution of the nerves, should be made with caution. This review is of articles written since 1965 because, in this period, there has been extensive use made of both the fluorescent histochemical technique for catecholamine identification and electron microscopy for identification of the nerves. Reference is made to earlier works when there is an obvious conflict between the results or when the studies are definitive and have not been enlarged upon or contradicted. There is general agreement that the lungs have a dual innervation, an excitatory innervation to the smooth muscle of the airways, blood vessels, and the glands, with an inhibitory innervation to the same structures. Stimulation of the vagus nerve constricts the airways (4-7), increases glandular or goblet-cell secretion (8, 9), and dilates the pulmonary vessels (10). These effects of vagal stimulation are blocked by atropine, which indicates that the released acetylcholine acts on muscarinic receptors located on these structures (11). In animals, stimulation of adrenergic nerves relaxes the smooth muscle of the airways (7, 12), constricts the pulmonary and bronchial blood vessels (10, 13, 14), and inhibits glandular secretions (15). Afferent nerves stimulated by pressure, stretch, or irritation have been partially characterized, but the anatomic basis for the initiation of these reflexes is not understood. T h e general pattern of the innervation of the lung is as follows: preganglionic fibers from the vagal nuclei descend in the vagus nerve to ganglia located around the airways and the blood vessels with postganglionic fibers from the ganglia innervating the smooth muscle of the airways, vasculature, and epithelium of the glands or goblet cells. Postganglionic fibers from the sympathetic ganglia enter the lung and innervate the same structures (16). Afferent endings, with their neurons in the mucosa, the smooth muscle, or the vagal nuclei, are located throughout the lung. These endings have been demonstrated by physiologic studies.

Dog General description. Daly and Hebb, in an excellent gross and light microscopic description of the innervation of the dog lung (17), point out that the vasomotor nerves are intermingled in one part of their course with sympathetic and vagal fibers to the heart, the bronchial vessels, and the airways. This intermingling of the nerves makes the canine lung very difficult to study from both a physiologic and an anatomic aspect (17). Airway smooth muscle. T h e smooth muscle of the airways is innervated by cholinergic nerves (18, 19), which produce bronchoconstriction when the nerves are stimulated (4-7, 11), and this bronchoconstriction may be augmented by 5-hydroxytrypamine (5). T h e cholinergic innervation is not equally distributed throughout the airways because stimulation of the vagus constricts the large airways, but has little effect on airways less than 0.8 mm in diameter (4). The diminished cholinergic innervation may be a consequence of few ganglia in the smaller airways (4, 20). Vagal stimulation also results in a weak vasodilator response in the pulmonary circulation (10). T h e effect of vagal stimulation on the bronchial glands is less clear, and although denervation produces a decrease in the glandular mass, it does not affect the normal clearance mechanism (8). Adrenergic innervation of the smooth muscle of the canine airways has been less clearly demonstrated than cholinergic innervation. Stimulation of the stellate ganglion decreases the amount of constriction produced by the cholinergic system (7), and propranolol given systemically increases the resistance of the small airways (21). T h e latter result could be explained by circulating catecholamines rather than by nerves in the small airways or, alternatively, by the local release of catecholamines from the adrenergic nerves around the bronchial arteries. Field stimulation of isolated tracheal strips has shown adrenergic beta and alpha responses in the presence of atropine (22), but morphologic studies of the muscle have shown adrenergic nerves mainly in the perivascular region and near the cartilage with only minimal fluorescence in the smooth muscle (22). Adrenergic innervation of the ganglia in the canine lung has been demonstrated (18, 23). Because the blood supply to the ganglia is from the bronchial vasculature, which has a dense adrenergic innervation, this might be expected (24). T h e adrenergic nerves enter, as well as encircle,

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the ganglia (23). Some neurons in the ganglia were believed to be adrenergic (18, 25). The significance of the latter finding of adrenergic neurons, if confirmed, is unclear, but the distribution of the adrenergic fibers to the ganglia is similar to the adrenergic innervation of the ganglia in the gastrointestinal tract (26, 27). Norepinephrine released from these nerves may modulate acetylcholine output (28) or affect cholinergic preganglionic nerves (29). In either case, the norepinephrine acts via alpha receptors (28,29). Vasculature. Histochemical techniques for the demonstration of cholinesterase and silver stains for the nonspecific demonstration of nerves have shown bundles of nerve fibers in the bronchial but not in the pulmonary vessels (19). With another technique, however, a network of fibers was found that surrounds the branches of the pulmonary vasculature (19). Fluorescent histochemistry for catecholamines has shown adrenergic nerves in the walls of the airways and surrounding blood vessels (19). Pulmonary arteries and the larger pulmonary veins have adrenergic nerves in their walls (13), but the smaller vessels, those less than 30 ^m in diameter, have none (18, 19). T h e bronchial vessels have a dense adrenergic innervation (19). Difficulty is experienced with the electron microscopy, but axons have been found in the wall of the pulmonary artery, and bundles of nonmyelinated fibers have been located along the outer edge of the media (19). Two types of vesicles have been found in the axon profiles: agranular vesicles 40 to 50 nm in diameter and dense-core, granular vesicles 50 to 100 nm in diameter, with some of these large vesicles containing cores of variable density (19). Fine nerve bundles encircle the origin of the pulmonary arterioles, and it has been suggested that these may play a role in the control of the blood flow (19). Thus, the pulmonary arteries and the bronchial arteries are innervated by cholinergic and adrenergic nerves, but the pulmonary veins have only adrenergic nerves (19). Adrenergic innervation of the pulmonary vasculature has also been demonstrated by fluorescent and electron microscopic techniques (13). Small, dense-core, granular vesicles have been found in the adventitia and media of the artery and in the adventitia of the vein. These axon profiles with the dense-core vesicles have been considered to be adrenergic (13). Axon profiles filled with small agranular vesicles have also been found in the arteries but not in the veins (13). These nerve terminals have been

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considered to be cholinergic (13). Stimulation of the adrenergic nerves or an infusion of norepinephrine constricts the vessels, as determined by an increase in pressure (10, 13, 14). T h e response to nerve stimulation is blocked by treatment with 6-hydroxydopamine but the response to norepinephrine is unchanged, and thus, because 6-hydroxydopamine destroys adrenergic nerve terminals, it has been concluded that release of norepinephrine from the adrenergic nerves produces vasoconstriction (13). Bundles of nonmyelinated axons have been described in relation to capillaries at the level of the alveoli and these axons are of 2 types (30). One type of axon, considered to be cholinergic, contains agranular vesicles 50 nm in diameter, as well as a few large moderately dense granules 100 nm in diameter (30). T h e other type of axon, which is considered to be adrenergic, contains 3 different vesicle populations: one with a very dense core and a diameter of 50 nm, a second with a moderately dense core and a diameter of 100 nm, and a third with a diameter of 85 to 100 nm with a moderately dense core (30). Some of the capillaries have a dual innervation, but most are not innervated at all, although it has been believed that the pericytes may respond to the nerves (30). Sensory receptors. Morphologic demonstration of nerve endings in the airways are few, particularly at the ultrastructural level (30). There are numerous descriptions of nerve endings in the pulmonary vessels, but these have been poorly characterized (17). Nerve endings, which have been described in other animals and believed to be sensory rather than motor, were not found in this particular study (30). Numerous receptors have been demonstrated by physiologic studies of the canine airways (31-43). T h e location of some of these receptors, such as the irritant receptor, has been determined; they are situated mainly in the lower trachea (42). T h e afferent fibers from the receptors, located in the extra-thoracic trachea, ascend mainly in the recurrent laryngeal nerves, and a section of these nerves interrupts 75 per cent of the afferent fibers (32). Characterization of receptors near the vasculature and the bronchial mucosa has been done with intrapulmonary vascular injections of histamine and by inhalation of histamine (35). Three types of receptors are generally recognized: a stretch, an irritant, and a juxtacapillary (or J) receptor (35). Most of the afferent fibers from the irritant receptors are of the C type, with a conduc-

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tion velocity of 0.8 to 2.4 meter per sec (44). blocked by atropine (22, 45). Electrical field T h e location of the stretch receptors is un- stimulation, in the presence of atropine, relaxes known, but they are believed to be arranged in and then contracts the muscle, and the contracseries within the smooth muscle of the membran- tion has been blocked by phentolamine, an eous portion of the trachea and the main bron- alpha-blocking agent (22). The relaxation has chi (36). These latter receptors exhibit a tonic been blocked by propranolol, a beta-blocking activity and respond more to positive than to agent, and it was concluded that both alpha and negative transmural pressure (37). The reflexes, beta receptors are present on the canine smooth which arise from stimulation of the receptors, muscle in the trachea and the bronchioles (46). are blocked by local anesthetics, and this block- No evidence for nonadrenergic or purinergic ade is a temporary, reversible phenomenon (41). innervation has been found in the canine airT h e number of irritant receptors increases from way smooth muscle (22), but the more distal the upper trachea to the lower bronchi and then airways have not as yet been studied. Studies of sharply decreases in the smaller airways (42). canine airways with an internal diameter of 1.5 T h e nature of the irritant receptors varies with mm have not shown nonadrenergic inhibitory the species of animal, and substances that stim- innervation (Russell, J. A.: Responses of isoulate the receptors in one species may not affect lated canine airways to electrical stimulation and the receptors in another species (43). The role acetylcholine, J Appl Physiol, 1978, 45, 690). of these receptors in the control of airway caliber in normal respiration is unclear, but an increase Cat in ventilation acts via the stretch receptors to General description. A very complete morphodilate the airways, whereas stimulation of the logic study of the innervation of the airway irritant receptors results in constriction of the smooth muscle of the cat has been performed by airways (38). Severance of the vagus nerve abol- Silva and Ross (47). They found a dense inishes these reflexes, but they will return if the nervation with both cholinergic and adrenergic lung is reinnervated (39). nerves and numerous ganglia in the peribronMorphologic summary. T h e canine lung re- chial tissue to the level of the small bronchi (47). ceives cholinergic innervation via the vagus The axon profiles of the nerves in the trachea nerve and the ganglia. Postganglionic fibers in- showed 2 types of granular vesicles, as well as nervate the smooth muscle of the airways, the agranular vesicles (47). One type of granular pulmonary arteries, the bronchial arteries, and vesicle is 30 to 60 nm in diameter, and the other possibly the perialveolar capillaries. The ques- type is 70 to 120 nm (47). The smaller granular tion of cholinergic innervation of the bronchial vesicles are destroyed by 6-hydroxydopamine, glands has not been answered. Postganglionic the larger ones are not affected (47). Some of adrenergic nerves from the stellate ganglion en- the ganglia show specific fluorescence for cateter the lung, intermingled with the vagus, and cholamines (47). T h e cholinergic and the adrenthe adrenergic nerves innervate the smooth ergic nerves have a similar distribution, and muscle of the airways, the pulmonary arteries, the innervation of the bronchioles is similar to and the larger pulmonary veins, but veins with that in the bronchi (47). T h e innervation of the a diameter of less than 30 ^m are not innervated. pulmonary blood vessels was not mentioned T h e bronchial arteries have a dense adrenergic (47). innervation. Adrenergic nerves, probably accomAirway smooth muscle. Stimulation of the vapanying the bronchial arteries that supply blood gus in cats constricted the trachea and the bronto the ganglia, encircle and may enter the chi with a maximal constriction of airways with ganglia in a manner similar to that of adrenergic a resting diameter of 0.8 to 2.0 mm and a minnerves in the myenteric plexus. Numerous re- imal constriction of airways less than 0.8 mm flexes have been demonstrated in the canine in diameter (4). T h e reason for the diminished lung, and their anatomic location has been constriction in the smaller airways is unclear, roughly determined by physiologic and pharma- but these authors have suggested that there may cologic techniques. Morphologic demonstration be a decrease in the number of efferent fibers of these endings is not available in sufficient de- in these airways as a consequence of fewer gangtail to warrant further comment on these end- lion cells in the distal airways and thus more ings. inhibition from adrenergic nerves (4). Physiologic studies. Electrophysiologic studVasculature. T h e pulmonary and the bronies of the trachealis muscle have demonstrated chial arteries of the cat have adrenergic nerves cholinergic contractile responses that are on the surface of the media and within the me-

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dia (48). No mention was made of the pulmonary veins in this study (48). Sensoiy receptors. Juxtacapillary or J type receptors have been demonstrated in the cat (49). These nerve endings have been stimulated by pulmonary congestion and other factors that alter the interstitial pressure (49). This study concluded that interstitial pressure increases with capillary pressure, and the increase in interstitial pressure stimulates the receptors (49). Thus, during exercise, which increases capillary pressure, stimulation of the receptors would signal a reflex inhibition of the limb muscles to terminate the exercise (49). Definite morphologic identification of the afferent nerve terminals has not yet been accomplished, but nerve endings, believed to be compatible with them, have been described (50, 51). In the epithelium of the lower trachea, carina, hilum, and distal airways, 3 types of axon profiles have been identified (50). One type contains agranular vesicles, a second has dense core vesicles with some clear vesicles, and a third has no vesicles (50). Some axon terminals near the lumen contain clear vesicles and mitochondria (50). The highest concentration of intraepithelial axons was found at the carina, and no intraepithelial axons were found in the distal airways (50). Most of the intraepithelial axons were found in the basal zone of the epithelium and appeared to be associated with ciliated cells, whereas in the trachea, these axons, which usually have no vesicles, were associated with the goblet cells and basal cells (50). In one submucosal gland an axon was found that contained clear vesicles and no mitochondria, and this was believed to be a motor nerve terminal (50). T h e authors concluded that they could not establish the functional nature of the endings from morphologic studies alone (50). In another study of the nerve endings in the epithelium of the trachea and the primary bronchi, nonmyelinated axons containing microtubules and mitochondria were observed throughout the entire epithelial layer, but were mainly concentrated in the basal portion just above the basal lamina (51). Most of the axons were under the basal cells, but others were adjacent to granulated cells or between columnar cells (51). In contrast to the previous study (50), no mention was made of axon profiles filled with vesicles, and the author postulated, on the basis of morphologic criteria, that the endings have a sensory function (51). Another study of the nerves in the cat lung concluded on the basis of degeneration studies that some of the

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fibers in the vagus originate in the ganglia and synapse with the stellate ganglion, and are thus part of a peripheral reflex arc of the autonomic system (52). This arc is similar to an arc that they described between the myenteric plexus and the prevertebral ganglia (52). T h e function of such a reflex arc is obscure. The vagus nerve in the cat contains approximately 30,000 fibers, most of which are nonmyelinated (53). Of 4,900 myelinated fibers, approximately 40 per cent are efferent, and these fibers, 12 Atm in diameter, enter the recurrent laryngeal nerve (53). This study showed that 85 per cent of the nonmyelinated fibers are afferent and that 60 per cent of the myelinated fibers are afferent, most of which, however, arise in the gastrointestinal tract (44). T h e number of fibers to the airways is approximately 6,000, and approximately 1,000 of these are efferent (53). This study points to one of the unresolved problems of the innervation of the lung and the gastrointestinal tract, which is the large discrepancy between the number of afferent and efferent nerve fibers. T h e implication is that the ganglia perform a more complicated function than that of simple relays. Recent morphologic (54) and electrophysiologic studies (55-57) of the ganglia in the myenteric plexus support this view. Morphologic summary. T h e general pattern of the innervation of the cat lung is similar to that in the dog except for adrenergic innervation. In the cat, the pulmonary and bronchial vessels, as well as the airway smooth muscle, are innervated by adrenergic nerves. No detailed physiologic or pharmacologic studies have been done on the innervation of the smooth muscle of the airways, and it is not known whether nonadrenergic innervation is present in the cat lung. Chicken General description. T h e morphology of the airways of the domestic chicken has been studied with the light microscope (58-62). T h e distribution of the nerves in the peribronchial and bronchial plexus is similar to the basic pattern in mammals (60). T h e primary bronchus is densely innervated, and the number of nerve fibers decreases towards the periphery (60). Ganglia are frequent and are largest in the peribronchial plexus of the primary bronchus and in the interlobular part of the peribronchial plexus of the tertiary bronchus (60). T h e bronchial plexuses of the primary and secondary bronchi have few, relatively small ganglia in their nerve bundles. Nerve endings were observed within the epithelium of the primary

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(9). This increase in secretions was blocked by atropine and was therefore cholinergic (9). Injection of the nodose ganglion with tritiated leucine, followed by examination of the trachea with autoradiography, showed uptake of the tritiated leucine in the nerve plexus external to the cartilage, as well as some uptake in the epithelium (69). This might indicate that some of the epithelial endings are afferent (69). There was also uptake of the tritiated leucine by a group of cells in the syrinx and in the primary bronchus (69). These authors concluded that the nerve endings in the trachea and primary bronchus are afferent and that the cells that took up the leucine resemble neuroepithelial bodies more than the granular cells described by Walsh and McLelland (70). In the air exchange areas of the chicken lung, there are bundles of axons, and most of these are nonmyelinated (71). Agranular and a few granular vesicles are seen in the axon profiles, and single axons are found between the endothelium and the epithelium (71). These axons contain agranular vesicles and a few microtubules and mitochonciria (71). These authors concluded that these axons do not show the structural features of afferent endings, and no function was ascribed to them (71). They also concluded that the chicken lung contains more nerve fibers in the gas exchange area than does the mammalian lung (71). Specialized cells in the epithelium of the primary bronchus of the chicken have been described (72). These cells contain dense-core, granular vesicles, and the cells are closely associated with nonmyelinated axons. These cells have been considered examples of neurite receptor cell complexes that might be either chemoreceptors or mechanoreceptors, but no definite conclusion has been reached (72). Physiologic studies. Bhatla and Richardson (73) have studied the primary bronchus of the chicken with physiologic and ultrastructural Sensory receptors. Numerous reflexes have techniques. T h e primary response of the smooth been described in the lungs of domestic fowl muscle to electrical field stimulation of the (68, 69), and rather detailed morphologic stud- nerves was relaxation, and this was not blocked ies, using both light and electron microscopy, by propranolol (73). T h e muscle relaxed with have been performed. Numerous nerve endings isoproterenol, and when it was relaxed, field have been described in the epithelium of chick- stimulation of the nerves produced a contracen airways, and many of these endings contain tion that was blocked by atropine (73). From vesicles (60, 67). T h e role of these endings is these studies, it was concluded that the smooth unknown, but they may be motor nerve endings muscle of the primary bronchus of the chickbecause stimulation of the descending, esopha- en, is controlled by nonadrenergic inhibitory geal branches of cranial nerves IX, X, and possi- fibers that are dominant over the cholinergic, a bly XII increased mucous secretions in the goose situation that is the reverse of what has been

bronchus and beneath the epithelium of the primary and tertiary bronchus, but, as the author states, "One can only speculate on the modalities of the possible endings seen in the present study" (60). Airway smooth muscle. T h e smooth muscle cells of the primary bronchus are parallel to the long axis of the bronchus (63). Numerous axons are seen between the muscle cells in the primary bronchus in comparison to the tertiary bronchus, and, on a count of axons per unit area, this was estimated as being 20 time's greater (63). Four types of vesicles were found in the axon profiles: type 1—large, irregular or oval granular vesicles as large as 120 nm in diameter; type 2—small, round granular vesicles 50 to 85 nm in diameter with a densely osmiophilic core 45 to 70 nm in diameter; type 3—small, granular vesicles 50 to 110 nm in diameter with an irregular, usually eccentric core 25 to 50 nm in diameter and type 4—agranular vesicles 40 to 55 nm in diameter (63). T h e axon profiles were of 3 basic types, depending on the type of vesicles that they contained (63). T h e first contained mainly type 4 agranular vesicles associated with cholinergic neurotransmission; the second, granular vesicles of one or more types and the third, numerous granular and agranular vesicles (63). Fluorescent histochemistry did not show adrenergic fibers in the smooth muscle of the primary bronchus (64). T h e ultrastructural study quoted previously (63) makes the point that axon profiles characteristic of adrenergic nerves were not seen (63), but states that this is contrary to pharmacologic studies, in which adrenergic agonists, either administered to the animal (65) or added to tissue baths that contained strips of bronchial muscle (66), relaxed the muscle. In neither of these 2 studies (65, 66) was there stimulation of the nerves, and thus only adrenergic receptors were demonstrated. Other studies have not demonstrated adrenergic nerves in the airways (67).

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found in humans (74). Ultrastructural studies showed numerous axon profiles filled with large, dense-core vesicles of the type associated with nonadrenergic or purinergic inhibition in the intestine (75). T h e smooth muscle also had cellto-cell connections, some of which were of the nexus or gap junction type (73). No adrenergic terminals were seen, nor could adrenergic fibers be demonstrated by physiologic techniques (73). In the epithelium, there were axon profiles that contained agranular and granular vesicles, as well as profiles that contained only mitochondria. The significance of these endings is unknown. T h e pattern of innervation in the chicken resembles that of mammals with an extrachondrial and a subchondral plexus of nerves, ganglia, and a variety of axon profiles, in both the smooth muscle and the epithelium, which contain agranular vesicles of a fairly constant size and granular vesicles of varying size. T h e smooth muscle, with cell-to-cell junctions of the nexus type, resembles the human airway muscle. This type of smooth muscle resembles that of the intestine and may be capable of myogenic activity. The innervation of the pulmonary vessels in the chicken has not been described in any of the studies. Mouse

General description. One of the most detailed studies of pulmonary innervation is that of Honjin (76). This excellent study reviewed the previous work on pulmonary innervation and discussed the limitations of the 4 different histochemical techniques used, but the study was done before the introduction of the fluorescent technique for catecholamines, and thus no comments were made on adrenergic innervation (76). The intrapulmonary nerves were shown to form 3 plexuses: a peribronchial, a perivenous, and a plexus around the bronchial arteries (76). Ganglia were found in the peribronchial and perivenous plexuses and nerve fibers were shown to terminate on the nerve cells in the form of pericellular endings (76). Nerve endings were found in the epithelium of the bronchi and within the smooth muscle of the bronchi, as well as in the wall of the pulmonary veins, and these endings were believed to be sensory. The ganglia are composed of 20 to 30 cells, with the larger ganglia surrounded by a delicate capsule of connective tissue, whereas the smaller ganglia have no capsule, a point that will be raised again in the present review. Isolated

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nerve cells were also seen, and no intrapulmonary ganglia were found beyond bronchi of the third order (76). Section of the vagi and subsequent study of the degenerated efferent fibers led to the author to conclude that some of the cell bodies in the ganglia are sensory in nature (76). Vasculature. T h e cardiac type muscle of the pulmonary vein in the mouse has a dual innervation with cholinergic and adrenergic type axon profiles (77). T h e axons are found in the media and near the intima, but most are in the adventitia (77). T h e axon profiles contained granular vesicles, 38 nm in diameter, and others contained agranular vesicles of the same size (77). T h e former are consistent with adrenergic nerves and the latter with cholinergic nerves. Further study of the pulmonary vasculature was not done, and the authors noted that the role of this cardiac type muscle in the pulmonary veins, despite its description in the nineteenth century, is still unknown (77). Sensory receptors. Groups of nonmyelinated nerves with as many as 9 axons have been demonstrated in the alveolar ducts (78). These nerves are located in the interstitium around the openings of the alveoli, and the nerves contain neurotubules and a few mitochondria (78). Similar, but smaller nerves, have also been demonstrated between the epithelial and the endothelial cells of the alveolar walls, and 2 distinct types of enlarged axons have been described (78). One contains small, dense mitochondria and is associated in some cases with type 1 pneumocytes (78). T h e other type, also located in the septa, is filled with dense-core vesicles 120 nm in diameter (78). These authors speculated that the axons with the small mitochondria are sensory and that the other type filled with vesicles might play a role in type 2 pneumocyte secretion (78). Another study of the bronchial epithelium showed single nonmyelinated axons between the epithelial cells, and these axons were associated with specialized epithelial cells that contained many dense granules (79). Because the axons contained numerous mitochondria, and resembled sensory nerves elsewhere, the authors believed that the axons are sensory in nature and they postulated that the granules in the epithelial cells are serotonin (79). Rat

General description. In a detailed light microscopic study of the rat lung, ganglia were found close to bronchi of large and intermediate

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size and arteries, and were more numerous in the region of the hilum (80). A plexus of nerves was demonstrated in the peribronchial region and another in the submucosa, with interconnection between the two demonstrable as far distally as the bronchioles (80). In the terminal bronchioles, only fine fibers were noted in the mucosa (80). The large pulmonary arteries were shown to have a dense adrenergic innervation with extension of the nerves into the media and the subendothelial region (80). Fluorescent fibers were also seen in the smooth muscle of the bronchi, but there was no communication of the fluorescent fibers with the plexus of nerves in the mucosa (80). An abundant fluorescent fiber network was also seen about the ganglia (25, 80). Vasculature. In a histochemical study for the demonstration of acetylcholinesterase and catecholamines coupled with nonspecific staining for nerves, large nerve trunks were found to enter the lung at the hilum, and these trunks contained myelinated and nonmyelinated fibers (81). Ganglia were found in the peribronchial tissue, and the nerves could be traced to these ganglia (81). Few nerve fibers were closely associated with the blood vessels, but nerves were shown in the bronchial arteries and the pulmonary veins with the pulmonary arteries being poorly supplied with nerves (81). The fluorescent reaction for catecholamines showed that a complex plexus of fibers was associated with the bronchial arteries and that this plexus followed the arteries into the bronchial walls and penetrated the vasa vasorum of the larger pulmonary vessels (81). Fluorescent nerve fibers were not found in the nerve plexus that accompanied the bronchial muscle or the pulmonary vessels (81). This study conflicts with other studies, in which the bronchial musculature, the pulmonary vasculature, and the bronchial arteries all showed adrenergic innervation (80, 82). Sensory receptors. T h e rat lung has been studied by electron microscopy in an attempt to localize the sensory receptors (83, 84). Nerves were encountered in only 2 of 80 blocks from 40 animals (84). T h e axons were found in the interstitium of the alveoli and contained agranular vesicles (83). Some axons, however, were shown to have a bulb-like shape that suggests a receptor function (83). Morphologic summary. T h e rat lung has cholinergic and adrenergic fibers present in the plexuses of the extrachondrial, perivascular, andsubmucosal region, and the general distribution of the nerves is similar to that of other animal

species. Adrenergic nerves supply the bronchial arteries, the pulmonary arteries, and the ganglia. There is conflicting evidence with regard to the adrenergic innervation of the airway smooth muscle (80-82). Nerve endings in the interstitium of the alveoli have been demonstrated with the electron microscope, and these endings are of 2 types: one with a sensory bulb-like formation and the other, an axon profile filled with agranular vesicles. T h e function of these endings is unclear. Nonadrenergic nerves have not been sought in the rat lung. Rabbit Airway smooth muscle and vasculature. A study of the innervation of the bronchi in 5 animals showed that the rabbit has few fluorescent fibers in the trachea and that these most probably go to blood vessels (85). T h e smooth muscle of the bronchi and that of the bronchioles are devoid of adrenergic fibers, but the bronchial arteries are innervated by a dense plexus of adrenergic nerves (85). Catecholamine-containing cells have been found in the ganglia of the bronchi (85). Another study of the rabbit showed adrenergic fibers on the outer surface of the media of the pulmonary arteries with direct innervation of the media demonstrable in some areas (48). Neuroepithelial bodies. An anatomic structure of some controversy, the neuroepithelial body, has been characterized morphologically in the rabbit (86). T h e cells of these bodies take up serotonin (5-HT) and L-Dopa, and it has been suggested that these bodies may play a role as chemoreceptors by release of their amine content, which in turn acts on nerves (86). T h e uptake of L-Dopa places these cells in the ADUP (amine precursor uptake decarboxylation) series (87), which is a series of neuroendocrine cells located throughout the body. Collections of these cells are referred to as neuroepithelial bodies, and it has been postulated that their function is that of receptor cells (88-90). Although they are associated with nerves in the epithelium, their exact function is unclear. Sensory receptors. Sensory receptors have been demonstrated in the rabbit lung by a variety of means, and the irritant type is involved in reflex bronchoconstriction (91). Afferent fibers are present in the vagus nerve, and the activity of these afferent fibers can be measured under a variety of stimuli (91). T h e afferent activity of these nerves may be involved in the modulation of breathing in the rabbit and other animals,

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but this is not the case in humans (92). Some of the afferent activity is in myelinated fibers and some of it is in nonmyelinated fibers, and the fiber type that carries the afferent activity varies with the type of stimulation (92). The significance of this variation between myelinated and nonmyelinated fibers in conduction of afferent activity is unknown. Another study showed a tonic inhibitory effect on the vasomotor center, which was carried in the afferent nerves from receptors in the heart and the lung (93). This study concluded that the afferent activity is carried in the nonmyelinated fibers because cooling of the vagus selectively abolished the activity of the myelinated fibers but not the activity of the nonmyelinated fibers (93). This paper contradicted the preceding study (92) on the question of which fibers in the vagus carry afferent activity. No electron microscopic studies of the nerve endings have been done, and thus the density of these endings, their type, and location are unknown. Guinea Pig

General description. T h e trachea of the guinea pig has been the subject of extensive pharmacologic study, particularly since the introduction of the tracheal chain preparation by Castillo and De Beer (94), but surprisingly little work has been done on the morphology of the guinea pig lung. Most pharmacologic studies agree with the general concept that parasympathetic and adrenergic nerves innervate the airway smooth muscle, with the cholinergic system being dominant (95-98). Coburn and Tomita (99) reinvestigated the innervation of the guinea pig trachea and demonstrated two very important facts. The tracheal smooth muscle is innervated by a nonadrenergic inhibitory system as well as the cholinergic system, and the adrenergic fibers are mainly in the proximal portion of the trachea (99). This localization of the adrenergic fibers to the proximal portion of the trachea was confirmed (100), and further evidence was obtained that showed that these nerves take up norepinephrine (101). T h e adrenergic innervation of the guinea pig airways was further investigated, and the entire lung was examined (102). In the lung, fluorescent fibers were associated with the vasculature, and even with preloading of the adrenergic terminals no demonstrable adrenergic fibers were seen in the smooth muscle of the airways (102). T h e authors found this surprising in view of the large amount of smooth muscle present in the airways, and they

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concluded that the guinea pig is similar to the sheep, the cow, and the goat in that the bronchi are sparsely innervated by adrenergic nerves and the bronchioles are not at all (102). T h e pulmonary artery, vein, and the arterioles, as well as the bronchial arteries, have an extensive adrenergic innervation with most of the fibers present at the medial adventitial boundary of thepulmonary, artery (102). They concluded that the adrenergic system has control of the airways only at the level of the trachea and that inhibition of the more peripheral airway smooth muscle is controlled either by non-neural factors or by the nonadrenergic inhibitory system (102). They further suggested that the nonadrenergic system may be an alternative to the adrenergic system (102). Nonadrenergic innervation. T h e guinea pig trachea was the first mammalian airway shown to contain nonadrenergic inhibitory fibers (99, 103-106). Ultrastructural studies of the trachea show abundant nerve endings filled with agranular vesicles compatible with cholinergic innervation and large granular vesicles (107), but as yet no axon profiles believed to be typical of nonadrenergic nerves have been found (75). The smooth muscle cells are separate from one another, and only the rare connection of the nexus type is seen; this was demonstrated only by freeze-fracture techniques (107). This type of cell connection, coupled with the demonstration of rhythmical contractions of the smooth muscle (108), indicates that it is probably of the single-unit type. This is similar to the smooth muscle of the canine trachea in which nexuses are not usually seen (45) but can be induced, as can rhythmical contractions, with the use of tetraethylammonium (109). T h e type of smooth muscle is important in a consideration of the neural control of the airways, and this will be treated in greater detail in the discussion of human pulmonary innervation. Reptiles and Amphibians

The lungs of the lizard (110, 111) and the toad (112, 113) have been studied with pharmacologic techniques (110, 113) and histochemical techniques (111, 112). T h e lizard has cholinergic nerves that are excitatory and adrenergic nerves that are inhibitory (110, 111). T h e smooth muscle in the lung demonstrates spontaneous activity (110, 111). T h e toad lung has excitatory fibers that are most probably cholinergic because they are blocked by atropine and inhibitory nerves that are adrenergic (112, 113).

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Ganglion cells in the toad lung contain adrenergic neurons, whereas none is demonstrable in the lizard lung (111). T h e inhibitory neurotransmitter in the toad lung is believed to be epinephrine rather than norepinephrine because this is the predominant amine in the lung (113). The smooth muscle in the toad lung also displays spontaneous activity (113). Nonhuman Primates

General description. Acetylcholinesterase-containing nerves have been demonstrated in connective tissue in the hilar region of the rhesus monkey (114). Clusters of ganglion cells were seen in this area before the nerves became associated with the bronchial tree, and ganglia were also present along the bronchial system (114). Connections exist between the extrachondrial and subchondrial plexuses of nerves, and these 2 plexuses extend to the small bronchi and fuse in the region of the bronchioles (114). In the submucosa of the bronchi there are acetylcholinesterase nerves that come from the subchondrial plexus of the bronchi, the perimuscular nerves, and the nerves associated with the bronchial artery (114). T h e submucosal plexus also appears to innervate the submucosal glands in the bronchi (114). Ganglia are common in the regions of bronchial bifurcation, and they are located external to the cartilage (114). The ganglion cells are acetylcholinesterase positive and are surrounded by acetylcholinesterase-positive nerve fibers (114). T h e authors of this study point out that the pattern of innervation in the monkey is different from that in nonprimate mammals in that the nerves that enter the monkey lung have no relationship to any definite structure until the level of the bronchi, where they become associated with the bronchial artery, the pulmonary vein, and the bronchial musculature (114). These authors state further that, although the nerve bundles are acetylcholinesterase positive, no fluorescence typical of adrenergic nerves was demonstrated, and therefore the myelinated and nonmyelinated nerves in these bundles are cholinergic, and the ganglion cells are secondary cholinergic neurons of the vagal system (114). Airway smooth muscle. The bronchi, large and small, and the bronchioles have dense adrenergic plexuses at bifurcation points, but the over-all adrenergic innervation is sparse (82). There is also a dense innervation of acetylcholinesterase-positive fibers in the airways (114). Vasculature. In the lung of the rhesus mon-

key, adrenergic nerves were shown to enter the lung parenchyma via the bronchial artery (82). These adrenergic nerves form a dense plexus at the adventitio-medial junction and appear as a varicose network (82). After several divisions, the nerves were found in the media, where they form a dense plexus (82). The plexus of adrenergic nerves associated with the bronchial arteries gives off branches to the muscle layers of the bronchi and bronchioles (82). The pulmonary arteries of the monkey lung have an adrenergic innervation, and the nerves are located at the adventitio-medial junction with a higher density of nerves at points of bifurcation (82). No adrenergic nerves, however, were observed to enter the media of the pulmonary artery (82). Adrenergic nerves also innervate both the large and small pulmonary veins, and the nerves penetrate the advential and medial layers (82). The bronchial arteries of the monkey therefore have a dual innervation of cholinergic and adrenergic nerves (82, 114). A physiologic study, with perfusion of the lung lobes isolated from Macaca mulata and a Papio species, showed a small increase in the calculated pulmonary vascular resistance, which was often followed by decreased resistance after stimulation of the stellate ganglion (115). In some preparations of the Macaca mulata lung, however, there was no change, whereas the Papio species all showed increased resistance (115). T h e conclusion was that in the Macaca mulata the response to adrenergic stimulation is a decrease in pulmonary vascular resistance mediated through beta receptors, whereas in the Papio species the response is an increase in resistance mediated through alpha receptors (115). Physiologic studies. There are few physiologic studies of the innervation of the nonhuman primate trachea and bronchi, but nonadrenergic inhibitory nerves, cholinergic nerves, and an absence of adrenergic nerves have been demonstrated in the baboon (116), and similar findings have been made in the tracheal smooth muscle of the rhesus monkey (Richardson, J. B.: Unpublished data). T h e innervation of the nonhuman primate airway smooth muscle, and to some extent that of the vasculature, is not completely understood, and no detailed electron microscopic studies are available that localize or identify nerves precisely within the lung. Humans

As stated previously observations of the nerves in the human lung were made some 300 years

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ago, and it is therefore surprising that so little is known about these nerves. T h e light microscopic work done in the late nineteenth and early twentieth centuries has been summarized (117, 118). In these reviews of the light microscopic work, the basic pattern of observed innervation consists of a parasympathetic supply from the vagus nerve and a sympathetic supply from the sympathetic trunk (117, 118). Larsell and Dow (117) made the statement that "the general features of this innervation are described in all textbooks of anatomy and are too familiar to require detailed attention." They pointed out that when the nerve trunks enter the lung at the hilum they became arranged in 2 basic plexuses: a periarterial plexus and a peribronchial plexus, with the latter subdivided into an extrachondrial and a subchondrial plexus according to their relation to the cartilagenous plates (117). T h e extrachondrial plexus lies external to the cartilage between the cartilage and the adventitial tissue, and the subchondrial plexus lies between the cartilage and the epithelium of the airway (117). T h e peribronchial plexus contains myelinated and nonmyelinated fibers, whereas the periarterial plexus contains only nonmyelinated fibers (117). Ganglion cells are scattered along the perichondrial plexus to the level of the smaller bronchi, and these ganglia are found mainly in the extrachondrial plexus, with only a few ganglion cells in the subchondrial plexus (117). Myelinated nerves with endings in both the epithelium and the smooth muscle were also described (117). Postganglionic, nonmyelinated, efferent fibers extend distally to the level of the alveolar duct where they terminate in a small smooth muscle band (117). It was believed that the glands are innervated by cholinergic nerves (117). T h e periarterial plexus starts as bundles of large nerve trunks, but these diminish in size so that at the level of the arterioles, there is only a single fiber (117). These fibers continue to the level of the capillaries (117). These authors pointed out that there appeared to be considerable intermingling between the periarterial plexus and the smooth muscle of the small bronchi, but stated that, although there appeared to be an intermingling, it did not mean that the nerves to the bronchi have a functional purpose (117). Innervation to the pleura of either a human infant or an adult lung, as has been described in the dog and the rabbit (20), was not demonstrated, but this was ascribed to a fault in the staining technique used (117). Afferent-

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type nerve endings were found in the epithelium of the airways and the smooth muscle where they form discrete "smooth muscle spindles" in the adventitia of the pulmonary artery and in the cartilage plates on the luminal side (117). Gaylor (118) also reviewed the previous literature on the lung and added his personal observations. He followed nerve fibers to the level of the terminal bronchioles, but was unable to demonstrate them distal to this (119). Innervation of the epithelium, of the smooth muscle, and of the outer aspect of the glands was demonstrated, but nerve endings were not seen within the glands or on the cell membranes (118). T h e sensory endings present in the smooth muscle were believed to respond to changes in tension and to be of importance in the Hering-Breuer reflex (118). Epithelial endings were believed to detect irritants (118). Axons from these endings were myelinated and terminated in the nodose ganglion or the brainstem (118). T h e complicated scheme of epithelial innervation with numerous collateral branches was believed to give rise to axonic reflexes with some of the collaterals terminating in the ganglia with a motor response from the ganglia (118). This remains a point of controversy (190). A study of 2 fetuses, 2 full-term infants, an 8month-old child, and one adult confirmed and enlarged some of the earlier light microscopic observations (120). T h e main pulmonary nerves enter the lung at the hilum and branch to accompany the bronchi and vessels (120). Afferent endings were described between epithelial cells, in the lamina propria beneath the basement membrane of the epithelium, in the smooth muscle, and in the perichondrium where they form tendril-like structures (121). Postganglionic, nonmyelinated fibers terminate in the smooth muscle, and a few fibers end in knob-like swellings applied to the outer surface of the gland acini, but it was believed that the major innervation of the glands comes from the peribronchial arterial plexus and was sympathetic in type (120). A plexus of thick and thin fibers accompanies the pulmonary arteries, and these fibers appear to be confined to the adventia or the outer media (120). T h e pulmonary veins, however, show a complex network of nerve fibers in the subendothelial region that was believed to be chemoreceptive in nature (120). T h e bronchial arteries are surrounded by a dense plexus of nerves, some of which terminate on the glands (120). Bronchial nerves were not traced beyond the level of the respiratory bronchioles (120).

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This confirmed the observation of Gay lor (118) that sensory nerves supply a definite area of the epithelium by giving off branches, and that there are distinct separations between the branches by noninnervated epithelium (120). Nerve endings believed to be sensory were found in the smooth muscle at the points of bifurcation of the bronchi, and these endings were believed to play a role in the initiation of the Hering-Breuer reflex (120). Three groups of ganglia were described: one in the peribronchial region, one in the intrabronchial or subchondrial region, and one in the perivenous region (120). All 3 groups of ganglia were believed to be part of the parasympathetic system (120). T h e nerve supply to the bronchial glands was considered to arise from the plexus around the bronchial arteries, and no evidence of innervation via the ganglia was found (120). Sensory nerve cells were located in the cervical and thoracic portions of the vagus nerve, and this confirmed the previous finding of Miiller (121). These neurons were believed to be present in addition to the neurons in the inferior vagal ganglion or nodose ganglion (120). T h e trachea receives branches from the recurrent laryngeal and the vagus nerve, and these form a network that is well developed on the posterior portion of the trachea, but only sparsely developed on the anterior portion (122). Ganglia are concentrated on the posterior aspect of the trachea, with the largest ganglion having 10 to 20 cells (122). T h e fibers that enter the trachea are mainly myelinated and when they penetrate the tracheal wall they lose their myelin sheath and divide to form a plexus around the ganglia (122). T h e smooth muscle, glands, and blood vessels have a rich, nonmyelinated nerve supply (122). Sensory endings in the muscle similar to those described in the bronchi (117, 118, 120) have also been found (122). T h e embryology and development of the innervation to the human lung have recently been reviewed (24). T h e following points in the development of the innervation are worthy of emphasis because they may be of importance in an understanding of smooth muscle control, the innervation of the glands, and the presence or absence of receptors. After the formation of the neural crest, neuroblasts in the region of the vagal, lower cervical, and upper thoracic ganglia multiply and migrate along pioneering nerves ventrally throughout the mesenchyma (24). Some of the ganglion cells form the pericardial

and the extrapulmonary plexus, and others migrate further to form the intramural ganglia of the thoracic and upper abdominal viscera (24). These neural-crest cells take up their positions in the wall of the future trachea and lung buds before the separation of the trachea from the gut, which occurs during the fourth or fifth week of gestation or in the 5- to 7-mm embryo (24). T h e blood supply to these ganglia comes from the bronchial arteries (24). T h e common embryologic origin of the nerves and smooth muscle of the gastrointestinal tract and the lungs may therefore result in a similar function of the 2 main components, the nerves and the smooth muscle. Recent studies of the gastrointestinal tract should be considered in relation to the function of these components in the lung. A histochemical, light microscopic study of the lungs from 25 newborns showed a loose plexus of fine nerves around the main blood vessels, but many of the medium-sized arteries and venules were devoid of nerve fibers (123). T h e lymphatic plexus of the hilum had a rich innervation that arose from both branches of the vagus (123). No nerve fibers were seen in the alveolar walls, but encapsulated nerve endings were seen in the visceral pleura and around the diaphragmatic and upper segments of the pleura (123). T h e bronchi had a rich supply of nerves to the muscle, the epithelium, and the glands (123). In a study of the innervation of 9 human fetuses, a chain of nerves and ganglia was found on each side of the trachea at the 85-mm stage, with the densest innervation on the posterior aspect of the trachea (124). Most of the ganglia were located in the extrachondrial plexus, and fibers were found that approached the acini of the glands (124). There are few ultrastructural studies of the human pulmonary innervation. Rhodin (125) described nonmyelinated axons with no vesicles in their profiles at the basal laminar region of the tracheal epithelium. T h e function of these endings is unclear, but with a lack of vesicles in the axon profiles, they resemble the putative afferent endings responsible for the sensory input more than the efferent motor endings (125). Afferent activity from the lung has been recorded in humans, and this activity has been compared to that observed in animal studies (126). Nonmyelinated axons have been described between the basal cells of human submucosal bronchial glands, and terminal axons have been found within the acini near serous cells and near myo-

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epithelial cells, but it is unclear whether these are motor or sensory, although the former possibility seems more plausible (127). T h e human tracheal epithelium, like that of other mammals, has been found to contain granulated cells with dense cores, 80 to 150 nm in diameter, which stain with silver techniques and show fluorescence with the light microscope (128). These cells are believed to be part of the APUD system (88) and may have a chemoreceptor function (128). Nonmyelinated nerve fibers have been found in the submucosal connective tissue and in the mucosa, and the axon profiles contain small agranular vesicles and mitochondria (128). Although the nerve ending suggests a contact with the granulated cells, this has not

been definitely shown, and the nerves are separated from the cells by at least 20 nm (128). It has been proposed that these cells have a chemoreceptor function and may play a role in smooth muscle tone, vascular regulation, and circulatory adaptation at birth because they are more common in the newborn than in the adult, in whom they have never actually been seen (128). From the preceding review of the innervation of the lungs in a variety of species, it is obvious that there is a variation in the innervation, both in type and quantity. It is also clear that little work has been done on the influence of this innervation on the glands, the epithelium, and the vascular and airway smooth muscle. T h e prob-

P&rasympathetic

Sensory

Sympathetic

preganglionic

afferent nerves

postganglionic

j

ABulzan 1979

Fig. 1. Schematic summary of the innervation of the airways. In the human lung, parasympathetic, preganglionic fibers descend into the vagus and terminate in the ganglia. The ganglia contain excitatory neurons that are cholinergic and inhibitory neurons that are nonadrenergic. Other neurons with an integrative function are probably also present. Glial cells (G) are present in the ganglia. Blood vessels and collagen are excluded from the neuropil. Postganglionic fibers to the smooth muscle are excitatory (e) or inhibitory (i). Excitatory fibers may also terminate in the glands. Sensory afferent endings are present in the epithelium and the smooth muscle. The neurons associated with these endings may be in the vagus or the vagal nuclei. Sensory neurons may also be present in the mucosa, and fibers from these neurons may terminate in the ganglia, as in the gastrointestinal tract. Sympathetic postganglionic fibers terminate on the ganglia, in humans and in other species, but adrenergic fibers to the glands or smooth muscle, although found in some mammals, have not been demonstrated in humans. In the epithelium, there are cells, such as the Kultschitzky type cell and the granular cell (A) found only in the chicken, whose functions are unknown. Nerves may be related to these cells. The human airway smooth muscle cells are connected by low resistance junctions, such as the nexus, and these connections may permit the muscle mass to act as a syncytium.

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lem of definite m o r p h o l o g i c identification of the sensory e n d i n g s in the l u n g is unresolved, alt h o u g h t h e physiologic evidence for their existence is firm. T h e d e m o n s t r a t i o n of a third cornc o m p o n e n t to the i n n e r v a t i o n , the n o n a d r e n e r gic or p u r i n e r g i c nervous system, has further c o m p l i c a t e d the p r o b l e m because b o t h the dist r i b u t i o n w i t h i n the l u n g a n d the n o r m a l function of this system are u n k n o w n . If this system is as i m p o r t a n t in the l u n g as it is in the gastrointestinal tract, t h e n t h e p r e s e n t theory of the control of t h e s m o o t h muscle of t h e airways will have to b e revised. T h e role of this third system in the vasculature is u n k n o w n at present a n d needs to b e investigated. It is also e v i d e n t that the type of s m o o t h muscle p r e s e n t in the airways needs to be investigated to d e t e r m i n e w h e t h e r it is s p o n t a n eously active or could, u n d e r conditions of disease, b e c o m e i n d e p e n d e n t of the inhibitory nerves p r e s e n t in t h e l u n g . U l t r a s t r u c t u r a l studies of t h e h u m a n l u n g have been concerned m a i n l y w i t h the e p i t h e l i u m of the distal p o r t i o n s or of the m a j o r airways, a n d little a t t e n t i o n has b e e n p a i d to t h e i n n e r v a t i o n of the smooth muscle o r t h e g l a n d s . A l t h o u g h work is b e i n g d o n e in this area, it is p r e m a t u r e to make any conclusions (129). W e are therefore still in m u c h the same p o s i t i o n as A l e x a n d e r (15) in 1933, w h e n , in a symposium o n t h e a u t o n o m i c nervous system he c o n c l u d e d , " I t is m o r e p r o b a b l e that o u r ideas c o n c e r n i n g t h e b e h a v i o u r of the a u t o n o m i c n e r v o u s system in m a n , a n d especially in disease, are e l e m e n t a r y . "

Acknowledgment T h e writer wishes to thank E. Zorychta and Claudia Ferguson for their help in the proof reading of this manuscript and Miss L. Surette for her patient help in typing it. T h e writer would also like to thank Mrs. Z. Blazina of the Osier Library of McGill University for her help in the translation of Thomas Bartholinus. References 1. Bartholinus, T.: De pulmonum substantia et motu diatribe. Accedunt Marcelli Malpighii de pulmonibus observatione anatomicae, Hafniae [Copenhagen], H. Godiani, 1663, p. 38. 2. Willis, T.: Facsimile of the anatomy of the brain and the description and uses of the nerves in the remaining medical works of that famous and renowned physician. Englished in 1683 by Samuel Pordage, Esq., W. Feindel, ed., McGill University Press, Montreal, 1965, vol. 2, 163. 3. Willis, T.: Of an asthma, in Pharmaceutice Rationalis, part 2, Dring, Harper and Leigh, Lon-

don, 1679, p. 82. 4. Nadel, J. A., Cabezas, G. A., and Austin, J. H.: In vivo roentgenographic examination of parasympathetic innervation of small airways. Use of powdered tantalum and a fine focal spot X-ray tube, Invest Radiol, 1971, 6, 9. 5. H a h n , H. L., Wilson, A. G., Graft, P. D., Fischer, S. P., and Nadel, J. A.: Interaction between serotonin and efferent vagus nerves in dog lungs, J Appl Physiol, 1978,44, 144. 6. Gold, W. M., Kessler, G. F., and Yu, D. Y. D.: Role of vagus nerves in experimental asthma in allergic dogs, J Appl Physiol, 1972,33,719. 7. Cabezas, G. A., Graf, P. D., and Nadel, J. A.: Sympathetic versus parasympathetic nervous regulation of airways in dogs, J Appl Physiol, 1971,57,651. 8. Brody, J. S., Klempfner, G., Staum, M. M., Vidyasagar, D., Kuhl, D. E., and Waldhausen, J. A.: Mucociliary clearance after lung denervation and bronchial transection, J Appl Physiol, 1972,52,160. 9. Phipps, R. J., and Richardson, P. S.: T h e nervous and pharmacological control of tracheal mucus secretion in the goose, J Physiol (Lond), 1976,255, 116. 10. Paulet, G., and Le Bars, R.: Effect of vasomotor innervation on the pulmonary blood mass in dogs, J Physiol (Paris), 1969,1, 160. 11. Widdicombe, J. G.: Regulation of tracheobronchial smooth muscle, Physiol Rev, 1963, 43, 1. 12. Ehinger, B., Falck, B., and Sporrong, B.: Possible axo-axonal synpases between peripheral adrenergic and cholinergic nerve terminals, Z Zellforsch Mikrosk Anat, 1970,107, 508. 13. Kadowitz, P. J., Knight, D. S., Hibbs, R. G., Ellison, J. P., Joiner, P. D., Brody, M. J., and Hyman, A. L.: Influence of 5- and 6-hydroxydopamine on adrenergic tansmission and nerve terminal morphology in the canine pulmonary vascular bed, Circ Res, 1976, 39, 191. 14. Aarseth, P., Nicolaysen, G., and Waaler, B. A.: T h e effect of sympathetic nerve stimulation on pulmonary blood volume in isolated perfused lungs, Acta Physiol Scand, 1971, 52,448. 15. Alexander, H . L.: T h e autonomic control of the heart, lungs and bronchi, Ann Intern Med, 1933,6,1033. 16. Nagaishi, C : Functional Anatomy and Histology of the Lung, University Park Press, Baltimore, 1972, p . 180. 17. Daly, J., and H e b b , C : Innervation of the lungs, in Pulmonary and bronchial vascular systems, Monographs of Physiological Society, 1966, 16, 89. 18. Blumcke, S.: Morphological aspects of the innervation of the lungs, Beitr Klin Eforsch Tuberk Lungenkr, 1968,138,229. 19. Fillenz, M.: Innervation of pulmonary and bronchial blood vessels of the dog, J Anat, 1970,

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106, 449. 20. Larsell, O.: T h e ganglia, plexuses and nerveterminations of the mammalian lung and pleura pulmonalis, J Comp Neurol, 1922,35,97. 21. Woolcock, A. J., Macklem, P. T., Hogg, J. C , and Wilson, N. J.: Influence of autonomic nervous system on airway resistance and elastic recoil, J Appl Physiol, 1969, 26, 814. 22. Suzuki, H., Morita, K., and Kuriyama, H.: Innervation and properties of the smooth muscle of the dog trachea, J p n J Physiol, 1976, 26, 303. 23. Jacobowitz, D., Kent, K. M., Fleisch, J. H., and Cooper, T.: Histofluorescent study of catecholamine-containing elements in cholinergic ganglia from the calf and dog lung, Proc Soc Exp Biol Med, 1973,144, 464. 24. Loosli, C. G., and Hung, K-S.: Development of pulmonary innervation, in Biology of Lung Series, C. Lenfant, ed., Marcel Dekker, Inc., San Francisco, 1977, p. 269. 25. Blumcke, S.: Experimental and morphological studies on the efferent bronchial innervation. I. T h e peribronchial plexus, Beitr Pathol Anat, 1968,137, 239. 26. Norberg, K. A.: Adrenergic innervation of the intestinal wall studied by fluorescence microscopy, Int J Neuropharm, 1964,3, 379. 27. Gabella, G.: Synapses of adrenergic fibers, Experimentia, 1971, 27, 280. 28. Kosterlitz, H. W.., and Robinson, J. A.: Inhibition of the peristaltic reflex of the isolated guinea pig ileum, J Physiol (Lond), 1957, 136, 249. 29. Wikberg, J.: Differentiation between pre- and postjunctional a-receptors in guinea pigs ileum and rabbit aorta, Acta Physiol Scand, 1978, 103, 225. 30. Tranzer, J. P., Thoenen, H., Snipes, R. L., and Richards, J. G.: Recent developments on the ultrastructural aspect of adrenergic nerve endings in various experimental conditions, Prog Brain Res, 1969,31,33. 31. Bradley, G. W., and Scheurmier, N.: Tracheal stretch receptor properties in vitro, J Physiol (Lond), 1977, 267, 48P. 32. Bartlett, D., Jr., Mortola, J. P., and Sant'Ambrogio, G.: Innervation of stretch receptors in the extra-thoracic trachea, J Physiol (Lond), 1977, 268, 36P. 33. Virdruk, E. H., H a h n , H. L., Nadel, J. A., and Sampson, S. R.: Mechanisms by which histamine stimulates rapidly adapting receptors in dog lungs, J Appl Physiol, 1977,43, 397. 34. Sant'Ambrogio, G., Bartlett, D., Jr., and Mortola, J.: Innervation of stretch receptors in the extra-thoracic trachea, Respir Physiol, 1977, 29,93. 35. Coleridge, H . M., and Coleridge, J. C : Impulse activity in afferent vagal C-fibres with endings in the intrapulmonary airways of dogs, Respir

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