Control Of Airway Smooth Muscle Research Articles

Chapter 14 - Neural control of the lower airways: Role in cough and airway inflammatory disease

Airway function is under constant neurophysiological control, in order to maximize airflow and gas exchange and to protect the airways from aspiration, damage, and infection. There are multiple sensory nerve subtypes, whose disparate functions provide a wide array of sensory information into the CNS. Activation of these subtypes triggers specific reflexes, including cough and alterations in autonomic efferent control of airway smooth muscle, secretory cells, and vasculature. Importantly, every aspect of these reflex arcs can be impacted and altered by local inflammation caused by chronic lung disease such as asthma, bronchitis, and infections. Excessive and inappropriate activity in sensory and autonomic nerves within the airways is thought to contribute to the morbidity and symptoms associated with lung disease.

Orexin-A Excites Airway Vagal Preganglionic Neurons via Activation of Orexin Receptor Type 1 and Type 2 in Rats.

Airway vagal nerves play a predominant role in the neural control of the airway, and augmented airway vagal activity is known to play important roles in the pathogenesis of some chronic inflammatory airway diseases. Several lines of evidence indicate that dysfunctional central orexinergic system is closely related to the severity of airway diseases, however, whether orexins affect airway vagal activity is unknown. This study investigates whether and how orexin-A regulates the activity of medullary airway vagal preganglionic neurons (AVPNs). The expression of orexin receptor type 1 (OX1R) and type 2 (OX2R) was examined using immunofluorescent staining. The effects of orexin-A on functionally identified inspiratory-activated AVPNs (IA-AVPNs), which are critical in the control of airway smooth muscle, were examined using patch-clamp in medullary slices of neonatal rats. Airway vagal response to injection of orexin-A into the magna cisterna was examined using plethysmography in juvenile rats. The results show that retrogradely labeled AVPNs were immunoreactive to anti-OX1R antibody and anti-OX2R antibody. Orexin-A dose-dependently depolarized IA-AVPNs and increased their firing rate. In synaptically isolated IA-AVPNs, the depolarization induced by orexin-A was blocked partially by OX1R antagonist SB-334867 or OX2R antagonist TCS OX2 29 alone, and completely by co-application of both antagonists. The orexin-A-induced depolarization was also mostly blocked by Na+/Ca2+ exchanger inhibitor KB-R7943. Orexin-A facilitated the glutamatergic, glycinergic and GABAergic inputs to IA-AVPNs, and the facilitation of each type of input was blocked partially by SB-334867 or TCS OX2 29 alone, and completely by co-application of both antagonists. Injection of orexin-A into the magna cisterna of juvenile rats significantly increased the inspiratory and expiratory resistance of the airway and consequently decreased the dynamic compliance of the lungs, all of which were prevented by atropine sulfate or bilateral vagotomy. These results demonstrate that orexin-A excites IA-AVPNs via activation of both OX1R and OX2R, and suggest that increased central synthesis/release of orexins might participate in the pathogenesis of airway diseases via over-activation of AVPNs.

Activation of α1-adrenoceptors facilitates excitatory inputs to medullary airway vagal preganglionic neurons.

In mammals, the neural control of airway smooth muscle is dominated by a subset of airway vagal preganglionic neurons in the ventrolateral medulla. These neurons are physiologically modulated by adrenergic/noradrenergic projections, and weakened α₂-adrenergic inhibition of them is indicated to participate in the pathogenesis and exacerbation of asthma. This study tests whether these neurons are modulated by α₁-adrenoceptors, and if so, how. In anesthetized adult rats, microinjection of the α₁A-adrenoceptor agonist A61603 (1 pmol) unilaterally into the medullary region containing these neurons caused a significant increase in airway resistance, which was prevented by intraperitoneal atropine (0.5 mg/kg). In rhythmically firing medullary slices of newborn rats, A61603 (10 nM) caused depolarization in both the inspiratory-activated and inspiratory-inhibited airway vagal preganglionic neurons that were retrogradely labeled, and a significant increase in the spontaneous firing rate. Under voltage clamp, A61603 significantly enhanced the spontaneous excitatory inputs to both types of neurons and caused a tonic inward current in the inspiratory-activated neurons along with significantly increased peak amplitude of the inspiratory inward currents. The responses in vitro were prevented by α₁A-adrenoceptor antagonist RS100329 (1 μM), which alone significantly inhibited the spontaneous excitatory inputs to both types of the neurons. After pretreatment with tetrodotoxin (1 μM), A61603 (10 or 100 nM) had no effect on either type of neuron. We conclude that in rats, activation of α₁-adrenoceptors in the medullary region containing airway vagal preganglionic neurons increases airway vagal tone, and that this effect is primarily mediated by facilitation of the excitatory inputs to the preganglionic neurons.

Increased Expression of RhoA in Epithelium and Smooth Muscle of Obese Mouse Models: Implications for Isoprenoid Control of Airway Smooth Muscle and Fibroblasts

The simultaneous rise in the prevalence of asthma and obesity has prompted epidemiologic studies that establish obesity as a risk factor for asthma. The alterations in cell signaling that explain this link are not well understood and warrant investigation so that therapies that target this asthma phenotype can be developed. We identified a significant increase in expression of the small GTPase RhoA in nasal epithelial cells and tracheal smooth muscle cells from leptin-deficient (ob/ob) mice compared to their wild-type counterparts. Since RhoA function is dependent on isoprenoid modification, we sought to determine the role of isoprenoid-mediated signaling in regulating the viability and proliferation of human airway smooth muscle cells (ASM) and normal human lung fibroblasts (NHLF). Inhibiting isoprenoid signaling with mevastatin significantly decreased the viability of ASM and NHLF. This inhibition was reversed by geranylgeranyl pyrophosphate (GGPP), but not farnesyl pyrophosphate (FPP), suggesting specificity to the Rho GTPases. Conversely, increasing isoprenoid synthesis significantly increased ASM proliferation and RhoA protein expression. RhoA expression is inherently increased in airway tissue from ob/ob mice, and obesity-entrained alterations in this pathway may make it a novel therapeutic target for treating airway disease in the obese population.

Protein Implicated in Nonsyndromic Mental Retardation Regulates Protein Kinase A (PKA) Activity

Mutation of the coiled-coil and C2 domain-containing 1A (CC2D1A) gene, which encodes a C2 domain and DM14 domain-containing protein, has been linked to severe autosomal recessive nonsyndromic mental retardation. Using a mouse model that produces a truncated form of CC2D1A that lacks the C2 domain and three of the four DM14 domains, we show that CC2D1A is important for neuronal differentiation and brain development. CC2D1A mutant neurons are hypersensitive to stress and have a reduced capacity to form dendrites and synapses in culture. At the biochemical level, CC2D1A transduces signals to the cyclic adenosine 3',5'-monophosphate (cAMP)-protein kinase A (PKA) pathway during neuronal cell differentiation. PKA activity is compromised, and the translocation of its catalytic subunit to the nucleus is also defective in CC2D1A mutant cells. Consistently, phosphorylation of the PKA target cAMP-responsive element-binding protein, at serine 133, is nearly abolished in CC2D1A mutant cells. The defects in cAMP/PKA signaling were observed in fibroblast, macrophage, and neuronal primary cells derived from the CC2D1A KO mice. CC2D1A associates with the cAMP-PKA complex following forskolin treatment and accumulates in vesicles or on the plasma membrane in wild-type cells, suggesting that CC2D1A may recruit the PKA complex to the membrane to facilitate signal transduction. Together, our data show that CC2D1A is an important regulator of the cAMP/PKA signaling pathway, which may be the underlying cause for impaired mental function in nonsyndromic mental retardation patients with CC2D1A mutation.

Bronchial inflammation induced PKCζ over-expression: involvement in mechanical properties of airway smooth muscle

Protein kinase C variants (PKCs) have been involved in the control of airway smooth muscle (ASM) tone, and abnormalities in PKC-dependent signaling have been associated with respiratory diseases such as asthma. In this study, the role of atypical PKCζ in airway hyperresponsiveness was investigated, using an in-vitro model of TNFα-treated human bronchi and an in vivo guinea pig model of chronic asthma. Our results demonstrated that PKCζ-specific inhibition produced a significant increase in isoproterenol sensitivity in TNFα-treated bronchi and ovalbumin (OVA)-sensitized guinea pig bronchi. The role of epoxy-eicosanoids, known to exert anti-inflammatory effects in lung, on PKCζ expression and activity in these models was evaluated. An enhanced PKCζ protein expression was delineated in TNFα-treated bronchi when compared with control (untreated) and epoxy-eicosanoid-treated bronchi. Measurements of Ca(2+) sensitivity, performed in TNFα-treated bronchi, demonstrated that treatment with myristoylated (Myr) PKCζ peptide inhibitor resulted in significant reductions of pCa-induced tension. Epoxy-eicosanoid treatments had similar effects on Ca(2+) sensitivity in TNFα-treated bronchi. In control and epoxy-eicosanoid-treated bronchi, the phosphorylated forms of p38MAPK and CPI-17 were significantly decreased compared with the TNFα-treated bronchi. An enhanced expression of PKCζ was ascertained in our in-vivo model of allergic asthma. Hence an increased Ca(2+) sensitivity could be explained by the phosphorylation of p38-MAPK, which in turn leads to phosphorylation and activation of the CPI-17 regulatory protein. This process was reversed upon treatment with the Myr-PKCζ-peptide inhibitor. The present data provide relevant evidence regarding the role of PKCζ in human and rodent models of airways inflammation.

A Shh/miR-206/BDNF Cascade Coordinates Innervation and Formation of Airway Smooth Muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

Muscarinic receptor antagonists, from folklore to pharmacology; finding drugs that actually work in asthma and COPD

In the lungs, parasympathetic nerves provide the dominant control of airway smooth muscle with release of acetylcholine onto M3 muscarinic receptors. Treatment of airway disease with anticholinergic drugs that block muscarinic receptors began over 2000 years ago. Pharmacologic data all indicated that antimuscarinic drugs should be highly effective in asthma but clinical results were mixed. Thus, with the discovery of effective β-adrenergic receptor agonists the use of muscarinic antagonists declined. Lack of effectiveness of muscarinic antagonists is due to a variety of factors including unwanted side effects (ranging from dry mouth to coma) and the discovery of additional muscarinic receptor subtypes in the lungs with sometimes competing effects. Perhaps the most important problem is ineffective dosing due to poorly understood differences between routes of administration and no effective way of testing whether antagonists block receptors stimulated physiologically by acetylcholine. Newer muscarinic receptor antagonists are being developed that address the problems of side effects and receptor selectivity that appear to be quite promising in the treatment of asthma and chronic obstructive pulmonary disease.

HCN channel blockers increase parasympathetic ganglionic transmission in guinea pig airways

Hyperpolarization activated cyclic nucleotide gated (HCN) channels regulate neuronal membrane potential. We assessed whether HCN channel blockers (ZD7288 or Cs+) alter vagal control of airway smooth muscle. In anaesthetized, ventilated guinea pigs bilateral electrical stimulation (32Hz, 10 sec) of the vagi produced voltage dependent (1‐12V), atropine sensitive airway contractions. The voltage producing 50% maximum contraction was reduced from 6.0±0.7 in vehicle (n=6) to 3.3±0.7 in ZD7288 (2mg/kg, iv, n=7) treated animals, P<0.05. Vagus nerve‐evoked contractions in vitro were potentiated by 5mM Cs+ (n=8). At stimulus intensities of 10.3±1.3 volts, tracheal contractions averaged 25.4±4.5 and 62.7±12.5 percent maximum before and after Cs+. Cs+ also evoked spontaneous rhythmical airway contractions (onset=17.0±4.9 mins, peak magnitude=29.6±8.2 percent maximal tracheal contraction). Spontaneous contractions were (a) abolished by 1μM atropine (n=3) and 1μM tetrodotoxin (n=3), (b) reversed by the ganglionic blocker hexamethonium (100mM, n=4) and (c) almost absent in tissues following 48 hours in organotypic culture. Preliminary immunohistochemical experiments show that preganglionic neurons in the nucleus ambiguus may express HCN1 and HCN4 channel subtypes. These data suggest that inhibition of HCN channels on vagal preganglionic neurons may increase ganglionic transmission in the airways.

Isolated porcine bronchi provide a reliable model for development of bronchodilator anti‐muscarinic agents for human use

In human airways, muscarinic acetylcholine receptors (mAChRs) exert a predominant role in the control of airways resistance and anti-muscarinic agents are currently included in the pharmacological treatment of chronic obstructive pulmonary disease (COPD). However, the development of more effective mAChR antagonists is hampered by considerable species variability in the ultrastrucural and functional control of airway smooth muscle, making extrapolation of any particular animal model questionable. This study was designed to characterize the mAChRs in a bronchial preparation from pigs, animals considered to provide close models of human biology. Smooth muscle bronchial strips were examined by electron microscopy in order to compare their neuromuscular structure with that of human bronchi and used to study the affinity of a series of selective mAChR antagonists, estimated as pKis in competition binding assays with NMS and pA2, by Schild analysis, in contractile experiments. Pharmacodynamic binding parameters and affinity profiles of a series of antagonists were consistent with the presence of a majority of M2 mAChRs along with a minor population of M3 mAChRs. Functionally, the highly significant correlation between postjunctional pA2 affinities and corresponding affinity constants at human recombinant M1-M5 subtypes indicated that smooth muscle contraction in porcine bronchi, as in human bronchi, was dependent on the M3 subtype. Based on the characterization of mAChRs, isolated porcine bronchi provide an additional experimental model for development of mAChR antagonists for the treatment of human airway dysfunctions.

Cholinergic Pathways in the Lungs and Anticholinergic Therapy for Chronic Obstructive Pulmonary Disease

Abundant data from animal models and humans support the hypothesis that changes at the level of parasympathetic neuronal control of airway smooth muscle result in increased bronchoconstriction in response to vagal stimulation, leading to airway hyperresponsiveness. Neuronal inhibitory M2 muscarinic acetylcholine receptors on parasympathetic nerves are responsible for limiting acetylcholine release from these nerves. In humans with asthma, and after pulmonary inflammatory events in experimental animals, these receptors are dysfunctional, which results in airway hyperresponsiveness. Although it is unknown what mechanisms underlie airway hyperresponsiveness in chronic obstructive pulmonary disease, loss of parasympathetic control of airway smooth muscle is thought to be a contributing mechanism. As such, anticholinergic therapy is used extensively and with a high degree of success in the treatment of this condition. The future for inhaled anticholinergic compounds for the treatment of chronic obstructive pulmonary disease appears to rest in their combination with other agents, such as beta2 agonists and phosphodiesterase-4 inhibitors. Nonselective anticholinergic agents might be the best choice, because M2 muscarinic receptors on airway smooth muscle inhibit the generation and accumulation of cyclic adenosine monophosphate. Adequate concurrent blockade of M3 muscarinic receptors would be expected to counteract the enhanced acetylcholine release that would result from blockade of neuronal inhibitory M2 muscarinic receptors.

An in vivo guinea pig preparation for studying the autonomic regulation of airway smooth muscle tone

The autonomic nervous system plays a primary role in regulating airway smooth muscle tone. Here, we describe the development of an in vivo guinea pig model that permits systematic studies of the autonomic control of airway smooth muscle. The model is based on preparations previously described and utilizes measurements of isometric tension in a perfused segment of extrathoracic guinea pig trachea in situ. It has the advantage that the autonomic innervation to the tracheal segment under study can be physiologically or pharmacologically isolated and studied independently from other mechanisms regulating airway smooth muscle tone. Initial experiments were conducted to optimize model conditions. Subsequent experiments were designed to highlight the usefulness of this preparation for studying parasympathetic regulation of airway caliber. The results of the study demonstrate the utility of this model for future studies into the neural regulation of bronchomotor tone and the mechanisms of airway obstruction and hyperreactivity associated with disease.

Ozone-induced hyperresponsiveness and blockade of M2 muscarinic receptors by eosinophil major basic protein.

Control of airway smooth muscle is provided by parasympathetic nerves that release acetylcholine onto M(3) muscarinic receptors. Acetylcholine release is limited by inhibitory M(2) muscarinic receptors. In antigen-challenged guinea pigs, hyperresponsiveness is due to blockade of neuronal M(2) receptors by eosinophil major basic protein (MBP). Because exposure of guinea pigs to ozone also causes M(2) dysfunction and airway hyperresponsiveness, the role of eosinophils in ozone-induced hyperresponsiveness was tested. Animals were exposed to filtered air or to 2 parts/million ozone for 4 h. Twenty-four hours later, the muscarinic agonist pilocarpine no longer inhibited vagally induced bronchoconstriction in ozone-exposed animals, indicating M(2) dysfunction. M(2) receptor function in ozone-exposed animals was protected by depletion of eosinophils with antibody to interleukin-5 and by pretreatment with antibody to guinea pig MBP. M(2) function was acutely restored by removal of MBP with heparin. Ozone-induced hyperreactivity was also prevented by antibody to MBP and was reversed by heparin. These data show that loss of neuronal M(2) receptor function after ozone is due to release of eosinophil MBP.

Neuroeffector mechanisms: The interface between inflammation and neuronal responses

There is a complex relation between inflammation and neural control of the airways. Cholinergic neurotransmission may be enhanced by inflammatory mediators; cholinergic nerves are the dominant neural pathway for bronchoconstriction in humans. Anticholinergic drugs are more effective in acute severe asthma than in chronic asthma, suggesting that cholinergic mechanisms may be important in exacerbations. Several possible abnormalities in adrenergic control in asthma have been proposed and may be caused by the inflammatory process. Adrenergic nerves do not have direct control of airway smooth muscle but may influence bronchomotor tone in several ways, such as adrenergic neural control of the bronchial vasculature or a secondary effect on cholinergic neurotransmission. Nonadrenergic noncholinergic (NANC) mechanisms mediate both bronchoconstriction and bronchodilation, and a defect in NANC bronchodilatation has been suggested to operate in severe asthma. Relatively little is known about the properties of airway sensory (afferent) nerves in human beings. They are thought to be involved in symptoms of cough and chest tightness, and the threshold for their activation is lowered in conditions of chronic inflammation. In addition, retrograde activation of sensory nerves by a local axon reflex, resulting in the release of peptides, may contribute to inflammation of the airways. Neurogenic inflammation is probably not relevant to mild asthma, however, but it may be more important in severe disease such as brittle asthma. (J A LLERGY C LIN I MMUNOL 1996;98:S73-83.)

Expression of multiple isoenzymes of protein kinase C in airway smooth muscle.

Protein kinase C (PKC) has been implicated in the control of airway smooth muscle (ASM) tone, and abnormalities in PKC-dependent signaling may be associated with asthma. PKC exists in different isoforms, but the pattern of their expression in ASM has not been previously reported. Accordingly, the purpose of the present study was to identify which isoforms of PKC are expressed in ASM. Tissue samples of canine ASM were homogenized and subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by immunoblotting with a range of antipeptide antibodies to PKC-alpha, -beta I, -beta II, -gamma, -delta, -epsilon, -eta, -theta, and -zeta. Positive controls were run in parallel with ASM. Immunoblots revealed a mixture of both calcium-dependent and calcium-independent isozymes: PKC-beta I and PKC-beta II were the only conventional isoforms detected; PKC-delta, PKC-epsilon, and the new muscle-specific isoform, PKC-theta, were all expressed in ASM, but the lung-specific isoform, PKC-eta, was not detected. The calcium- and phospholipid-independent isoform, PKC-zeta, was also present. Thus, expression of a wide variety of both calcium-dependent and calcium-independent isoforms suggests a complex, multifunctional role of PKC in ASM.

Early ontogeny of the vagus nerve: An analysis of the medulla oblongata and cervical spinal cord of the postnatal rat

Retrograde transport of cholera toxin conjugated with horseradish peroxidase in the postnatal rat has revealed remarkable features of dendritic fields of vagal motor neurons in the medulla oblongata and cervical spinal cord during the period of early development (0–10 days). At birth, vagal motor neurons in the dorsal motor nucleus of the vagus, nucleus ambiguus, nucleus retroambigualis, nucleus dorsomedials and the spinal nucleus of the accessory nerve are small with relatively few, unbranched processes. The span of the dendritic tree is much smaller than that found in adult animals. By the postnatal Day 2 there are marked changes in the soma as well as in the dendritic tree of these neurons. There is dispersion of the cell bodies within the neuropil as well as an expansion of the total area of the brain stem occupied by these motor neurons and their dendritic processes which show extensive growth and branching. By postnatal Day 3 the most extensive proliferation of these neurons is seen and appears to represent the peak of dendritic growth of vagal motor neurons such that the area occupied by the dendritic tree of a single neuron is three times that seen in an adult rat. This proliferation gradually decreased during the subsequent seven days of early development (i.e. Days 4–10) so that by Day 10 the dendritic span of vagal motor neurons was reduced to about twice the adult size. This growth progressively decreased from Days 10 to 30 at which time adult levels were reached. Ultrastructural examination of these horseradish peroxidase labeled dendrites showed a positive correlation between the number of dendritic processes and the number of axo-dendritic synapses. This was accompanied by an increase in the number of identifiable synaptic junctions. These morphological complexities observed during the period of early development of vagal motor neurons indicate that the vagus nerve undergoes dramatic changes during the period of early development including the establishment of numerous synaptic contacts between vagal afferents and efferents in the brainstem. A number of these changes occur in developing dendritic fields of vagal motor neurons during the first three days of neonatal life. It is reasonable to assume that developmental abnormalities during this “critical period” could produce significant functional changes in the pattern of respiration as well as in the control of airway smooth muscle.

Cyclooxygenase inhibitors increase canine tracheal muscle response to parasympathetic stimuli in situ

To determine whether cyclooxygenase inhibitors alter parasympathetic control of airway smooth muscle in situ, we pretreated anesthetized dogs with intravenous indomethacin, meclofenamate, or normal saline and measured the isometric contraction of tracheal muscle in response to electrical stimulation of the vagus nerves. Indomethacin and meclofenamate increase the response of airway smooth muscle to parasympathetic stimulation. In subsequent experiments to determine the site of action of cyclooxygenase inhibitors, we found that indomethacin does not alter the response of tracheal muscle to intra-arterial acetylcholine (a muscarinic agonist) but does augment the response to intra-arterial dimethylpiperaziniumiodide (a nicotinic agonist). Moreover, the response to parasympathetic stimulation after pretreatment with a combination of indomethacin and BW755C (a combined cyclooxygenase-lipoxygenase inhibitor) does not differ significantly from the response after indomethacin or meclofenamate alone. We conclude that cyclooxygenase inhibitors increase the sensitivity of the contractile response of tracheal smooth muscle to parasympathetic stimulation, that they exert their effect on the postganglionic parasympathetic neuron, and that their effect is prejunctional. The effect appears secondary to a decrease in cyclooxygenase products rather than to an increase in lipoxygenase products. These findings suggest that endogenous cyclooxygenase products may modulate parasympathetic control of airway smooth muscle in vivo. They may relate to the mechanisms that underlie airway hyperresponsiveness, by which mediators of inflammation modulate airway responsiveness and by which nonsteroidal anti-inflammatory drugs induce severe bronchoconstrictor responses in some persons who have asthma.

Cholinergic control of airway smooth muscle.

Cholinergic control of airway smooth muscle.

Presynaptic control of airway smooth muscle.

Presynaptic control of airway smooth muscle.

Respiratory Physiology Innervation of Airway Smooth Muscle

Our understanding of the control of airway smooth muscle contraction, a topic of great practical importance, has been significantly advanced in the recent past. Particular aspects of the anatomy and physiology of this system are summarized in this section. The macroscopic and microscopic circuitry of the nerve supply to the smooth muscle of the larger airways is discussed in the article by Gabella. Most afferent fibers are vagal ones that originate in the nodose ganglion, whereas most efferent fibers arise from ganglia in the walls of the large airways. While the principal output to the muscle fibers is cholinergic and excitatory, there are also adrenergic fibers, as well as nonadrenergic, noncholinergic fibers. The precise course and site of terminal synapses of the nerves from the sympathetic and parasympathetic systems are not clear; for example, it is not known how many sympathetic fibers synapse in the paratracheal ganglia and how many pass through these ganglia. In his article, Coburn describes the function of the paratracheal ganglia in the airway. These ganglia integrate inputs from parasympathetic, sympathe­ tic, and nonadrenergic/noncholinergic nerves and provide local control of airway smooth muscle to a much greater degree than was appreciated until recently . Kalia first summarizes the microcircuitry of the motor outflow from the brain stem nuclei to the smooth musculature via the ganglionic network of the