Anatomy and neurophysiology of cough: CHEST Guideline and Expert Panel report

Abstract

Bronchopulmonary C-fibers and a subset of mechanically sensitive, acid-sensitive myelinated sensory nerves play essential roles in regulating cough. These vagal sensory nerves terminate primarily in the larynx, trachea, carina, and large intrapulmonary bronchi. Other bronchopulmonary sensory nerves, sensory nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature regulate cough patterning and cough sensitivity. The responsiveness and morphology of the airway vagal sensory nerve subtypes and the extrapulmonary sensory nerves that regulate coughing are described. The brainstem and higher brain control systems that process this sensory information are complex, but our current understanding of them is considerable and increasing. The relevance of these neural systems to clinical phenomena, such as urge to cough and psychologic methods for treatment of dystussia, is high, and modern imaging methods have revealed potential neural substrates for some features of cough in the human.

Figure 1 –

Peripheral mechanisms of cough. A dual-sensory neuron model subserving cough. Extensive studies in animals and humans support the concept that at least two subtypes of primary sensory neurons can induce coughing when stimulated. C-fiber nociceptors, which have their terminals in and around the mucosa surface of the airways, are sensitive to a wide variety of inhaled or locally produced chemical mediators, which may either activate or sensitize nociceptor nerve endings. Mechanically sensitive “cough receptors” are positioned beneath the epithelium in the large airways and are relatively insensitive to most chemical mediators (with the exception of low pH) but are exquisitely sensitive to punctate stimuli delivered to the mucosal surface (for example, inhaled particulate matter). A number of peripherally acting compounds that block nociceptor and or mechanoreceptor activation to reduce coughing are listed in the gray box. TRPA1 = transient receptor potential A1; TRPV1 = transient receptor potential vanilloid 1.

Figure 2 –

Representative traces of single-unit recordings from airway vagal afferent nerve subtypes in anesthetized rats. A, Airway C-fibers are quiescent during tidal breathing and relatively unresponsive to lung inflation. However, C-fibers respond vigorously to IV injected capsaicin. B and C, Rapidly adapting receptors (RARs) (B) and slowly adapting receptors (SARs) (C) are sporadically active during the respiratory cycle. Neither subtype of mechanoreceptor responds to capsaicin, but both respond intensely when the lungs are inflated. Note that RARs are easily differentiated from SARs by their rapid adaptation during sustained lung inflation. ABP = arterial BP; AP = action potential; Cap = capsaicin; P_t = tracheal pressure. (Reprinted with permission from Ho et al.¹¹)

Figure 3 –

Structural organization of airway nerve terminals in guinea pigs and humans. A-A′′, The arrangement of a cough receptor terminal in the guinea pig airways. Heavy chain neurofilaments (A) are expressed in the cough receptor axon and major branch points, indicative of the myelinated nature of these sensory nerve subtypes. These major branch points give rise to a more complex terminal structure (A′) that is defined by the expression of an isozyme of the Na⁺/K⁺ ATPase containing the α3 subunit (α3 ATPase). B, Staining of nerve fibers innervating the human airways revealed using the pan-neuronal marker protein gene product 9.5 (PGP9.5) (original magnification × 20). C, Some of these fibers express heavy chain neurofilament proteins and are comparable to the structures in guinea pigs that give rise to cough receptor terminals (original magnification × 10). Scale bars represent 50 μm.

Figure 4 –

A, Time-synchronized acoustic recording; B, esophageal pH impedance. Acoustic recording shows a period of speech followed by two cough sound waveforms. The gray area in the impedance trace shows a retrograde fall in esophageal impedance associated with a fall in pH below 4 in the distal pH channel, indicative of an acid reflux event. As the cough sounds commence within 2 min of the start of the reflux event, they are considered associated. The cough event marker is created by the patient pressing a button on the impedance device to document the coughing. (Data from Smith et al.⁸⁹)

Figure 5 –

Central mechanisms regulating cough. Airway sensory neurons project to the brainstem, where they terminate predominately in the nTS. Projections from the nTS can reflexively induce coughing by modifying activity of the respiratory CPG, a collection of neurons that generates respiratory rhythm in the VRG of the brainstem. Output from the CPG via MNs provides the drive to respiratory muscles needed to elicit the cough motor pattern. Superimposed on this reflex pathway is a complex higher brain network that also receives inputs from the nTS in the brainstem. Higher brain processing gives rise to respiratory sensations and emotions associated with airway irritation as well as enabling a higher level of motor control over the basic reflexive cough pathways. Centrally acting antitussive agents likely affect both brainstem and higher brain processes to modify coughing. CPG = central pattern generator; GABA = gamma-aminobutyric acid; MN = motor neuron; NMDA = N-methyl-D-aspartate; nTS = nucleus tractus solitarius; VRG = ventral respiratory group.