What are pulmonary C fibres?

Pulmonary C Fibers: Unmyelinated Juxtacapillary Receptors

Pulmonary C fibers are unmyelinated fibers also called juxtacapillary receptors. These specialized afferent nerve fibers represent the majority of vagal afferents innervating the airways and lungs 1.

Characteristics of Pulmonary C Fibers

Pulmonary C fibers have several distinctive properties that differentiate them from other airway receptors:

• Conduction velocity: < 2 m/s (unmyelinated) 1
• Location: Found in close proximity to pulmonary capillaries (hence "juxtacapillary") 1
• Mechanical sensitivity: Relatively insensitive to mechanical stimulation and lung inflation 1
• Chemical sensitivity: Highly responsive to various chemical stimulants including:
  • Capsaicin
  • Bradykinin
  • Protons (low pH)
  • Transient receptor potential vanilloid 1 (TRPV1) activators
  • Transient receptor potential ankyrin 1 (TRPA1) activators 1

Subtypes and Distribution

Pulmonary C fibers can be further subdivided based on their:

• Ganglionic origin: Nodose (inferior vagal) vs. jugular (superior vagal) ganglia
• Sites of termination: Bronchial vs. pulmonary circulation 1
• Neurochemical properties: Substance P expression varies between subtypes 1

In dogs, for example, pulmonary C fibers may be unresponsive to histamine, while bronchial C fibers are activated by it 1. In guinea pigs, C fibers from the jugular ganglia innervate both intrapulmonary and extrapulmonary airways and express neuropeptides like substance P, while those from nodose ganglia terminate almost exclusively in intrapulmonary airways and rarely express substance P 1.

Functional Role in Respiratory Physiology

Pulmonary C fibers play important roles in respiratory defense mechanisms:

• They remain quiescent during normal breathing but activate in response to chemical irritants 1, 2
• When stimulated, they can trigger:
  • Cough reflex
  • Bronchoconstriction
  • Mucus hypersecretion
  • Apnea followed by rapid shallow breathing
  • Bradycardia and hypotension (part of the pulmonary chemoreflex) 1, 2, 3

Clinical Significance

The sensitivity of pulmonary C fibers can be markedly elevated in inflammatory airway conditions 2, 4. This hypersensitivity is often mediated by inflammatory mediators like prostaglandin E2 4, which can directly affect these sensory terminals, leading to:

• Enhanced cough responses
• Chest tightness
• Altered breathing patterns
• Respiratory discomfort during normal activities like mild exercise 2, 3

Understanding the mechanisms of pulmonary C fiber activation may lead to novel therapeutic approaches for respiratory symptoms in inflammatory airway diseases 3.