Pathways and importance of the cough reflex

This chapter is most relevant to Section F10(v) from the 2017 CICM Primary Syllabus, which expects the exam candidates to be able to "explain the pathways and importance of the cough reflex". This topic has appeared in Question 13 from the first paper of 2014 and Question 8 from the second paper of 2020. It is therefore explored here because it may one day bedevil an unprepared exam candidate. The second time it came up, the pass rate was still only 62%.

Polverino et al (2012) give an excellent brief overview of this reflex without overflowing with unnecessary detail. If unnecessary detail becomes necessary for whatever reason, the review by Canning (2006) offers an abundance of it. Given the relative lack of interest regarding this tpic from the CICM examiners, only a brief summary is offered here, resembling an answer to a hypotherical SAQ which asks the candidates to "explain the pathways and importance of the cough reflex".

In summary:

Stimulus for cough:

• Chemical and biological stimuli:
  • Acids, eg. gastric contents
  • Biological pathogens
  • Mediators associated with inflammation
• Mechanical stimuli:
  • Aspiration of liquids
  • Solids, eg. inhaled particles or accumulated secretions 

Purpose and importance of cough reflex:

• Protective function
  • Defense against foreign material in the airway
• Pathological consequences
  • Damage to the mucosa with persistent or unproductve cough
• Diagnostic purpose
  • Evidence of intact medullary function

Afferent pathway of the cough reflex arc: Three main classes of receptor:

• Rapidly Adapting Receptors:
  • Respond to dynamic lung inflation, bronchospasm, or lung collapse
  • Sporadically active throughout the respiratory cycle
• Slowly Adapting Stretch Receptors
  • Sensitive to the mechanical forces
  • Participate in the Hering-Breuer reflex
• C-Fibers
  • Essentially, just nociceptors, similar to to those in the skin
  • Respond to noxious chemical and mechanical stimuli

These receptors connect to the medullary control centre by vagus nerve fibres:

• Bronchial mucosa: pulmonary, auricular, pharyngeal, superior laryngeal, gastric nerve branches of the vagus
• Diaphragm: cardiac and esophageal branches of the vagus

Central integrated control of cough:

• Located at the caudal two-thirds of the nucleus tractus solitarius

Efferent pathway of the cough reflex arc:

• To the diaphragm: via the phrenic nerve
• To the abdominal muscles: via the spinal motor nerves
• To the larynx: via the laryngeal branches of the vagus, from the nucleus ambiguus

Process of cough:

• Sensory phase: afferent fibres conduct mechanoreceptor and chemoeceptor stimuli to the central interator in the medulla, and a cough reflex is triggered
• Inspiratory phase: glottis opens and a deep breath is inhaled
• Compressive phase: glottis closes and expiratory muscles forcibly contract; the intrathoracic pressure may transiently rise to over 100 cm H₂O.
• Expulsive phase: the glottis opens and rapid airflow begins; the bronchial tissues oscillate due to the rapid turbulent flow, which loosens the secretions.

In the event that one needs to read more about the actual procedure of coughing, one may wish to explore the excellent article by Stuart Brooks (2011) in which the author for some reason uses his spouse as a model for what happens during a cough: "...now, her face is cerulean-colored as she forcibly emits a staccato-like exhalation...".