Describe the cough reflex.

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College Answer

Overall, this question was reasonably well answered. Those that performed well had suitably detailed knowledge and structured their responses which generally included a definition and purpose of the reflex as well as the identification and a description of the afferent, integrator/controller, and efferent limbs of the reflex. This structure allowed a logical platform for the elucidation of the detail required in the answer, including types of stimulus, receptors, nerves (for both limbs of the reflex) and the muscles used in the phasic response to be clearly articulated.

Discussion

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.

References

Polverino, Mario, et al. "Anatomy and neuro-pathophysiology of the cough reflex arc." Multidisciplinary respiratory medicine 7.1 (2012): 5.

Brooks, Stuart M. "Perspective on the human cough reflex." Cough 7.1 (2011): 10.