Respiratory Tract Receptors, Cough Reflex & Related Anatomy

Respiratory Tract Receptors, Cough Reflex & Related Anatomy

Non Chemical Influences on Respiration

Vagal Receptors

Ventilation Reflexes• Slow adapting receptor mediation

– Hering-Breuer Reflexes• Inflation reflex: prolongation of expiration caused by steady lung inflation• Deflation reflex: decreased duration of expiration caused by marked lung

deflation

• Rapidly adapting receptor mediation (RAR’s)– Mechano & chemoreceptors within the trachea & larynx causing

bronchoconstriction, mucus secretion, hyperpnoea, cough• Bronchial & Pulmonary C-fibres (J or juxtacapillary fibres)

– Arguable whether they activate or inhibit cough but definitely play a role in the cough response

– Respond to hyperinflation, airborne and blood borne mediators (pulmonary chemoreflex response to substances in the blood eg bradykinins, prostaglandins, histamine) causing apnoea -> tachypnoea -> bradycardia -> hypotension

Cough Reflex

• The cough reflex arc is complex !• Controlled by its nerve centre in the medulla

(arguable)• The reflex arc is served by afferent fibres of the vagus

(includes recurrent and superior laryngeal nerves, trigeminal and glossopharyngeal nerves. Sigma opioid receptors (decrease cough) are also present and are a target of cough suppressants

• Stimuli to receptors innervated by these nerves may precipitate the cough response.

Phases of cough• An initial phase where afferent nerves suppress the normal

respiratory drive within the brainstem• Contraction of the diaphragm, external intercostals and extrinsic

muscles of inspiration, bronchioles dilate. The deep inspiration component is greatest within the bronchi whereas it is least in the larynx (makes sense – you don’t want to inhale a foreign object before you cough it out but want a big lung full to expel matter deep within the bronchi)

• Closure of the glottis, constriction of the vocal folds• Compressive Phase: Forceful contraction of the abdominals

and internal intercostals• Expulsive Phase: Partial reopening of the epiglottis

producing an explosive jet of air

http://www.wrongdiagnosis.com/symptoms/night_cough/book-causes-16b.htm

Sites for Eliciting Cough• Stimulation of vagal afferents including

– Cough receptors of CN X are located in the larynx, trachea, bronchi, GI tract, external auditory canal, tympanic membrane whereas the oropharynx, nose & paranasal sinuses can elicit reflexes via CN’s V & IX.

– Vagal innervation from the segmental bronchi and upwards elicits a cough reflex. Chemoreceptors predominate within the bronchi whereas mechanoreceptors predominate in the trachea and larynx region.

– Irritation of bronchioles and alveoli does NOT elicit a cough (consider this method of clearing debris – the shearing forces produced in the small airways in insufficient to dislodge debris trapped in the mucous/surfactant layers), therefore, emphysema, oedema, congestion, sarcoidosis might not elicit cough.

– The deep inspiration component is greatest within the bronchi whereas it is least in the larynx (makes sense – you don’t want to inhale a foreign object before you cough it out but want a big lung full to expel matter deep within the bronchi)

– The efferent pathway of this arc stems from the medullary centre through CN’s X & XI, phrenic, intercostals, thoracoabdominals and sympathetic spinal motor nerves, which innervate the larynx, intrinsic and accessory muscles of respiration, abdominals & bronchioles

Arnold’s Reflex

Efferent Nerves (Inspiratory Phase)

More Efferent Nerves (Inspiratory)

Efferent Nerves – Epiglottis Closure

Efferent Nerves (Compressive/Expulsive Phases)