Introduction to the mechanism of cough Flashcards
Define the three phases of a cough.
- Inspiritory phase: Deep inspiration causes stretch reflex activastion -> stronger elastic recoil of lung to aid expiration
- Compressive & Expulsive phase: Glottis closes while expiratory muscles contract -> rapid increase in pressure -> expul;sive flow when glottis opens
- Recovery phase: Restorative inspiration
Describe how bronchoconstriction and mucus secretion help the defensive function of the expulsive phase of a cough
Bronchoconstriction increases linear velocity of airflow (constriction ?increases pressure) and lessen the inflow of the irritants deeper into the lung.
Mucus secretions trap irritants and are cleared through mucociliary transport and cough. It also acts as a barrier between the irritants and the airway wall.
Outline the function of coughing
- Defense
- Foreign objects
- Mucous clearance
- Works with mucociliary clearance
- Compensates when ↓cilia function or ↑hypersecretion of mucus
Outline the afferent neural pathway for cough
Cough receptors are stimulated
Stimulation of cough receptors in the LRT sends signals through the vagus nerve
Stimulation of cough receptors in URT sends signals through the superior laryngeal nerve
Signal reaches the medulla cough centre then gets to the motor cortex.
Name two type of stimulation on cough receptors and give some examples
- Mechanical
- Dust, mucus, food/drink
- Chemical
- Inflammatory mediators, capsaicin
Name the cough receptors found in the lungs
- RASRs (Rapidly Adapting Stretch Receptors)
- Aδ
- C-fibre receptors (chemical; nociceptors)
- C
Wihere are RASRs found? What are their features and what kind of stimuli activates them?
Naso-pharynx, Larynx, trachea, bronchi
Myelinated fibres (Aδ)
Mechanical, chemical, inflammatory mediators
Wihere are C-fibre receptors found? What are their features and what kind of stimuli activates them?
Larynx, trachea, bronchi, lungs
Unmyelinated,‘free’ nerve endings
Chemical, inflammatory mediators
What can C-fibre receptors (nociceptors) release
Neuropeptides:
SP
NKA
CGRP
SP = Substance P; NKA = Neurokinin A; CGRP = Calcitonin gene-related peptide
What are the two subtypes of vagal afferent neurones and what kind of fibres’cough receptors are they associated to
-
Jugular ganglia neurones
- Aδ, C
-
Nodose ganglia neurones
- Aβ, Aδ, C
Once the cough receptors are stimulated, they send signals up the afferent pathway into the brainstem. Describe the central mechanism (centres involved) in regulating cough.
Signal reaches nucleus tracus solitarius
Sends signals to sub-cortex for processing
Signal reaches motor cortex
Signal to CPG/VRG then to motor neurones
Controls respiraatory muslces
In chronic cough what is activated in excess
Midbrain pathways
Outline the efferent pathways of cough
Motor cortex
Medulla
Glottis and expiratory muscles
What are the primary outflows of cough (efferent)
Diaphragm
Pharyngeal muscles
Intercostal muscles
Abdominal muscles
What are the secondary outflows of cough
Bronchoconstriction
Mucus Secretion
Muscles opposing mechanical stress of cough
Heart Rate
Define a gough
A defensive airway reflex which generates high flow velocities to expel mucus or other foreign bodies from the lungs
What are the three phases of cough
-
Inspiritory phase
- Enhanced contraction of diaphragm
-
Compressive phase
- Expiration against a closed glottis
-
Expulsive phase
- Forceful contraction of expiratory muscles with an open glottis
Define a chronic cough
Cough that has occured for more than eight weeks
Define cough hypersensitivity syndrome
Chronic cough triggered by low exposure levels of:
- Thermal
- Mechanical
- Chemical
How can you assess a patient with cough hypersensitivity syndrome
Capsaicin stimulation
Inhaled at increasing concentrations
Number of coughs recorded
What factors can increase the reaction to capsaicin
Post-viral infection
Asthma
COPD
rhinitis
GORD
Pulmonary fibrosis
What neuronal factors can increase cough
- Increased chemical mediators (bradyknin, PGE2)
- Increased receptors (TRPV1)
- Increased neurotransmitters (SP, neurokinins)
- Increased nerve densities
Name some anti-tussives used for chronic cough
Central acting: Opiates
Peripherally acting: Moguistine, levodopropizine
Disease specific antitussives (PPI, ICS)
Cough hypersensitivity syndrome is treated as a sensory neuropathy. How is it managed?
SALT review
Pharmacology: Amitryptiline, Gabapentin
Experimental: TRPV1 blockers, P2X3 ATP Channel blocker
Mechanism of action for amitryptiline in cough
Inhibits reuptake of inhibitory neurotransmitters (i.e. augments inhibitory influences)
Inhibits NMDA receptors
Mechanism of action for gabapentin in cough
Activate GABAB receptors
