Sinus & Nasal Mucosa Overproduction
Allergic or infected nasal lining floods the sinuses with watery mucus.
- ~1 L/day: Normal mucus turnover (swallowed unnoticed)
- 2–4×: Rhinitis mucus increase (baseline output)
- 2: Common triggers (allergic + infectious)
- Minutes: Onset after exposure (histamine-driven)
Allergic rhinitis pathway
Allergen binds IgE on mast cells, releasing histamine fast.
Histamine dilates vessels, mucosa swells and weeps fluid.
Sinusitis pathway
Infected sinus lining secretes thicker, sometimes purulent mucus.
Turbinate mucosa response
Goblet cells and glands ramp up secretion within the nose.
Mucus Drips Down the Posterior Pharynx
Gravity and cilia move excess mucus backward, not forward, out the nose.
- Nasopharynx: Drip pathway (to oropharynx)
- ~1 cm/min: Mucociliary flow (posterior direction)
- Throat clearing: Sensation reported (classic symptom)
- Lying down: Worsens when (nighttime cough)
Posterior pharyngeal wall
Excess volume overwhelms anterior drainage, mucus falls backward.
Cilia and gravity
Mucociliary escalator normally clears forward; overload reverses flow.
Supine position lets gravity pool mucus deep in the throat.
Symptom correlation
Patients feel a persistent drip and frequent throat clearing.
Mucus Contacts Cough Receptors
Pooled mucus touches sensory nerve endings lining the larynx and throat.
- Rapid-adapting: Receptor type (mechano/chemo)
- Vagus nerve: Innervation (CN X afferents)
- Mechanical + chemical: Stimulus type (mucus contact)
- Highest: Density (larynx, carina)
Cough receptor anatomy
Sensory nerve endings sit just under the laryngeal mucosa.
Mucus as an irritant
Thick or acidic mucus mechanically and chemically triggers firing.
Receptor stimulation builds until a firing threshold is crossed.
Threshold behavior
Below threshold, only tickling or urge-to-clear sensation occurs.
Cough Reflex Arc Fires
An afferent signal to the brainstem returns as a motor cough command.
- Vagus (CN X): Afferent nerve (sensory limb)
- Brainstem: Integration site (nucleus tractus solitarius)
- Phrenic + spinal: Efferent nerves (motor limb)
- <1 second: Reflex latency (signal to cough)
Afferent limb
Vagal fibers carry the irritation signal up to the brainstem.
Brainstem integration
Cough center processes input and commands a motor response.
One reflex arc: sensory in, motor out, cough produced.
Efferent limb
Diaphragm and laryngeal muscles contract to expel air forcefully.
Chronic Cough Until Rhinitis Is Treated
The cycle repeats hourly until the underlying mucus source is controlled.
- #1: Cause of chronic cough (post-nasal drip)
- Antihistamine: First-line therapy (or nasal steroid)
- Days: Symptom relief onset (with treatment)
- Persistent: Untreated course (weeks to months)
Why the cough persists
Mucus keeps forming, so receptors keep firing, cough repeats.
Treatment breaks the loop
Antihistamines or nasal steroids shrink mucosa and cut mucus.
Less mucus reaching the pharynx means fewer cough episodes.
Resolution
As inflammation subsides, drip and cough frequency taper off.