Understanding Cough: Reflex Mechanism, Common Triggers & When to See a Doctor

Cough is a protective reflex expelling air at high velocity to clear airways.

Reflex Arc

1. Receptors: C-fibers (chemosensitive) and Aδ-fibers (mechanosensitive) in larynx, trachea, bronchi.
2. Afferent: Vagus nerve → nucleus of tractus solitarius (NTS).
3. Central pattern generator: "Cough center" in lateral medulla coordinates motor output.
4. Efferent: Phrenic (diaphragm), spinal motor (abdominals, intercostals), laryngeal adductors/abductors.
5. Execution: Deep inspiration → glottic closure → pressure build-up → explosive glottic opening → high-velocity airflow (up to 500 mph).

Common Triggers

• Mucus/postnasal drip (URI, allergies, sinusitis)
• Irritants: Smoke, pollution, cold air, strong odors
• Infections: Viral (URI, flu, COVID), bacterial (pertussis, pneumonia)
• Asthma (cough-variant: dry cough only)
• GERD (microaspiration, vagal reflex)
• Medications: ACE inhibitors (bradykinin accumulation — 10–20% of users)
• Interstitial lung disease, heart failure, malignancy (less common, serious)

Duration Classification

• Acute: <3 weeks (usually viral)
• Subacute: 3–8 weeks
• Chronic: >8 weeks (requires workup)

Red Flags for Urgent Evaluation

• Hemoptysis (coughing blood)
• Weight loss, night sweats, fever >38°C
• Dyspnea at rest, new wheeze
• Voice change, dysphagia
• Immunocompromised

Cough is a symptom, not a diagnosis. Chronic cough warrants systematic evaluation.