Cough Reflex

What Is the Cough Reflex?

The cough reflex is an automatic, protective sequence in which the body detects something irritating in the airways and expels it with a sudden, forceful breath out. It keeps the larynx, trachea and large bronchi clear of mucus, dust, food, fluids and microbes, and it is one of the most important ways the lungs protect themselves.

Reflex and voluntary at the same time

Most reflexes, such as the knee-jerk, cannot be started on purpose. Coughing is unusual because it can be triggered in two ways: as a true reflex, when receptors in the airway are irritated, and as a voluntary act, when a person decides to clear the throat or cough. The two share the same muscles and the same brainstem circuits, but the voluntary version starts in the cerebral cortex.

Why the body needs a cough

The airways already have a constant cleaning system, the mucociliary escalator, in which beating cilia carry a sheet of mucus up toward the throat, as described on our cilia and mucus page. Coughing is the backup. It takes over when too much mucus builds up, when the cilia are damaged, or when something enters the airway suddenly, such as a crumb or a drop of water that goes down the wrong way.

Explained simply

Think of the windpipe as a hallway with a door at the top, the glottis. When something is stuck inside, the body first takes a big breath to fill the hallway with air, then shuts the door and squeezes hard. When the door bursts open, the rush of air blows the unwanted thing up and out, like a puff that sends dust flying off a table.

Where Does the Cough Reflex Originate?

The cough reflex originates in sensory nerve endings in the lining of the larynx, trachea and large bronchi, with the greatest sensitivity at the larynx and at the carina, the ridge where the trachea divides into the two main bronchi. Smaller airways have far fewer cough receptors, and the alveoli have none.

Main trigger sites

• Larynx: the most sensitive site; touching the area around the vocal cords triggers an immediate, strong cough.
• Trachea and carina: the lining of the windpipe, and especially the carina, responds strongly to mechanical irritation.
• Large bronchi: receptors are concentrated at the points where the airways branch, where inhaled particles tend to land.
• Other sites: the pharynx, the external ear canal, the pleura and the esophagus can also trigger coughing through nerves that share connections with the vagus nerve.

Types of cough receptors

Two broad groups of sensory nerve fibers start a cough. Mechanically sensitive fibers respond within milliseconds to touch, particles and mucus moving across the airway lining, and they are best placed to catch objects that are breathed in. Chemically sensitive fibers, including slow C-fibers, respond to substances such as acid, smoke, cold air and chemicals released during inflammation. Inflammation can make both groups more sensitive, which is why the airways feel irritable during and after a cold.

The ear cough reflex

A small branch of the vagus nerve, often called Arnold's nerve, supplies part of the external ear canal. In some people, touching the ear canal, for example during an ear examination, sets off a brief cough. This shows how widely the vagus nerve's sensory network spreads beyond the lungs.

What Is the Cough Reflex Pathway?

The cough reflex pathway is a reflex arc of five parts: airway receptors detect the irritant, sensory fibers in the vagus nerve carry the signal to the medulla, brainstem circuits generate the cough pattern, motor nerves carry commands to the breathing and laryngeal muscles, and those muscles produce the cough.

Cough reflex pathway nerves

The sensory (afferent) limb runs almost entirely in the vagus nerve, the tenth cranial nerve. Signals from the larynx above the vocal cords travel in the internal branch of the superior laryngeal nerve, while those from below the vocal cords and from the trachea travel in the recurrent laryngeal nerve, and those from the bronchi travel in the pulmonary branches of the vagus. The cell bodies of these sensory neurons sit in the nodose and jugular ganglia, just below the skull. The glossopharyngeal nerve carries signals from the pharynx.

The brainstem center

The vagal fibers end in the nucleus tractus solitarius, a relay station in the medulla oblongata. From there, signals pass to the brainstem's breathing network, the same groups of neurons that set the normal rhythm described on our control of breathing page. These neurons reorganize their usual rhythm into the cough pattern: a deep breath, a hold and a blast. Higher brain regions also receive the signal, producing the conscious urge to cough.

The motor limb

The motor (efferent) limb uses several nerves at once. The phrenic nerve drives the diaphragm during the deep breath in. Spinal nerves activate the intercostal muscles and the abdominal muscles that create the pressure. The recurrent laryngeal nerve closes and then opens the glottis, and vagal fibers to the airways can narrow the bronchi slightly and increase mucus secretion.

What Are the Stages of the Cough Reflex?

The cough reflex has three main stages: the inspiratory stage, in which a deep breath fills the lungs; the compressive stage, in which the glottis closes and the breathing muscles squeeze the chest; and the expulsive stage, in which the glottis opens and air bursts out. A short relaxation phase follows before normal breathing returns.

Cough reflex steps

1. Inspiratory stage: the diaphragm and external intercostals draw in a breath that is usually deeper than normal. The extra volume stretches the breathing muscles to a more powerful length and gives the cough more air to work with.
2. Compressive stage: the vocal folds snap shut and the epiglottis helps seal the larynx. For about 0.2 seconds, the abdominal and internal intercostal muscles contract hard against the closed airway, and pressure in the chest can rise to 100 mmHg or more.
3. Expulsive stage: the glottis opens suddenly. Air rushes out at very high speed, carrying mucus and particles toward the mouth. The vibrating vocal folds and airway walls produce the familiar sound of a cough.
4. Relaxation: the expiratory muscles relax, and breathing returns to normal or a new cough begins.

Single coughs and bouts

Coughs often come in bouts, with several expulsive blasts after a single deep breath, each starting from a smaller lung volume. Successive coughs from lower volumes squeeze progressively smaller airways, helping move mucus up from deeper in the bronchial tree. The function of the glottis in this process is explained on our glottis vs epiglottis page.

How Does a Cough Clear the Airways?

A cough clears the airways by creating a short burst of very fast airflow that shears mucus and particles off the airway walls and carries them upward, a process made more powerful by the narrowing of the trachea and large bronchi under high chest pressure.

Dynamic compression

During the expulsive stage, the high pressure around the airways squeezes the trachea and large bronchi. The flexible back wall of the trachea, made of the trachealis muscle rather than cartilage, bulges inward and narrows the tube. For the same flow of air, a narrower tube means higher speed, just as water speeds up when a hose nozzle is partly covered. This fast-moving air drags mucus along the airway wall.

Working with the mucus layer

A cough works best when mucus is present in the right amount and consistency. A thin layer of mucus can be swept along by the airflow, while very thin, watery secretions or very thick, sticky mucus are cleared less efficiently. The respiratory epithelium produces this mucus, and the cilia bring it into the large airways, where coughing is most effective.

Where a cough is effective

Because the speed of air is highest in the large central airways, a cough mainly clears the larynx, trachea and the first few generations of bronchi. It does little in the small bronchioles, where airflow is slow and clearing relies on cilia and on alveolar macrophages. Particles that reach the alveoli are removed by these cells, not by coughing.

The cough and the spread of germs

A cough propels droplets and smaller aerosol particles from the airways into the air. This protects the person coughing but can carry infectious microbes to others, which is how many respiratory infections, including tuberculosis and whooping cough, spread.

Can You Control the Cough Reflex?

Yes, partly. Because the cough pathway connects to the cerebral cortex, people can start a cough on purpose, and they can often delay or hold back a reflex cough for a while, although a strong stimulus in the larynx usually overrides voluntary control.

The urge to cough

Before most reflex coughs, people feel an urge to cough, a tickling or irritating sensation in the throat or chest. This sensation comes from signals that reach higher brain regions in parallel with the brainstem reflex. It allows the brain to weigh whether to cough, which is why someone can often hold back a cough during a quiet performance.

Cough during sleep

The cough reflex is strongly damped during sleep. Most coughs at night happen after the person has briefly woken up, even if they do not remember it. This is one reason why liquids that enter the airway during deep sleep are less likely to cause an immediate cough.

Changes across life

In newborns, especially premature babies, irritation of the larynx tends to cause a pause in breathing and swallowing rather than a strong cough, and the adult-type cough reflex matures over the first months of life. In older adults the reflex may become less sensitive and the cough less forceful, and conditions affecting the brain, such as stroke, can weaken it further. A weak cough makes it harder to clear material that enters the airway, which increases the risk of aspiration.

Cough Reflex vs Gag Reflex: What Is the Difference?

The cough reflex expels material that has already entered the airway by blasting air out of the lungs, while the gag reflex stops material at the back of the mouth by contracting the throat muscles, preventing it from being swallowed. Both protect the airway, but they use different triggers, nerves and muscles.

The gag reflex

The gag, or pharyngeal, reflex is triggered by touching the back of the tongue, the tonsil area or the back wall of the pharynx. Its sensory signals travel mainly in the glossopharyngeal nerve (cranial nerve IX), and its motor commands travel in the vagus nerve (cranial nerve X) to the muscles of the pharynx and soft palate, which contract to push the object forward. Its strength varies widely between people, and some healthy people have very little gag reflex.

The sneeze reflex

The sneeze is a close relative of the cough. It also uses a deep breath in, a build-up of pressure and a sudden blast out, but it is triggered by irritation of the nasal lining through the trigeminal nerve, and much of the air is directed through the nose to clear the nasal cavity. Our page on why we sneeze explains it in detail.

Other protective reflexes

• Laryngeal closure: the vocal folds snap shut when liquid or food touches the larynx, sealing the airway even before a cough starts.
• Expiration reflex: a sharp breath out without a deep breath first, triggered from the vocal folds to push material out before it can be breathed further in.
• Swallowing: the epiglottis folds down and the larynx rises, guiding food into the esophagus rather than the trachea; see trachea vs esophagus.

Common Conditions That Affect the Cough Reflex

Many conditions affect the cough reflex, either by stimulating it too often, as in infections and airway diseases, or by weakening it, as in some nerve and muscle disorders. Our respiratory diseases section covers these conditions, and a cough that lasts for weeks, brings up blood or comes with breathing difficulty should be checked by a doctor.

Acute, subacute and chronic cough

Doctors often group coughs by how long they last. In adults, an acute cough lasts less than 3 weeks and is most often caused by a cold or other respiratory infection. A subacute cough lasts 3 to 8 weeks, often lingering after an infection. A chronic cough lasts longer than 8 weeks.

Conditions linked to frequent coughing

• Infections: colds, bronchitis, pneumonia, croup and whooping cough inflame the airway lining and sensitize cough receptors.
• Asthma and COPD: inflamed, narrowed airways and extra mucus trigger frequent coughing.
• Postnasal drip and reflux: mucus from the nose and sinuses, or acid from the stomach, can irritate the throat and larynx.
• Smoke and pollutants: tobacco smoke and irritant gases stimulate the chemically sensitive fibers.

Conditions linked to a weak cough

Stroke, Parkinson's disease, muscle diseases, spinal cord injury and heavy sedation can weaken the cough by reducing sensation, coordination or muscle strength. A weak cough lets mucus and inhaled material collect in the airways and raises the risk of chest infections.