In one breath
Reflexes from the lungs, airways and chest wall fine-tune each breath and protect the lungs from harm. Slowly adapting stretch receptors give the Hering–Breuer reflex, which ends inspiration once the lungs are well inflated; rapidly adapting irritant receptors trigger cough and gasps; and C fibers, including the J receptors, cause rapid shallow breathing. In sleep the waking drive is lost, so breathing leans on CO₂, and the slack upper-airway muscles of REM sleep can block the airway (obstructive sleep apnea).
Builds on: Respiratory centers (neural control) · Leads to: Abnormal breathing patterns
What non-chemical control means
- Reflexes from the upper airways, lungs and chest wall feed back to the respiratory centers. They fine-tune breathing and protect the lungs from things in the environment.
- The sensors are mechanoreceptors in the lungs, airways and chest wall, working alongside the chemoreceptors that track blood pH and gas tensions.
Receptors for respiratory reflexes
There are three types, carried mainly by myelinated and unmyelinated fibers in the vagus:
| Receptor | Fiber | Main effect |
|---|---|---|
| Slowly adapting (SAR) | myelinated | ends inspiration (Hering–Breuer) |
| Rapidly adapting (RAR) | myelinated | cough, gasp |
| C fiber endings | unmyelinated | rapid shallow breathing |
Slowly adapting receptors
- Site: in the smooth muscle of the conducting airways; endings of myelinated afferents.
- Also called pulmonary stretch receptors, because airway stretch excites them.
- They fire as airway transmural pressure rises, so they track lung volume.
- They are slow to adapt: they keep firing while the stretch is held.
- Their firing cuts inspiration short and lengthens expiration; this is the Hering–Breuer reflex.
Hering–Breuer reflex
Hering and Breuer described it in 1868: inflating the lungs made the tidal volume smaller and the breathing rate faster.
| Reflex | Trigger | Response |
|---|---|---|
| Inflation reflex | steady rise in lung volume | longer expiration; strong inflation can cut inspiration short |
| Deflation reflex | marked deflation | shorter expiration |
- Receptors: slowly adapting stretch receptors in the lung tissue and airways.
- Afferent path: the vagus, to the medullary inspiratory neurons.
A slip in your book
Your book names the vagus as both the afferent and the efferent path. Only the afferent path is vagal. The efferent path is the motor supply of the breathing muscles, the phrenic and intercostal nerves, whose firing the medulla cuts short.
Significance:
- Protection: it guards against overinflation without upsetting normal alveolar ventilation.
- More important in infants, where it controls tidal volume in ordinary quiet breathing (eupnea).
- In adults it comes into play only when tidal volume is much larger than normal. In the awake adult, the stronger central control overrides it.
- Worth knowing, though not in your pages: in adults it probably does not switch on until tidal volume is more than about three times normal (roughly 1.5 L per breath).
Other roles of slowly adapting receptors:
- A larger-than-normal breath excites them, and they trip the inspiratory off-switch, shortening inspiration.
- They help set expiratory time, expiratory muscle activity and the FRC.
- Beyond breathing, they relax airway smooth muscle, lower vasomotor tone and speed the heart.
Rapidly adapting receptors
- Site: endings of myelinated afferents in the larger conducting airways.
- Adaptation: a sudden, held inflation makes them fire fast, but the firing drops quickly while the new volume is kept (your book puts the fall at about 20% in the first second).
- Chemicals: histamine, serotonin, bradykinin and prostaglandins, the local mediators of allergy and inflammation, excite them strongly.
- Irritant receptors: smoke, dust, ammonia and other noxious agents set them off, as do acute congestion and inflammation of the airways.
Significance:
- They detect disease of the airways: irritation, congestion and inflammation.
- The stiffer the lung (the lower its compliance), the more they fire, so they signal changes in compliance. Since they are nearly silent in quiet breathing, this matters mainly in disease.
- Their effects are excitatory: coughing, gasping and prolonged inspiration.
C fiber endings
Unmyelinated (C) fibers end among the alveoli and in the conducting airways. There are two groups:
| Type | Lie in | Most sensitive to |
|---|---|---|
| Pulmonary C fibers | beside the alveoli | edema, congestion, embolism |
| Bronchial C fibers | airway walls | chemical agents |
- Pulmonary C fibers are reached from the pulmonary capillary blood, hence their other name, juxtapulmonary capillary (J) receptors. The Indian physiologist Dr A. S. Paintal described them. Chemicals such as the products of inflammation excite them less.
- Bronchial C fibers are reached from the bronchial circulation.
- Shared triggers: injury to the lung, strong inflation, sudden congestion of the pulmonary vessels and some chemicals.
Effects of stimulation:
- Rapid, shallow breathing, bronchoconstriction, more airway secretion and depression of the circulation.
- Sudden strong stimulation: apnea, a marked fall in blood pressure and an abrupt loss of skeletal muscle tone.
Applied: J receptors in lung congestion
Fluid in the lungs, as in pulmonary edema, and pulmonary emboli stimulate the J receptors, which drive the rapid, shallow breathing these patients show. Worth knowing, though not in your pages: J-receptor firing is thought to add to the breathlessness of left heart failure.
Chest wall proprioceptors
- They tell the centers how the chest is moving and how tense its muscles are, which matters most in effortful breathing.
- Muscle spindles: many in the intercostals, few in the diaphragm. Through feedback onto the motor neurons they adjust each breath (see Muscle spindle).
- They also help you sense how hard you are breathing.
Other proprioceptors:
- Breathing quickens when joints move, whether you move them or someone else does (active or passive movement).
- The sensory fibers carrying this traffic from muscles, tendons and joint ligaments up to the thalamus send side branches (collaterals) to the brainstem respiratory centers.
- So more proprioceptive traffic means more breathing. This is one cause of the extra breathing in exercise.
Receptors in the upper airways
Receptors in the nasopharynx, trachea and bronchi change breathing during coughing and sneezing.
Coughing
Cough can be voluntary or reflex. Its job is to clear irritants from the respiratory tract.
- A deep inspiration.
- Forced expiration against a closed glottis, raising intrapleural pressure to 100 mmHg or more.
- The glottis opens suddenly and the air bursts out (your book gives about 600 miles per hour).
- Worth knowing, though not in your pages: the afferents run in the vagus. Guyton gives the outflow as 75–100 miles per hour, the more usual textbook figure.
Applied: cough after lung transplant
In a heart–lung transplant the donor trachea is joined to the recipient’s just above the carina, and the nerves to the donor lungs never regrow.
- Kept: cough from the trachea, whose nerve supply is untouched; the resting breathing pattern, yawns and sighs, which therefore do not need lung nerves.
- Lost: cough from the smaller airways, and the Hering–Breuer reflex.
Sneezing
- Usually involuntary; it clears dust and irritants from the upper airway.
- It works like a cough, with one difference: the glottis never closes.
- Worth knowing, though not in your pages: the trigger is in the nose, the afferents run in the trigeminal nerve, and the uvula drops so that much of the air leaves through the nose.
| Point | Cough | Sneeze |
|---|---|---|
| Control | voluntary or reflex | usually involuntary |
| Clears | respiratory tract | upper airway |
| Glottis | shut, then bursts open | open throughout |
Afferents from the viscera
- During swallowing and vomiting, breathing stops for a moment and the glottis closes, so food or vomit cannot enter the airway. The reflex runs on visceral and diaphragmatic afferents.
- Worth knowing, though not in your pages: the pause during swallowing is called deglutition apnea. In the pharyngeal stage the swallowing center inhibits the respiratory center at whatever point of the cycle it has reached, while the larynx rises, the epiglottis tips back over the laryngeal opening and the vocal cords close.
Hiccup
- A sudden spasm of the inspiratory muscles, the diaphragm among them, pulls in a quick breath.
- The glottis shuts abruptly at that moment, and the closure makes the “hic”.
- Its purpose is unknown, though it occurs both before and after birth.
- A firm breath-hold usually ends it. Stubborn (intractable) hiccups may need drugs: centrally acting analgesics or dopamine antagonists.
Yawning
- A reflex deep breath in, drawn out and often noisy, that tends to spread from person to person (contagious).
- Its mechanism is unknown. It is seen in the fetus, fish, tortoises and mammals.
Its probable functions:
- Clears CO₂ and rouses you: it is thought to follow a build-up of CO₂ in the blood. It removes CO₂, improves tissue oxygenation and briefly livens up a tired person, by an unknown mechanism.
- Heralds sleep: people yawn just before sleep, and often when short of sleep.
- Opens the lungs: the deep breath and stretch may inflate underventilated alveoli and so prevent atelectasis, though experiments have not proved this.
- Helps venous return: probably by boosting the thoracic and abdominal pumps, so it may be good for the heart.
- Nonverbal communication: it sends a message to others, in animals and in humans.
Afferents from baroreceptors
- Besides the cardiovascular centers, baroreceptor afferents from the carotid sinus, aortic arch, atria and ventricles also reach the medullary respiratory centers.
- They usually inhibit breathing, but only mildly and briefly.
- The hyperventilation of chemoreceptor stimulation does not work through baroreceptors.
Afferents from higher centers
- Pain and emotion change breathing through the limbic system and hypothalamus, whose fibers act on the brainstem centers.
- Voluntary breathing comes from the neocortex, whose fibers bypass the brainstem centers and reach the spinal respiratory motor neurons directly (see Respiratory centers (neural control)).
Breathing during sleep
- Sleep begins when the brainstem reticular formation stops exciting the brain. The same RAS keeps up a steady excitation of the medullary respiratory neurons, so losing it in sleep would be expected to damp breathing overall.
- As sleep deepens and the waking stimulus fades, breathing is depressed. As sleep lightens, breathing picks up again, driven by the CO₂ that built up in the meantime.
- Exaggerated, this waxing and waning becomes Cheyne–Stokes breathing, usually in slow-wave sleep (see Abnormal breathing patterns).
| Sleep stage | Breathing |
|---|---|
| Slow-wave, stages 1–2 | depth waxes and wanes |
| Slow-wave, stages 3–4 | slow, deep, regular |
| REM | rapid, irregular |
Responses to CO₂, hypoxia and irritants in sleep
| Stimulus | Response in sleep |
|---|---|
| CO₂ | reduced, yet the main drive in slow-wave sleep |
| Hypoxia | reduced in slow-wave and REM sleep |
| Airway irritants | apnea and airway dilation |
- CO₂ in slow-wave sleep: sensitivity falls mainly because the waking stimulus is lost. Without the waking drive, CO₂ becomes the chief stimulus holding the medulla steady, so a disease that blunts the CO₂ signal lets breathing sag in slow-wave sleep.
- REM sleep: breathing is run by the behavioral control systems, and its rapid, irregular pattern is not altered by hypoxia.
- Arousal: in sleep, hypercapnia wakes you more readily than hypoxia.
- Irritants: the same irritant that makes you cough, breathe fast and narrow your airways when awake tends instead to stop breathing (apnea) and widen the airways during sleep.
Upper airway obstruction in sleep
- In REM sleep skeletal muscle tone falls, including the laryngeal and pharyngeal muscles, and the tongue relaxes.
- The upper airway can then block, which causes snoring.
- Repeated, prolonged blocks cause marked hypercapnia and hypoxemia, which wake the sleeper again and again.
Sleep apnea syndrome
| Type | Cause |
|---|---|
| Obstructive | pharyngeal tone lost in REM sleep; airway shuts in inspiration |
| Central | medulla sends less drive to phrenic motor neurons |
Obstructive sleep apnea (in adults):
- Obesity contributes.
- Each apnea ends with a brief awakening and a spell of normal breathing; sleep returns, and so does the apnea, in cycles all night.
- The broken nights bring morning headache and fatigue, and sleepiness during the day (day somnolence).
Draw it: the Hering–Breuer reflex arc
Draw the lungs with a stretch receptor in the airway smooth muscle, labelled “slowly adapting receptor”. Run an afferent line up the vagus to the DRG in the medulla, and mark it (−) on the inspiratory neurons. From the medulla draw two efferent lines down: the phrenic nerve to the diaphragm and the intercostal nerves to the external intercostals. Beside the arc write “lung inflates → SARs fire → inspiration switched off, expiration prolonged”.
Exam-answer skeleton: "Hering–Breuer reflex" (short note)
- Definition: stretch of the lungs reflexly ends inspiration and prolongs expiration (Hering and Breuer, 1868).
- Receptors: slowly adapting pulmonary stretch receptors in the airway smooth muscle, with myelinated fibers.
- Reflex arc: vagal afferents → DRG → phrenic and intercostal nerves to the inspiratory muscles (diagram).
- Inflation reflex (longer expiration; strong inflation stops inspiration) and deflation reflex (shorter expiration).
- Significance: prevents overinflation; controls tidal volume in infants; works in adults only with very large breaths; lost after lung transplant.
- Other roles of the receptors: inspiratory off-switch, expiratory time, FRC, airway relaxation, heart rate.
Asked in exams
- Deglutition apnoea: Dec 2016, 2 marks