Hering-Breuer and the respiratory reflexes

Last updated on Jan 13, 2025

You're now thinking about your breathing.

You weren't before, but you are now.

Sorry.

It's okay, you can just breathe normally. Except you can't, because you're still thinking about it.

Normal breathing is called eupnea - think about that instead.

Humans are strange creatures in many ways, and one of them is that we have extremely powerful conscious control of our breathing allowing us to choose how and when we breathe - within survivable limits of course.

Now, unless you suffer with Ondine's curse, you fortunately don't need to consciously think about your breathing all the time, it's mostly automatic.

Negative feedback

Pretty much every homeostatic mechanism in the body is driven by a negative feedback loop, where too much of something switches the machine off, and too little switches it on again.

The same applies to breathing, with a variety of inputs to the respiratory centres in the medulla and pons providing highly sensitive, breath-to-breath adjustments to the respiratory rate and depth, to keep all the numbers where they should be.

The medulla

This is the main protagonist of the respiratory show, with direct neural connections to the muscles of respiration.

It has two main regions of interest:

The dorsal respiratory group stimulates inspiration
The ventral respiratory group stimulates expiration

The pons

Sits just below the medulla.

Controls rate of respiration

Also has two main regions to do this:

The apneustic centre - stimulates inspiration for long, deep breaths
The pneumotaxic centre - inhibits inspiration, coordinating timing and respiratory rate

These two work as antagonists and thus maintain a steady homeostatic breathing pattern.

You also have a bunch of reflexes built in, most of which pass through the medulla, with or without a bit of pontine poking.

Pulmonary reflexes

As a general rule, the neuromuscular reflexes that have been pre-programmed into your body by billions of years of evolution are pretty nifty.

They allow an action or effect to be initiated quickly and efficiently by the organs involved, without the input of the higher decision making components slowing things down.

Maybe the NHS needs some reflexes.

If you think of the lungs as a delicate sponge of incredibly thin tissue, suspended in a constantly moving cage of muscle and bone, responsible for keeping everything else alive, it then makes sense that it has rather a few safety reflexes to stop everything going wrong rather rapidly.

What receptors are there in the respiratory tract?

Larynx

Cold
Irritant
Pressure
C-fibre

Lower airways

Slowly adapting stretch receptors
Rapidly adapting stretch receptors
C-fibres
Neuroepithelial bodies

Meet Doctors Hering and Breuer

In 1868 these spritely German-and-Austrian-respectively gentlemen noticed that animals would briefly stop breathing if you inflated their lungs for a sustained period while anaesthetised.

Before getting bored and wandering off to become the founder of psychoanalysis with his buddy Freud, Breuer and his chum Hering decided to investigate this further.

The Hering-Breuer reflex protects mammalian lung from overinflation.

Birds, reptiles and amphibians have similar mechanisms, but not quite as swish as ours.

How it works

The lung inflates
Stretch receptors in the bronchial and bronchiolar walls activate the vagus nerve
This relays to the inspiratory area of the medulla and the apneustic centre of the pons
The inspiratory centre is directly and indirectly* inhibited - *the apneustic centre is prevented from activating the inspiratory centre
This inhibits the motor neurons supplying the inspiratory muscles
It also stimulates motor neurons to the expiratory muscles

The net result is that the inspiratory effort is reduced and then terminated as the lungs reach maximum safe levels of inspiration, preventing overstretching and potential volutrauma.

💡

It is of particular importance in neonates and premature babies

At the same time, the afferent pathways send projections over to the cardiac vagal motor neurons in the nucleus ambiguus as well as the dorsal motor vagal nucleus.

This reduces vagal tone to the heart
This causes a transient increase in heart rate while the lungs are fully inflated
This is called sinus arrhythmia

An entirely personal anecdote

When extubating a patient, many anaesthetists (ourselves included) recommend having some PEEP on board, and even squeezing the bag to fill the lungs as you take the tube out.

The proposed benefits are:

Recruits the lung and reduces atelectasis
Fills the FRC with oxygen such that any silliness (laryngospasm) isn't quite as stressful to manage
Activates the stretch receptors and the HBR, possibly reducing the likelihood of a big inspiration and coughing fit but this is anecdotal at best

The deflation reflex

This is the other much less famous lung reflex named by the same dynamic duo.

This one aims to shorten exhalation when the lung is deflated, and is activated by a combination of stretch receptors and proprioceptors that tell the brain how inflated the lung is.

Again, travels via the vagus but only stimulates the inspiratory centre of the medulla.

Doesn't appear to be overly important in humans but may play a role during exercise.

A really nerdy bit you definitely don't need

Breuer didn't know whether the inflation reflex (the Hering-Breuer reflex proper) and the deflation reflex (the other bit) were part of one reflex arc, or two separate mechanisms.

In theory, if stretch causes vagal inhibition of the inspiratory centre to reduce inspiratory effort, surely the simple removal of this vagal tone will cause an increase in inspiration?

Henry Head (see below) had a terribly clever idea.

He gave a few unhappy rabbits bilateral traumatic pneumothoraces, and compared the resulting inspiratory effort to others, in whom he simply cut the vagus nerves.

The logic goes like this:

If it's all mediated through simple vagal tone, then cutting the vagus should give a maximal response
If there's something else going on, triggered by the deflation of the lungs themselves, then a bigger response will be seen here

He found a much stronger inspiratory effort to be generated by the pneumothorax, clearly suggesting the existence of an entirely separate reflex pathway.

So there you go.

This is of no exam value whatsoever.

Other reflexes to know about

Cough reflex

Reason for existence

To clear objects causing potential obstruction and irritants from the airway

How it works

Mucosal receptors in larynx, trachea and bronchi detect foreign irritants
Afferents via vagus to medulla
Efferents via multiple pathways resulting in rapid inhalation, glottis closure, forced expiration against closed glottis, then glottic release

Sneeze reflex

Reason for existence

To clear irritants from nose and nasopharynx

How it works

Mucosal receptors in nasal mucosa
Afferents via trigeminal to the caudal ventral respiratory group in the brainstem
Efferents to various targets resulting in a deep inspiratory effort, followed by a forced expiration with varying glottic involvement (a.k.a. the Dad sneeze)

Apneustic reflex

Reason for existence

To help regulate depth and rhythm of breathing

How it works

Activation of the apneustic centre causes prolonged inspiration
The pneumotaxic centre then inhibits the apneustic centre, and terminates inspiration

J-receptor reflex

Juxtapulmonary capillary receptors are found in the pulmonary interstitium, right next to the capillaries, and detect interstitial pressure

They're also called pulmonary c-fibre receptors

Reason for existence

To adjust respiratory mechanics to deal with excessive fluid or increases in pressure in the lung (we think)
Particularly activated in pneumonia, heart failure and pulmonary embolus

How it works

Afferents through nonmyelinated fibres via vagus nerve
Induces apnoea, followed by rapid shallow breathing, bradycardia and hypotension

Chemoreceptor reflexes

These are the main drivers of your day-to-day respiratory effort.

Reason for existence

To adjust ventilation to maintain homeostatic levels of CO2 and oxygen in the blood

How it works

Peripheral chemoreceptors respond to PCO2 and pH via the carotid bodies (glossopharyngeal) and aortic bodies (vagus nerve)
Central chemoreceptors in the medulla respond to CO2 levels in the CSF
These activate the respiratory centres in the brainstem, causing an increase or decrease in minute ventilation as required

Dive reflex

Most babies will reflexively hold their breath if you dunk them into cold water.

Please don't dunk babies into cold water.

Reason for existence

Avoid drowning

How it works

Cold stimulation of the face sends afferents via the trigeminal nerve
This induces a reflex bradycardia, apnoea and peripheral vasoconstriction

Laryngeal reflex

Different to the cough reflex.

Reason for existence

Stimulation of laryngeal receptors by a potentially airway-compromising foreign body causes the glottis to slam shut and protect the delicate trachea and bronchi
Particularly useful during swallowing

How it works

Laryngeal irritant receptors
Trigger afferents via superior laryngeal nerve

Head's paradoxical reflex

Sometimes, stretch of the lung tissue and respiratory muscles induces further inspiration.

This is one of the very few examples of positive feedback in an otherwise homeostatic physiological system.

Henry Head - the world's most appropriately named neurologist - himself wrote of this reflex 'I am totally at a loss to account for its appearance'

It was later found that entirely different receptors were responsible for this reflex:

Rapidly adapting pulmonary stretch receptors (RARs) increase inspiratory effort
Slowly adapting receptors (SARs) are responsible for the Breuer-Hering reflex

Reason for existence

Great for babies taking their first breath
Also useful during exercise, and in preventing atelectasis during normal quiet breathing*

*If you've ever wondered why you'll just randomly take a big breath out of nowhere - it's probably this.

What's the difference between apneustic breathing and Cheyne-Stokes?

Apneustic breathing

A prolonged period of gasping inspiration
Followed by a brief and insufficient expiratory effort
A rate of 1 to 2 breaths per minute
Caused by pontine trauma, stroke or ischaemia

Cheyne-Stokes breathing

A cyclical respiratory pattern that includes periods of apnoea
A progression of increased rate and depth of respiration followed by smaller, shallower breaths
Can occur while awake but usually occurs during sleep or when unconscious
Can be associated with cardiac or renal failure, and overdose

So what's agonal breathing?

Slow, shallow, irregular respiratory pattern as a result of hypoxic brain injury
Often progresses to apnoea

References and further reading

Authors to follow

If you're interested in reading more about this topic then we suggest following the works of John Widdicome who has written extensively on the topic and is enjoyable to read.