by Alice Campbell-Smith

The notion of a sigh as one of life’s most crucial reflexes has been brought forward by Californian scientists.

This key life sustaining process preserves lung function and is linked to neurons in the brain. Interestingly, it also correlates with emotional states.

So, what are the mechanisms involving the lungs and brain which are responsible for the extra exhaling?

A small set of various neurons act in a similar way to that of a button. When activated can ‘push’ into effect various breathing outputs, one of which can be a sigh. These neurons regulate different types of breath; from sighs and coughs to laughs and cries. The alveoli – small sacs in the lungs that form clusters similar in shape to a small bunch of grapes, then fill up with air, inflating and controlling the exchange of oxygen and carbon dioxide (O₂ and CO₂), and are key structures for the process of respiration.

Researchers report that the brain stem involved in the control of breathing, sleep and heart rate is instrumental in orchestrating a sigh.

We can even experience a startling rate of up to 12 sighs an hour!

Why is the sigh, essentially an extra breath for an already inflated lung, so important?

This process has been found to rely on the smallest set of neurons linked to fundamental human behaviour. So, the key may well be in the simplicity.

Prof. Jack Feldman, at University College Los Angeles said, “One of the holy grails in neuroscience is figuring out how the brain controls behaviour.”

Working on laboratory mice allowed the team to screen the brain patterns of 19,000 brain cells which could be linked to genetic activity. Interestingly, mice can sigh up to 40 times an hour, over 9 times more than humans. To answer questions about the differences in fundamental life supporting mechanisms amongst species, protein and gene expression studies were conducted.

Two laboratories collaborated in the discovery that peptides triggered another set of nerve cells to activate the mice’s muscles and result in a sigh.

In humans, sighing becomes part of the survival mechanism. It maintains the lungs’ health, whereby if one does not sigh often enough, the alveoli can slowly collapse and lead to lung failure.

In addition, sighing certainly corresponds to emotional states. When stressed, (as we have all experienced with coursework or exams) our sigh rate increases. Perhaps regions of the brain involved in emotional states of stress are going on to release sigh neurotransmitters, however it is not yet known for sure.

This research pieces together a ventilation puzzle which could provide much needed information about respiratory system injuries. This is important for clinical settings and provides a direct application for doctors and patients.

However, the study offers no explanation of why people sigh when anxious, sad or stressed. This could perhaps be an important area for future research, promoting collaboration between neuroscientists and psychologists.

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