This simple reflex action may play a more significant role than we thought.
You should never smile at a crocodile, according to the 1950s song, because rather than smiling back at you it’s contemplating your potential as an appetiser.
Yawning at a crocodile, on the other hand, is something these prehistoric critters might relate to. Because, like humans, crocodiles are prone to yawning and we suspect that, also like humans, they don’t quite know why this reflex action actually occurs.
But researchers at the University of New South Wales, reckon they might be a step closer to understanding at least the physiological functions of yawning.
According to the findings of an imaging study published last month in the journal Respiratory Physiology & Neurobiology, the Aussie boffins reckon that yawning could be a mechanism for helping important fluids move out of the brain.
Using real-time MRI scans, the team were able to see what happened inside the head and neck when people yawned. They then compared that with the effect of normal and deep breathing.
The results, based on a small-scale group of 22 participants, showed that yawning triggered a specific manoeuvre in which cerebrospinal fluid (CSF) and venous blood moved out of the skull together, whereas during deep breathing cerebrospinal fluid flowed into the skull.
CSF fluid is a clear liquid that surrounds the brain and spinal cord, helping cushion and protect the brain and spinal cord from injury, as well as carrying nutrients in and waste products out of the area.
MRI scans were taken at the level of the C3 vertebra, a crossroads in the upper neck where blood and CSF pass as they travel to and from the brain. The scans of the subjects contagiously yawning were then compared with those when they simply took a deep breath as if pretending to yawn.
According to the researchers, the fact that CSF and venous blood flows away from the skull during yawning, but flows in the opposite direction when deep breathing, was quite a surprise.
“There has been speculation that yawning can help clear waste from the brain, but so far there has not been solid proof,” the research team lead Professor Lynne Bilston told media.
“Our research suggests that yawning can play a role in cleaning brain fluid, which would most likely happen close to bedtime.”
The UNSW research team said this finding could be important for further studies into neurodegenerative diseases such as Alzheimer’s and Parkinson’s, which have been potentially linked to the build-up of waste products in and around the brain that could be a result of impaired CSF flows.
Interestingly, the boffins also found that each person yawned in a unique, but consistent way, involving a complex movement of the tongue.
They likened the yawns to a fingerprint and suggested folks might one day be able to be identified just on the basis of how they yawned.
So once again, your mother was right. You should always cover your mouth when yawning in public, particularly if you’re planning something nefarious.
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