What is the neurophysiology of yawning?

Welcome to part two of my mini-course on yawning.  In this blog entry, I want to give you a brief overview of the physiology and neurochemistry of yawning, because it is quite complex and interesting.  As I mentioned in the previous blog entry, there are many theories of yawning.  Common to many of them is this idea that yawning serves an adaptive function of some kind, with the most common theory being that yawning helps with some kind of physiological regulation.  So, the place to begin, then, is to look for the physiological causes of yawning.  This will be by no means an exhaustive review of the neurophysiology of yawning, so perhaps I will revisit the topic in future blog entries.

Perhaps the best understood physiological cause of yawning has to do with the significant number of neurotransmitters and hormones that are involved in the behaviors.  Guggisberg, Mathis, Schnider, and Hess (2010), in their review, point to studies that show that dopamine, acetylcholine, glutamate, serotonin, nitric oxide, adrenocorticotropic hormone (ACTH), oxytocin, and steroid hormones play some role in yawning.  Many of these chemicals work in the diencephalon, particularly the hypothalamus and thalamus, as well as projections from those areas to the hippocampus, pons, and medulla oblongata.  We shall see, in an upcoming lecture on sleep, that serotonin and acetylcholine in particular are involved in wakefulness and sleep, so their relationship to yawning, which is often associated with drowsiness, should come as no surprise.

Lets take a closer look at the role of dopamine and oxytocin in yawning behavior.  Dopamine agonists induce yawning accompanied by penile erection when injected into rats (Daquin, Micallef, & Blin, 2001).  In particular, the D3 receptors located in the paraventricular nucleus of the hypothalamus are activated, and those receptors, in turn stimulate the release of oxytocin, a modulating hormone involved in orgasm, anxiety, and maternal behavior (Daquin, Micallef, & Blin, 2001).  One interesting theory of why we yawn is that it produces an arousal or alertness spike, particularly during moments of low arousal, possibly caused by boredom.  The involvement of dopamine and oxyocin in yawning certain supports the idea that yawning is related to arousal.  I have often speculated about that after many a late night studying, when I would yawn so much my eyes would water.  I wondered if this was some attempt by my body to keep me a little more awake and alert since I was up well past my normal bedtime.  The role of dopamine might seem puzzling at first, since dopamine is involved in the brain’s reward system.  However, the primary function of the dopaminergic reward systems in the brain seems to be to draw your attention to particular stimuli in the environment, thus ensuring that you make the connection between that particular stimulus and the reward that often follows it.  In that sense, then, the role of dopamine makes more sense, if you look at this neurotransmitter as being part of an attention or alerting system.

One interesting aspect of oxytocin is that it is implicated in social behavior, particularly maternal behavior, social recognition, and pair bonding.  Cooper et al (2012) found that EEG recordings of the brain during contagious yawning show results that are consistent with mirror neuron activity.  Mirror neurons are found in the frontal lobes, and fire either when we perform a specific activity or we watch someone else perform a specific activity.  It doesn’t take much imagination to see why researchers were interested in the role of mirror neurons and yawning.  Anyone who has ever spent time in a boring lecture or service and watched yawns travel from person to person would make the connection if they knew about mirror neurons.

So we have links between oxytocin, which is involved in social behavior, mirror neurons which are probably intimately involved in our ability to imitate and empathize and can be triggered in social settings, and yawning.  This is an interesting puzzle.  The physiological links are there, and there are certainly theories about yawning that suggest the behavior plays a communicative role, which would explain the underlying brain physiology and neurochemistry.  I’ll be taking a look at the communicative function of yawning in the next blog post.  For now, I wonder if these links are just happenstance, meaning that our brain structure is, at some level, involved in solving social problems.  Yawning may serve a communicative function, or it may just be a behavior that got caught up in the machine of social communication as it evolved, but that we shouldn’t necessarily read more into it than that.

Aside from mirror neurons in the frontal lobes, the hypothalamus, as well as other subcortical structures in the forebrain, also seem to be involved in yawning, as are “lower” structures in the hindbrain, particularly the pons and the medulla oblongata.  The hypothalamus is a major regulatory structure in the brain, responsible for regulating eating and drinking as well as playing a role in sleep and wakefulness and weight.  That evolutionary processes, which tend to be conservative and recycle, might direct some control over yawning to this area, particularly if yawning is involved in arousal, also makes sense.  It may be a way of regulating arousal.

This is all, of course, a bit of speculation on my part.  It is an interesting exercise to take all the pieces of knowledge and try to fit them together into some coherent understanding of a particular behavior.  That is, however, often very difficult, as evidence sometimes is contradictory.  For example, according to Daquin, Micallef, & Blin (2001) report that a dopamine antagonist which works on the negative symptoms of schizophrenics did not suppress experimentally induced yawning (via an injection of apomorphine) even though it worked at the level of the D3 receptor.  This evidence muddies the water as far as the role of the D3 dopamine receptor in yawning, and serves to remind us, once again, of the brain’s sheer complexity even when it comes to a seemingly simple behavior such as yawning.

References

Cooper, N. R., Puzzo, I., Pawley, A. D., Bowes-Mulligan, R. A., Kirkpatrick, E. V., Antoniou, P. A., & Kennett, S. (2012). Bridging a yawning chasm: EEG investigations into the debate concerning the role of the human mirror neuron system in contagious yawning. Cognitive, Affective & Behavioral Neuroscience, 12(2), 393-405. doi:10.3758/s13415-011-0081-7

Daquin, G., Micallef, J., and Blin, O. (2001). Yawning.  Sleep Medicine Review, 5(4), 299-312. doi:10.1053/smrv.2001.0175

Guggisberg, A. G., Mathis, J., Schnider, A., & Hess, C. W. (2010). Why do we yawn? Neuroscience And Biobehavioral Reviews, 34(8), 1267-1276. doi:10.1016/j.neubiorev.2010.03.008

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