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- Abstract
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- Yawning has fascinated scientists for
centuries. Hippocrates included it I his list of
"useful natures" and Hindus used to regard
yawning as a religious "offence". Some ancient
superstitions saw yawning as the escaping of the
soul. However, scientists today are still amazed
with the extent to yawning tells us about many
other neurological conditions (e.g. Thompson,
2010). The importance of yawning in helping us
understand other neurological mechanisms has led
to La Société Française de
Neurologie et La Société des
Neurosciences convening the first international
conference on yawning at l'Hôpital de la
Pitié-Salpêtrière, Paris,
France in June 2010 to which the author was
invited as one of the invited Speakers.
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Thompson
Cortisol Hypothesis : all the
publications
- Introduction
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- It has been commonly thought that yawning
replaces important oxygen in the blood by
expanding the lungs and stretching the muscles.
Indeed, stretching the lungs potentially
increases their capacity and also increases
wakefulness (Provine, 1986).
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- The curious nature of yawning has led
researchers to question these beliefs especially
since yawning has been evidenced in so many
animals as well as in humans. Yawning has been
seen even in rodents, and in the order of
reptiles known as the Testudines (which is the
crown group of the super order of Chelonia) such
as turtles, and in pigs.
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- A possible reason for yawing is that muscle
movements and compressions stiumulate the
carotid body, receiving shunted blood, giving
rise to an increase in pressure and hormones
release (Matikainen & Elo, 2008). Stretching
the muscles involved in yawning implicates the
locus coeruleus, paraventricular nucleus of the
hypothalamus, and reticular activating system
(Provine, 1986). However, others believe that
neural mechanisms are responsible for the
mechanical action as well as the aetiology
(Thompson, 2010).
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- Discussion
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- In pre-term and near term infants, the
incidence of yawning and also the frequency,
decreases during the day and with age (Giganti,
et al., 2007). This is probably due to circadian
and homeostatic control of sleep and
wakefulness.
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- In non-primates, the incidence of yawning is
higher before than after sleep (Walusinski &
Deputte, 2004). In particular, in rats,
cholinesterase inhibitors such as E2030, can
induce yawning. Scopolamine, (a centrally acting
anti-muscarinic drug), has been seen to block
E2030-induced yawning but not E2020-induced
yawning. This suggests that central cholinergic
and dopinergic mechanisms may be involved in
E2020-induced yawning (Ogura, et al.,
2001).
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- Drug-induced yawning may serve as a
compensatory brain cooling mechanism when
natural regulation fails (Prasad, 2008). Gallup
and Gallup (2007) have suggested that yawning
may regulate brain temperature in Multiple
Sclerosis (MS). Although patients often
experience disturbed sleep patterns with MS,
there is evidence of irregular thermoregulation
(Fleming & Pollak, 2005).
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- The yawn may be a physiological trait
emerging from a vestigial reflex that
coordinates aggressive social behaviour (Prasad,
2008). However, Walusinki and colleagues (2010)
have shown that the onset of yawning can
coincide with involuntary rising of the
paralysed arm in stroke, leading the authors to
coin the term "parakinesia brachialis
oscitans".
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- Patients with depression have elevated brain
temperature levels; with mania, brain
temperature tends to fall (Salerian, et al.,
2008). This has led researchers to investigate
whether reducing the dose of antidepressant
lowers brain temperature and reduces the
incidence of yawning (Gutiérrez-Alvarez,
2007).
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- The link between brain-stem ischaemia and
excessive yawning is also well documented
(Wimalaratna & Capildeo, 1988) and suggests
that neural mechanisms are involved.
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- Contagious yawning
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- Fifty per cent of us yawn within 5 minutes
of seeing another person yawn. It is suggested
that yawning is an innate action that recognises
a particular behavioural state. Indeed, Mental
Attribution Theory provides an explanation along
the lines that we simulate another's behaviour
if we empathy with their perceived emotional
state. Hence, if we are aligned with them at
some emotional level, then we might actually
yawn after they yawn.
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- This attribution of empathy is being
explored by the author in other contexts, such
as with artistic and film appreciation, fashion
choice, and the level of cortisol in the blood
(correlated with fatigue and cold). It will be
interesting to see if yawning is correlated with
any of these factors and if so, if it is because
of the attribution of perceived (and shared)
levels of emotional status.
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- Some authors have suggested that there are
"mirror neurons" present that may be responsible
for contagious yawning (Cooper, et al., 2008).
Others support the theory that yawning shares
the neural networks with empathy (Walusinki,
2006; Thompson, 2010). Higher representational
states may engage neurophysiological structures
to produce the yawn (Sarnecki, 2008).
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- Linking neurological disorders
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- There is an historical lack of clarity of
the neurotransmitters implicated in Parkinson's
disease namely, dopamine was thought to be the
only one implicated and later on, also
serotonin. Likewise, in Alzheimer's disease,
E2020 (acetylcholine esterase inhibitor) is
known to have an affect on memory functioning
and has been introduced as an "anti-dementia"
drug but E2020-induced yawning is not blocked by
Scopolamine, as stated earlier.
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- Serotonin is implicated in depression
because of its depletion. Monoamine oxidase
inhibitor-A, is active for serotonin (Jansen
Steur, 1997). Dopamine is thought to be
implicated in contagious yawning by activating
oxytocin in the hypothalamus and hippocampus.
Yawning is now thought to be influenced by the
neurotransmitters acetylcholine (active in
memory functioning); gamma amino butyric acid
(GABA); Adreno-Cortico-Trophic Stimulating
Hormone (ACTH); and may be others.
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- Yawning may also increase arousal and
self-awareness and connects consciousness as
well as unconscious interoception to higher
mental functions (Walusinski, 2006).
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- Yawning and trauma
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- Olivier Walusinski (2006b; 2007) and
Walusinski and colleagues (2010) have been
influential in re-writing our knowledge of the
"yawn", particularly with his descriptions of
parakinesia brachialis oscitans. Furthermore, he
has proposed that mapping of the neural network
for yawning may be possible from knowledge of
stroke localisation. This invites an exciting
new area of research potentially linking
together several neurological disorders.
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- Conclusion(s)
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- Yawning research is interesting because it
offers scientists and neurologists that yawning
may represent a warning of underlying
neurological problems. With the advent of modern
technology and understanding, we still do not
have a clear picture of why yawning happens in
some neurological cases. On the other hand, we
do know that yawning can indicate neurological
trauma which makes it an important area of
further in-depth investigation.
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