-
- Abstract
- Yawning apparatus and the exact location of
the yawn reflex remains controversial. Yet
yawning is a significant behavioural response
that may potentially be a new diagnostic marker
of neurological disease such as multiple
sclerosis. Evidence of brain cooling following
yawning supports the Thompson Cortisol
Hypothesis which postulates the association
between cortisol, electrical nerve activity and
yawning within the known stress-response system,
the Hypothalamus-Pituitary- Adrenal-axis.
Changes in cortisol levels are important because
they present as a new potential diagnostic tool
in the early diagnosis of neurological symptoms.
-Thompson SB
The dawn of the yawn: is yawning a warning?
linking neurological disorders. Medical
Hypotheses 2010;75(6):630-633
- -Thompson
SB Born to yawn? Cortisol linked to yawning:
A new hypothesis. Medical Hypotheses
2011;77(5):861-862
- -Thompson
SB, Zisa L Yawning: Thompson Cortisol
Hypothesis Discussed. 12-2011. http://www.webmedcentral.com
- -Thompson
SB Is Yawning A Warning, Neurologically?
12-2011. http://www.webmedcentral.com
- -Thompson
SB.Yawning, fatigue, and cortisol: Expanding
the Thompson Cortisol Hypothesis. Med
Hypotheses.Med Hypotheses. 2014;83(4):494-6
- -Thompson
SB. Pathways to yawning: making sense of the
Thompson Cortisol Hypothesis. Medical Research
Archives 2015(3)
- -Thompson
SB, Simonsen M. Yawning As a New Potential
Diagnostic Marker for Neurological Diseases.
Journal of Neurology and Neuroscience.
2015;6(3):22
- 1. INTRODUCTION
- 1.1 Myths about yawning
- Yawning manifests itself in many different
ways making it difficult to research because its
origin has historically and apparently become so
allusive to researchers. Yawning is accessible
to many of us and has been anecdotally
associated with sleepiness; lack of oxygen;
headaches; excessive fatigue (Catli et al.
2014); anxiety; after taking antidepressants;
and as a contagious form, after seeing animals
and humans yawning (Prasad, 2008; Sarnecki,
2008; Provine, 2012).
-
- Association between yawning and brain
temperature regulation has been proposed because
of the link between excessive yawning and brain
temperature fluctuations in people with multiple
sclerosis (MS) (Gallup & Gallup, 2007;
Thompson, 2010). Circadian rhythm and the
control of brain temperature is the
responsibility of the hypothalamus which is
linked to the pituitary gland, and the adrenal
glands. Normally, the
hypothalamus-pituitary-adrenal (HPA) axis
produces sufficient hormones to protect against
stress and to provide the body with readiness
for physical exertion (Thompson, 2014a).
Cortisol is the naturally produced hormone that
protects the body against being stressed
physically and psychologically, and recently has
been reported in association with yawning in a
number of experimentally-controlled trails
(Thompson & Bishop, 2012; Thompson, 2014a;
Thompson, Frankham & Bishop, 2014; Thompson,
Rose & Richer, 2014).
-
- 1.2 Contagious yawning
- Mental Attribution Theory (Platek et al.
2003; Norscia & Palagi, 2011) has been
presented as the reason for contagiously yawning
when we are empathic to others who yawn, and
even to animals that we care about, who yawn in
our presence. Yawning happens in the womb
(Reissland et al. 2012) and is more frequent in
new- born babies than toddlers since sleep
deprivation increases the chances of us yawning
which may make us more vulnerable to the effects
of stress and fatigue (Giganti et al.
2007).
-
- 2. CORTISOL AND YAWNING
- 2.1 Cortisol as a stress hormone
- It is understood that cortisol acts to
protect our body against stress, both physical
and psychological stress loadings. It also
regulates the other hormones released within the
HPA-axis. It is suggested that, as part of its
stress- protection and stress-response, cortisol
elicits yawning by increasing the electrical
activity of the nerves in the muscles around the
jaw line, giving rise to yawning (Thompson &
Zisa,2011).
-
- 2.2 Thompson Cortisol Hypothesis
- The Thompson Cortisol Hypothesis (Thompson,
2010; Thompson & Bishop, 2012; Thompson,
Frankham & Bishop, 2014; Thompson, Rose
& Richer, 2014) is the first evidence-based
report that links cortisol with yawning, and
shows that cortisol rises when we yawn. It is
suggested that the rise in cortisol level
triggers our yawning response. Implications of
this research are that yawning is the mechanism
for controlling hormone regulation and
hypothalamus temperature regulation.
-
- Reports of brain-stem ischaemic stroke
patients have indicated that spontaneous
yawning, gives rise to parakinesia brachialis
oscitans in which the paralyzed arm may rise on
yawning (Wimalaratna & Capildeo, 1988;
Thompson, 2010; Walusinki, Neau &
Bousslavsky, 2010). Recently, swallow reflex and
yawning have also been postulated to be
temporally related (Kimiko et al. 2014).
-
- 2.3 Yawning and the motor cortex
- From the Functional Magnetic Resonance
Imaging studies reported, yawning seems to
involve the frontal and parietal lobes, insula
and amygdala (Krestel et al. 2013) though it is
probable that there is a threshold level of
cortisol to be reached following fatigue,
empathy, or sleep deprivation, to elicit
yawning. Electrical Myographical (EMG) nerve
activity in the jaw muscles is increased with
yawning, and is also associated with rises in
cortisol levels (Thompson, 2013; Thompson,
2014b). EMG feedback as well as hormone level
changes within the HPA-axis continually regulate
cortisol and adrenaline production within this
feedback closedloop.
-
- The motor cortex is likely to be involved in
this feedback loop since it controls movement
including jaw-line muscle activation. Other
influences in this mechanism may be the amygdala
(Norscia & Palagi, 2011), especially during
empathic (or contagious) yawning where
psychologically we are influenced by our mood
and the sense of belonging to a particular
social grouping. Hence, we respond by yawning
because others (or animals) are yawing in our
presence (Guggisberg et al. 2010).
-
- For the motor cortex to be involved in
yawning, it is probably stimulated by cortisol
production. For this to occur, the motor cortex
needs to be in communication with the
hypothalamus which is triggered by cortisol and
regulates brain temperature. Since brain
temperature may be lowered following yawning, it
is postulated that the change in brain
temperature is communicated, via the
hypothalamus (within the HPA-axis), to the motor
cortex so that yawning can be ceased.
-
- Symptom relief such as in lowering brain
temperature, in people with multiple sclerosis,
has been evidenced by Gallup and Gallup (2007).
Excessive yawning is a common symptom of
multiple sclerosis due to excessive fatigue
(Fleming & Pollak, 2005).
-
- 2.4. Yawning and the brain-stem
- The brain-stem is an evolutionary structure
known to be of importance for our vital
functions. Ischaemia in this region clearly
affects these functions, such as in stroke, and
tracts to the motor cortex are the likely method
of communicating status of movement but also
feedback about yawning responses. This is
probable because of the unusual yawning and
involuntary responses of the upper paralyzed
limb of brain- stem ischaemic stroke patients
(Walusinski, et al. 2010), but also because
jaw-line muscle movement is governed by the
motor cortex (Thompson, 2011;2013).
-
- It is theorised that cortisol plays an
important role in regulating, indirectly, the
movement of limbs (through musculature) via
feedback to the motor cortex, or more precisely,
via the hypothalamus that communicates with the
motor cortex.
-
- In this instance, it is suggested that the
brain stem fails to act on the change in levels
of cortisol and allows upper limb musculature
(signalled by the motor cortex because of rises
in cortisol) to contract, and raise the upper
limb. In the stroke patient, the cortisol levels
are inadequately detected thus resulting in the
movement in the paralyzed limb (Wimalaratna
& Capildeo, 1988; Walusinski, 2009). In
contrast, on the unaffected side of the body,
movement is inhibited, because the signal (rise
in cortisol) is recognised within the brain stem
structure.
-
- Comparison between pathways illustrated,
allows postulation that the brain-stem is
involved inhibition of movement. Reacting to
cortisol levels detected by the hypothalamus,
the brain-stem acts to inhibit movement of the
upper limb when it is not required to move.
Failure of this inhibition may be evidenced in
parakinesia brachialis oscitans (Walusinki,
2006; 2007) following brain-stem ischaemic
stroke.
-
- Of course, hormones do not act singularly
and it is acknowledged that this scenario may be
a simplification because the hypothalamus, in
particular, contributes to the HPA-axis which
also governs secretion of adrenaline. However,
it is highly plausible that in brain stem
ischaemia, it is the loss of neuronal detection
systems that would normally communicate with the
motor cortex, that are responsible for the
strange movement in the paralyzed upper limbs of
stroke patients. It is plausible that the
detection system relies on the monitoring of
cortisol levels, which when faulty, result in
yawning without the recognition of falling
cortisol levels after yawning.
-
- 2.5 Implications for MS
- In MS, it is the incomplete innervation and
loss of the myelin sheath around nerves that is
largely responsible for the uncoordinated
movements seen in people with the disorder.
Since the cause of fatigue in MS is not well
understood, an evaluation of the attentional
network during intrinsic and phasic alerting
tasks has been performed (Périn et al.
2010).
- Importantly, the rise in temperature seen in
MS patients is coexistent with fatigue together
with excessive yawning (Gallup & Gallup,
2008; 2010). It is postulated, therefore, that
cortisol levels rise when yawning occurs in
people with MS, just as it is evidenced in
healthy people (Thompson & Bishop, 2012),
but at higher levels than seen in the healthy
population because of the presence of excessive
fatigue which is frequently seen as a common
symptom of MS (Gallup & Gallup, 2008).
-
- Brain scans performed before and after
induced fatigue and yawning may well be the
conclusive answer to theories suggesting
communication between motor cortex, HPA- axis,
and brain-stem regions, and potentially
supported by cortisol assay. An on-going
multi-centre study conducted at Bournemouth
University by the author together with French
neuroscientists at Université Paris X
Ouest Nanterre La Défense, Hôpital
Universitaire Amiens, and Jules Verne
Université de Picardie, France (Thompson
et al. 2014) may help towards illuminating such
theories.
-
- 3. CONCLUSION
- Yawning and cortisol is of continued
interest to neuroscientists, clinical
practitioners, neurologists and theorists.
Excitement has been re-kindled with the
suggestion that cortisol, and yawning, may
present as the next new potential diagnostic
biomarker for neurological disease detection.
Clearly, further research is indicated but it is
important in pursuing this possibility if it
means that there may be a potential use of one
of our oldest observed behaviour, yawning, that
for centuries, philosophers such as Hippocrates
(Vigier, 1620) has upheld for inclusion in their
list of "useful natures".
-
-
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