Yawning is often noted in medical seminars
and conferences - be they surgical, orthopaedic,
gastro-enterological, endocrinological or
neurological. Yet, this condition receives
little coverage by professors in medical
schools. On the contrary, most lecturers have an
adverse reaction to it - when the overworked
medical student opens his mouth to raise the
topic, the fierce pathology lecturer demands
that it be covered up.
Yawning - what is the physiology underlying
it? Why is it that when you yawn, the whole
world yawns with you? And what are the
implications of the "infectiousness" of
yawning?
Yawning, a brainstem-mediated bodily
response that is common to all vertebrates, is
observed in human fetuses as early as at 15
weeks of intra-uterine life. A yawn involves a
deep inspiratory breath and slow expiration,
accompanied by wide opening of the jaw, and
sometimes by limb-stretching and lacrimation.
The bronchial stretching on inspiration
stimulates a cholinergic response that reduces
peripheral vascular tone, thereby increasing
peripheral blood flow. Venous return is also
increased by the drop in intra-thoracic
pressure. The blood-rich lateral pterygoid
muscles (involved in jaw-opening) and the soleus
muscle (leg-stretching) also contract,
expellingbloodandincreasing blood flow to the
circulation. Now you see what the physiologists
at medical school left out...
Askenasy' proposed thaty awningbrings about
an increase in oxygenation of the "yawn centre"
of the brain, which is located in the brainstem
close to the ascendant activatory reticular
system (AARS). This may be the body's reflex to
combat drowsiness and maintain the required
level of alertness. This may explain why Olympic
athletes tend to yawn more frequently just
before their events. Drowsiness (as a result of
boredom, mediated via connections from the
pre-frontal area to the AARS) is generally the
most common trigger for yawning. However, the
"yawn centre" is also stimulated by a wide range
of other cortical and subcortical input,
hormones and neurotransmitters.
Yawning is noted to be a symptom of many
neurological and psychiatric illnesses, such as
encephalitis, cerebral hypoxia, CNS neoplasia,
schizophrenia, and depression.
It is also seen in people suffering from
Eustachian tube disorders, gastric and biliary
illnesses, and different syndromes of hormonal
imbalance involving adreno-corticotrophin (ACTH)
and melanocyte-stimulating hormone (MSH).
Yawning is one of the symptoms of drag overdose
(from substances such as naloxone, sodium
valproate, imipramine, serotonin and
pentobarbital), and of withdrawal (from drags
such as heroin, morphine, ketamine, methadone,
and pentazocine). As you can see, then, the
dethroned Olympic sprinter Ben Johnson may have
been yawning for more than one reason.
Why then, is yawning "infectious" among
normal healthy individuals, and is the
explanation of increasing cerebral
oxygenationand alertness sufficient to justify
the persistence of this primal (and seemingly
useless) behaviour over time?
The question of why yawning is "contagious"
has been monkeyed around with. Anderson et al
showed that adult chimpanzeesyawnedmore
frequently whenviewingvideos of
otherchimpanzeesyawning, compared to when
watching videos of chimps opening their mouths
for other purposes (e.g., snarling and eating).
Half of the adult human subjects also showed
this same response to videos of adults yawning.
Yet interestingly, neither infant chimpanzees
nor human children under the age of five showed
any sign of being "infected" by others'
yawning.
It has been suggested by these scientists
that yawning is a sub-conscious form of empathic
behaviour; a manifestation of our origins as
social beings. The ability to empathise requires
a certain maturity of intellect, and children
below the age of five are apparently unable to
"put themselves in another's shoes". This
probably explains why the sight and sound, of
tired, yawning parents woken from sleep in the
middle of the night has little effect in
inducing guilt or sleep in the minds of their
little offspring. Research has also shown that
"potential lovers", or people attracted to each
other, had an increased tendency to induce each
other to yawn than other individuals with less
in common.
This presents the possibility of using
"suggestibility to yawning" as a research tool
to assess the mental and social make-up of adult
human beings. Would autistic patients, for
example, who have a decreased or abnormal
ability to relate to others, be less susceptible
to infectious yawning? Perhaps a
"cross-infectivity index" for yawning could be
used to assess the level of subconscious empathy
between future life-partners - dating agencies,
take note!
Yawning may also have served as a social
signal between members of ancient human
communities: collective loud yawning and
stretching serving as a wake-up call in the
morning, and signifying bed-time at night. It is
possible that, like speech, which evolved from
involuntary noises, through short signals, to
complex language over evolutionary time, yawning
could be trained to be produced voluntarily. In
fact, Dr Anderson has shown that macaque monkeys
can be trained to yawn on-demand.
If one day, humans could be trained
similarly to yawn, and internally suppress yawns
at will, our society would reachnew levels of
graciousness. No longer would we give boring
lecturers grief by initiating yawning epidemics
in class. We would be models of courtesy and
attentiveness even in the face of the most
long-winded conversationalists. And children
could be trained to yawn themselves to sleep at
eight o'clock on the dot, giving their parents
much-needed reprieve.
The power of the yawn is such that even
thinking or reading about yawning may itself
induce yawning... just in case you go away
thinking that the yawns you have had over the
past few minutes were due to the quality of this
article.
REFERENCES
Askenasy
JJ Is yawning an arousal defense reflex? J
Psychol 1989;123(6):609-621
Anderson
JR, Myowa-Yamakoshi M, Matsuzawa T.
Contagious yawning in chimpanzees The Royal
Society Biology Proc R Soc Lond B Biol Sci
2004;271 Suppl 6:S468-470 Contagious yawning in
chimpanzees. Proc Biol Sci
2004;271(S6):468-70.
Platek
SM, Critton SR, Myers TE, Gallup GG.
Contagious yawning: the role of self-awareness
and mental state attribution Cogn Brain Res
2003;17(2):223-227 Contagious yawning: the role
of self-awareness and mental state attribution.
Brain Res Cogn Brain Res 2003;17:223-7.
