Yawning is a
phylogenetically old, stereotyped event that
occurs alone or assiociated with stretching
and/or penile erection in humans and in animals
from reptiles to birds and mammals under
different conditions.
Yawning is a common
physiological
event that has
been described since antiquity. Hypocrates
described yawning as an exhaustion of the fumes
preceding fever. Modern medicine did not pay a
great deal of attention to yawning until the
eighties, with advances in
neuropharmacology.
Yawning can be divided
into 3 distinct phases: a long inspiratory
phase, a brief acme and a rapid expiration,
frequently but not always associated with
stretching, tears, shivering, obstruction of the
eustachian canal (causing a reduction in
audiologic acuity), followed with a feeling of
comfort. The average duration of the yawn is 5
s, (range, 3 to 45 s). The earliest appearance
of yawning was observed in
a 15-week-old embryo.
(see embryology)
This
semi-voluntary event
increases vigilance and aims to alert when
drowsiness occurs ( In
animals it
subserves behaviour related to stressful
situations). Yawning probably has an important
role for social communication as well. Excessive
or pathological
yawning, "chasm",
is defined as a compulsive,
repetitive action which is not trigered by
"physiological" stimuli such as fatigue or
boredom, discribed in cases of frontal lobe
tumors, encephalitis, progressive supranuclear
palsy, following thalamotomy, after
electroconvulsive therapy and as an early
manifestation of vasovagal response (and a
variety of others pathologie-states)
and many drugs.
The complex neuronal
reflex system of yawning appears to be located
in a reticular brainstem system closely related
through the diencephalo-hypothalamic network
with large associative cortical
areas.
The
neuro-pharmacology
of yawning is complex and knowledge of its
mechanisms is incomplete. While under the
control of several neurotransmitters, yawning is
largely affected by dopamine. Dopamine may
activate oxytocin production in the
paraventricular nucleus of the hypothalamus,
oxytocin may then activate cholinergic
transmission in the hippocampus, and finally
acetylcholine might induce yawning via the
muscarinic receptors of the effectors. In fact,
this scheme is over simplified. Many other
molecules can modulate yawning, such as nitric
oxide, glutamate, GABA, serotonine, ACTH, MSH,
sexual hormones and opium derivate peptides,
hypocretin.
Dopamine involvement in yawning could have
practical applications in the study of new drugs
or the exploration of neurological diseases such
as migraine or psychosis.
Maintaining or attaining a particular
level of arousal is an important matter in the
life of most vertebrates, and yawning, to the
extent that it serves as a means for doing so,
should be seen as an important part of
adaptative behavior. The physiological,
ontogenic and phylogenic findings reviewed here
are consistent with this view. (R
Baenninger)
Treat this website as a field guide
to the terra incognita of yawning, a source of tips
about where to find yawning, how to study it, and
what it means. You will not find a tidy series of
experiments that drive inexorably (and with an
intellectual flourish) to a Grand Unified Theory of
Yawning. The yawn project is, instead, a
catch-as-catch-can interdisciplinary work in
progress. Some pieces of the yawning puzzle fit
nicely into place, while others are parts of a yet
unknown whole.
Useful advice about the process is
offered by the embryologist Hans Spemann: " I
should like to work like the archeologist who
pieces together the fragments of a lovely thing
which are alone left to him. As he proceeds,
fragment by fragment, he is guided by the
conviction that these fragments are part of a whole
which, however, he does not yet know. He must be
enough of an artist to recreate, as it were, the
work of the master, but he dare not build according
to his own ideas. Above all, he must keep holy the
broken edges of the fragments; in that way only may
he hope to fit new fragments into their proper
place and thus ultimately achieve a true
restoration of the master's creation. "
adapted from R Provine.
«...seeing
a dog and horse and man yawn, makes me feel how
much all animals are built on one structure
» Charles
Darwin 1838 ,
notebook
Yawning is a stereotyped behaviour present in
most mammals from rodents to humans and has been
described since antiquity. Hippocrates
considered yawning to be an exhaustion of the
fumes preceding fever. Modern medicine did not
pay much attention to it until the 1980s, when,
with advances in neuropharmacology, yawning
proved to be a valuable tool for the assessing
dopaminergic activity and the pharmacological
properties of new drugs. However, its precise
role in human physiology is still unknown and
its mechanisms remain unclear.
Yawning occurs after waking up, before
eating, before sleeping, and in passive
activities when it is necessary to maintain a
certain level of vigilance. It is then followed
by an acceleration of the
electroencephalographic rhythms. It does not
serve a primary respiratory function and it
clearly has a non-verbal communicative status.
Nevertheless, it is also a clinical sign in
intracranial hypertension, migraine, or
iatrogenic side effects of dopaminergic drugs
and serotonin reuptake inhibitors. In basal
ganglia disorders, yawning is reduced in
patients with Parkinson's disease, and occurs
more often in patients with Huntington's disease
and supranuclear palsy than in controls. In
healthy volunteers, apomorphine induces yawning
which is also observed at the beginning of the
''on'' periods in Parkinson's disease.
The anatomical structures known to be
implicated in the occurrence and control of
yawning are the paraventricular nucleus of the
hypothalamus(PVN), the hippocampus, the
reticular formation, the neostriatum, and the
cranial (V, VII, IX, X, XI, XII),
cervical(C1&endash;C4), and dorsal nerves.
Yawning is probably a reflex answer of the
brainstem reticular formation aimed to increase
the cortical level of vigilance. Dopamine and
oxytocin are the main neurotransmitters
implicated in its modulation. Indeed yawning
induces sensory efferents from the terminals of
the fifth facial nerve to the reticular
formation or the PVN through the spinothalamic
and hypothalamic tracts. Stimulation of the
dopamine D2 receptors of the PVN activates the
oxytocin neurones that project either to the
pons (reticular formation, locus coeruleus), to
the hippocampus, to the insula, or to the
orbitofrontal cortex, leading to the transient
feeling of wellbeing that follows yawning. This
pathway is modulated by acetylcholine,
serotonin, opioid peptides, sexual hormones, and
orexin.
Contagious yawning is an even more
intriguing phenomenon. It is triggered by
seeing, hearing, or even thinking about someone
else yawning. Contagious yawning does not occur
in species that do not recognise themselves in
mirrors or in infants younger than two years
old. The phenomenon has been investigated with
functional magnetic resonance imaging, which
implicated the precuneus or the posterior
cingulate regions, functional regions associated
with the identification of selreferent
information, a primitive form of empathy.