Remarkably little interest has been paid to
yawning in research, even though it is an
everyday phenomenon. Observation suggests that
yawning is a semivoluntary act associated with
boredom and fatigue, and it seems to be
contagious in a room of similarly affected
people. But why do fetuses yawn, and what is the
physiological purpose of yawning?
The yawning reflex can be triggered by drugs
or by psychological, hormonal, or neurological
stimuli. Yawning occurs at as early as 15
weeks in an embryo, and newborn babies
usually yawn shortly after they take their first
breath. All vertebrates
yawn, and yawning persists in people who are
in a vegetative state. Such universality
suggests that yawning has a defensive or
protective function.
The reflex arc is thought to be in the
reticular system of the brainstem,
and it involves the respiratory neurons in the
medulla; the motor nuclei of the 5th, 7th,10th,
and 12th cranial nerves; the phrenic nerves; and
the motor supply to the intercostal muscles.
Neurological lesions tell us much about the
functioning of the brain; they confirm that the
yawning center is part of the reticular system
because people with pontine
lesions who cannot open their mouths at will
are able to yawn in response to appropriate
stimuli. The reticular complex is closely
connected to cortical areas, the subcortical
limbic center, and the hypothalamus, which
explains why boredom, fatigue, and the act of
observing others yawning can provoke the reflex
through the prefrontal associative area.
Endorphins
play an important role in yawning. Apomorphine,
a dopaminergic receptor agonist, so reliably
stimulates yawning that apomorphine-induced
yawning is used as a test of the integrity of
dopaminergic function in patients with
neurological disorders such as Parkinson's
disease. Increased serotoninergic activity
also induces yawning, acting in concert with
cholinergic influences. This increase in
cholinergic activity acts through negative
feedback to decrease serotonin and noradrenaline
concentrations and thereby reducing the tendency
to sleep. Yawning, therefore, has an alerting,
antisleep effect, which is probably
enhanced by the generalized stretching movements
that often accompany a yawn.
Studies
have shown that yawning predicts a subsequently
increased level of activity. However, the rate
of yawning is not dependent on the amount of
previous sleep or the time of waking or retiring
to bed. More yawning occurs during the week than
at weekends. Yawning is a feature of some
pathological states. The frequency of yawning is
reduced in Parkinson's disease, and it may occur
more often in epilepsy and in states of opiate
withdrawal. In patients who have had a stroke
affecting the pyramidal tract in the internal
capsule region, stretching movements of the
hemiplegic arm
may occur during spontaneous or apomorphine
induced yawning. Yawning induced cerebral
ischemia (in the form of a transient ischemic
attack) in a patient who had undergone bypass
surgery when the accompanying vigorous
movement of the jaw muscle occluded the temporal
and middle cerebral arteries. In comatose
patients the onset of yawning predicts the
imminent return of wakefulness. Yawning has been
associated with a minor degree of hypoxia in
patients with cerebrovascular disease. This has
led to the idea that the function of yawning is
to generate a large inspiration to overcome
borderline hypoxia and hypercapnia. If this were
true, the incidence of yawning would be reduced
by breathing high
concentrations of oxygen and would be
increased by raising the partial pressure of
carbon dioxide.
Studies have shown that this does not occur,
and yawning is not a major clinical feature in
patients with acute or chronic ventilatory
failure. From a behavioral point of view, when
yawning seems to be contagious,
similar external stimuli (or the lack of
stimuli) are likely to affect all of those
present. However, yawning can be induced in
those who are not bored just by thinking about
it. These socioenvironmental influences indicate
a communicative function, and the reflex may act
as a nonverbal cue in generating social
cohesion.
We can probably conclude that yawning has an
important role as an arousal mechanism to
maintain alertness and defend against the onset
of sleep. It can be affected by a range of
behavioral influences, but much still needs to
be learned about this everyday
activity.
