Yawning is such a commonplace physiological
occurrence that very little is known about its
aetiology.
It is a reflex action whose pathways reach
no higher in the central nervous system than the
basal ganglia. The act itself consists of a
tonic contraction of several muscle groups
resulting in a deep inspiration, dilatation of
the pharynx, and depression of tongue and lower
jaw.
The physiological effects of the deep
inspiration include an increase in venous return
to the heart and, probably more significant, the
opening of pulmonary alveoli which may have
closed during a prolonged period of quiet
breathing. If yawning is impossible, as in a
patient on a ventilator, disseminated alveolar
collapse may occur; this is the cause of the
venoarterial shunting and arterial hypoxaemia
seen in this situation.
The 'purpose' of the associated facial
contortions is less easy to determine. It has
also been noticed that the sense of smell is
more acute during a yawn; this is probably as a
result of a large bolus of air being brought
into contact with an unusually exposed
nasopharynx. The importance of an acute sense of
smell for wild animals is clear and it has been
postulated that this reflex may have had a
survival value for primitive man.
The stretching of arms, commonly associated
with yawning, is known as 'pandiculation' and is
also a reflex act. This information comes as a
surprise to some but is conclusively proved by
the fact that the paralysed arm in hemiparesis
may demonstrate pandiculation even when no
voluntary movement is possible. It remains to
give an account of the afferent side of this
reflex arc. This can be based only on personal
experience and everyday observations. Boredom
and drowsiness certainly provoke yawning as does
the sight or sound of someone else's yawn.
This remarkable contagiousness of yawning is
well known but no satisfactory explanation for
it appears ever to have been offered. Very
occasionally yawning, especially when occurring
very frequently, may be evidence of organic
disease. It may be an epileptic phenomenon or
occur following attacks of encephalitis along
with other disturbances of respiration such as
hyperventilation and Cheyne-Stokes breathing.
Paroxysms of yawning may also be caused by
cerebral tumours, especially those situated in
the posterior fossa, and yawning can be
regularly produced in an opiate addict by the
injection of a narcotic antagonist.
