- J.
Barbizet has done a useful service in
reminding us how little is known about the
everyday occurrence of yawning. In a short
article he concisely and clearly covers almost
all that can be said about it from the
physiological, psychological, and clinical
points of view. The act of yawning is
involuntary and unfolds itself in a fairly
consistent way, beginning with an active
inspiratory movement that is chiefly remarkable
for the enormous dilatation of the pharynx. At
the height of the inspiration there are
associated facial movements and then very often
generalized stretching of the limbs. This is
followed by the third phase of passive
expiration. Such movements are seen also in many
animals, and there is possibly a connexion
between it and, for example, certain movements
of birds' beaks accompanied by the flapping of
wings.
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- Drowsiness and boredom are the usual
precipitating causes of yawning. It has been
observed in some neurophysiological experiments
but usually in association with sleep. No
"centre" for yawning has been detected by direct
stimulation, nor have the conditions been
defined that will invariably produce it.
Barbizet considers it likely to be related to
the activity of the brainstem reticular
formation, whose function is intimately related
to the level of wakefulness of the brain. In
support of this is the fact that yawning is
commonly observed in association with lesions of
the brainstem or lesions indirectly compressing
it. It is among the sequelae of encephalitis
lethargica, and is occasionally seen as an aura
of epilepsy. However, A. P. Heusner cites
various authors who describe the complex
yawning-stretching act in anencephalic infants,
which would put the controlling centre, if it
exists, at the level of the lowest centre of
respiration.
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- The possible functional significance of
yawning is unsettled. The movement may increase
venous circulation to the heart, and as a result
of increased cardiac output the brain might
benefit, but direct observations of the effect
of yawning on the cerebral circulation are
missing. Heusner finds a systemic
vasoconstriction and transient tachycardia,
probably the same as that obtained by any deep
inspiration.
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- In many ways the most interesting and
tantalizing aspect of yawning is its relation to
psychological factors and in particular the way
in which it seems to be infectious in a group.
The act is a part of the complex language of
gesture that long preceded speech as a means of
communication in the prehistory of man, a
subject greatly enlarged by the work of
ethologists such as Lorenz and Tinbergen. In
some unknown way animals are tuned to respond to
significant signals of their own species.
Yawning is a relatively simple signal, and the
perception of its social meaning-boredom,
presumably-may hardly require any learning by
environmental experience. We seem to know more
about the social significance of yawning than
the part, if any, it plays in the homoeostasis
of the body.
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-
- SIR.-Your annotation (Journal, December 27,
p. 1585) on the article by Dr. Barbizet
about yawning does not quite cover all the
ground. There is the psychological conflict
which Dr. Barbizet hints at: the conflict
between the need to remain within an irksome
environment and the desire to escape from it (to
sleep or other activity). But aside from this
there is also a functional respiratory
consideration.
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- Yawning may be considered as a paroxysmal
sigh, combining the thoracic and abdominal
muscles of respiration with the phylogenetically
primitive glossopharyngeal elements. By thinking
of a yawn in this way, one aspect of its
functional significance may be clearer, for
occasional sighing is a regular and necessary
accompaniment to the muscular hypotonia of
sleep.
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- Without an occasional sigh the compliance of
the chest falls and the work of breathing is
increased. This fall of compliance is readily
observed in cases of prolonged apnoea treated by
mechanical artificial ventilation. If a
pressure-controlled machine is used and the
pressure of the inspiratory phase is kept
constant for an hour or so a gradual fall of
tidal volume is observed. However, if a few
artificial sighs are interposed, by increasing
the inflating pressure for two or three breaths
(so that the tidal volume approaches the
inspiratory capacity), then the compliance
reverts to its previous level. Presumably the
act of sighing opens up alveoli which had become
shut by surface tension forces, restoring the
lung volume to its normal value.
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- It is of some interest that this restoration
of compliance does not occur if the patient is
receiving noxious stimuli while under light
general anaesthesia, with or without complete
curarization. Here the attempted reaction is the
opposite of a sigh or a yawn, the patient
responding to noxious stimulation from without
by an expiratory rather than an inspiratory
effort.-I am, etc.,
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- Royal Victoria Hospital, P. R. BROMAGE.
Montreal.
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- BROMAGE PR. Total respiratory compliance in
anaesthetized subjects and modifications
produced by noxious stimuli. Clin Sci (Lond).
1958;17(2):217-36.
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