-
- How
yawning switches the default-mode
network to the
attentional network
- by
activating the cerebrospinal fluid
flow
- Olivier Walusinski
-
-
- In reference to the origins of yawning, last
addressed in these pages by Gupta and
Mittal,[1] there are recent discoveries
characterizing the lymphatic system of the brain
and calvarium which may be relevant. Lymphatic
channels from the brain had previously been
identified only in conjunction with the cranial
nerves exiting the skull, and anatomists and
physiologists had erroneously concluded that the
brain lacked the ubiquitous lymphatic
circulation common to other organs. It was
proposed that metabolic waste byproducts were
chiefly drained from the cerebrospinal fluid
(CSF) into the venous circulation through the
arachnoid granulations. Recent studies by
Louveau et al. and Louveau et al.[2]
suggest that an organized lymphatic circulation,
present in the dural sinuses, also collects CSF
from the parenchyma and then empties into the
deep cervical lymph nodes of the rat.
Furthermore, the "glymphatic" system of fluid
transport through the paraarteriolar space, into
the central nervous system (CNS) through
aquaporin channels on astrocytes (glia), and
then across the brain interstitium through
convection and into the perivenous space, has
been characterized.[3]
-
- It is also recently reported that there is a
marked enhancement of flow and clearance of CNS
metabolites during sleep through the
"glymphatic" system.[4] This increased
lymphatic flow presumably results in increased
circulation into the venous structures of the
deep cervical and subcranial lymphatics,
possibly including the oropharyngeal lymph
nodes.
-
- The yawn is a stereotypical coordination of
respiratory and muscular movements that is
retained across evolutionarily diverse species
of vertebrates. Its origin remains uncertain,
but most often correlates with sleep debt or
resolution of the sleeping process. If one of
the (vestigial) physiological functions of sleep
is to clear metabolic byproducts from the brain
interstitium, one might conjecture that the
build-up of such metabolites in central nuclei
might initiate the yawning reflex. The yawn
itself consists of a (1) deep respiratory
effort, associated with (2) extension of the
neck and wide expansion of the jaw and
subsequent (3) contraction of the tongue and
tension of the pharyngeal orifice. This may be
associated with raising of the arms and muscular
clenching of the extremities. One may speculate
that this mechanism might, respectively,
initiate increased venous return through
lowering of the intrathoracic pressure as well
as through increasing return venous flow from
the contracted musculature of the extremities,
coordinated thereafter with propulsion of
cervical lymphatic fluid into the central
vessels through flexion of the neck, contraction
of the tongue and expansion of the jaw, and
contraction of platysma and neck
musculature.
-
- Testing the hypothesis that yawning promotes
increased clearing of CNS-derived fluid into the
central venous structures should be feasible
(although perhaps chiefly academic) and might be
accomplished through radionucleotides instilled
through the CSF or real-time magnetic resonance
imaging flow imaging.
-
- References
-
- 1. Gupta S, Mittal S. Yawning and its
physiological significance. Int J Appl Basic Med
Res 2013;3:11-5.
-
- 2. Louveau A, Smirnov I, Keyes TJ, Eccles
JD, Rouhani SJ, Peske JD, et al. Structural and
functional features of central nervous system
lymphatic vessels. Nature 2015;523:337-41.
-
- 3. Iliff JJ, Wang M, Liao Y, Plogg BA, Peng
W, Gundersen GA, et al. A paravascular pathway
facilitates CSF flow through the brain
parenchyma and the clearance of interstitial
solutes, including amyloid ß. Sci Transl
Med 2012;4:147ra111.
-
- 4. Xie L, Kang H, Xu Q, Chen MJ, Liao Y,
Thiyagarajan M, et al. Sleep drives metabolite
clearance from the adult brain. Science
2013;342:373-7.
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