A conjecture as to the physiological origins of yawning

Dear Sir, 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. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.