The body, etc.

The inspiratory muscles, including the intercostal and neck muscles, also spasm, while the expiratory muscles are inhibited. (7) These sudden contractions cause a rapid intake of air (“hic”), followed by the immediate closure of the glottis or vocal cords (“up”). (8)

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As many of us have probably experienced, a range of stimuli can cause these involuntary contractions. The physical stimuli include anything that stretches and bloats the stomach, (9) such as overeating, rapid food consumption and gulping, especially of carbonated drinks. (10) Emotionally, intense feelings and our responses to them, such as laughing, sobbing, anxiety and excitement, can also be triggers. (11)

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This list is not at all exhaustive; in fact, the range of stimuli is so large that hiccups might be considered the common thread between a drunk man, a Parkinson’s disease patient and anyone who watches The Notebook. The one thing that alcohol, (12) some neurological drugs (13) and intense sobbing (14) do have in common is that they exogenously stimulate the hiccup reflex arc. (15) This arc involves the vagal and phrenic nerves that stretch from the brainstem to the abdomen which cause the diaphragm to contract involuntarily. (16) According to Professor Georg Petroianu from the Herbert Wertheim College of Medicine, (17) many familiar home remedies for hiccupping – being scared, swallowing ice, drinking water upside down – interrupt this reflex arc, actually giving these solutions a somewhat scientific rationale.

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While modern research has successfully mapped out the process of hiccups, their purpose is still unclear. As of now, the hiccup reflex arc and the resulting diaphragmatic spasms seem to be effectively useless.

Of the existing theories for the function of hiccups, the most prominent seems to be that they are a remnant of our evolutionary development, (18) essentially ‘vestigial’; in this case, a feature that once served our amphibian ancestors millions of years ago, but now retain little of their original function. (19) In particular, hiccups are believed to be a relic of the ancient transition of organisms from water to land. (20) When early fish lived in stagnant waters with little oxygen, they developed lungs to take advantage of the air overhead, in addition to using gills while underwater. (21) In this system, inhalation would allow water to move over the gills, during which a rapid closure of the glottis – which we see now in hiccupping – would prevent water from entering the lungs. It is theorised that when descendants of these fish moved onto land, gills were lost, but the neural circuit for this glottis closing mechanism was retained. (22) This neural circuit is indeed observable in human beings today, in the form of the hiccup central pattern generator (CPG). (23) CPGs exist for other oscillating actions like breathing and walking, (24) but a particular cross-species CPG stands out as a link to human hiccupping: the neural CPG that is also used by tadpoles for gill ventilation. 

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Tadpoles “breathe” in a recurring, rhythmic pattern that shares a fundamental characteristic feature with hiccups: both involve inspiration with closing of the glottis. (25) This phenomenon strengthens the idea that the hiccup CPG may be left over from a previous stage in evolution and has been retained in both humans and frogs. However, the CPG in frogs is still used for ventilation, while in humans, the evolution of lungs to replace gills has rendered it useless. (26)

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Based on this information, it seems hiccupping lost its function with time and the development of the human lungs, remaining as nothing more than an evolutionary remnant.

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However, we cannot discredit hiccupping as having become entirely useless as soon as gills were lost. Interestingly, hiccupping has only been observed in mammals – not in birds, lizards or other air-breathing animals. (27) This suggests that there must have been some evolutionary advantage to hiccupping at some point, at least in mammals. A popular theory for this function stems from the uniquely mammalian trait of nursing. (28) Considering the fact that human babies hiccup in the womb even before birth, this theory considers hiccupping to be almost a glorified burp, intended to remove air from the stomach. This becomes particularly advantageous when closing the glottis prevents milk from entering the lungs, aiding the act of nursing. (29)

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Today, we reduce hiccups to the disorder and disarray they bring to our day. But, next time you are hit with a bout of hiccups, take a second to find some calm amidst the chaos and appreciate yet another fascinating evolutionary fossil, before you hurry to dismiss them. After that, feel free to eat those lemons or gargle that salty water to your diaphragm’s content.