Why Do We Hiccup and Sneeze? Reflexes With Very Different Jobs

A sneeze is a protective reflex that expels irritants from the nasal passages. Irritation of the nasal lining, by dust, pepper, pollen, infection or inflammation, triggers sensory nerve endings that signal through the trigeminal nerve to a region in the brainstem, which then coordinates a sequence: a deep inspiration, closure of the glottis, contraction of the chest and abdominal muscles to build pressure, and a sudden release through the nose and mouth. Measurements of the resulting airflow have been revised downward from older claims, with careful studies putting typical velocities well below the frequently quoted figure of a hundred miles per hour, and high-speed imaging showing that the expelled material forms a turbulent multiphase cloud carrying droplets much further than ballistic calculations predict, which is why the airborne transmission literature has paid it so much attention. Droplets range from large ones that fall quickly to fine aerosols that remain suspended, and that size distribution rather than the peak speed is what determines infectious risk.

Sneezing has a set of triggers that are not irritants at all and reveal something about how the reflex is wired:

• •The photic sneeze reflex, in which sudden bright light causes sneezing, affecting something like a fifth to a third of people, inherited as a dominant trait, and probably caused by crosstalk between the optic and trigeminal pathways
• •Sneezing on a full stomach, a less common inherited reflex with the appealing name of snatiation
• •Sneezing triggered by plucking eyebrows or other manipulation near the nose, again through trigeminal branches
• •A reflex in some people triggered by sexual arousal or orgasm, documented in case reports and not well explained
• •The eyes close involuntarily during a sneeze, which is part of the coordinated reflex rather than a protection against eyeballs popping out, a claim that has no basis
• •Suppressing a sneeze by pinching the nose and closing the mouth traps the pressure and has caused documented injuries including ruptured eardrums and, in one case report, a torn throat

A hiccup is a sudden involuntary contraction of the diaphragm and the muscles between the ribs, drawing air in sharply, followed about thirty-five milliseconds later by abrupt closure of the vocal folds, which stops the airflow and produces the sound. The timing is the peculiar part: the closure occurs so quickly that the inhalation achieves almost nothing, so a hiccup does not move air usefully and appears to serve no respiratory function. The reflex arc involves the phrenic and vagus nerves and a central component in the brainstem. Triggers include rapid eating, swallowing air, carbonated drinks, alcohol, sudden temperature changes and emotional excitement, all of which involve distension or irritation of the stomach or oesophagus. Almost everyone experiences brief bouts. Persistent hiccups lasting more than a couple of days are a medical matter, since they can indicate irritation anywhere along the nerve pathway, including from chest, abdominal or central nervous system disease, and prolonged cases have caused serious weight loss and exhaustion.

Several explanations have been proposed and none is settled. The most discussed evolutionary account notes that the motor pattern closely resembles the gill ventilation of amphibians and lungfish, in which water is pushed across gills by a similar sequence of contraction and glottal closure, and suggests hiccupping is a retained ancient circuit with no current function. A second proposal points out that the pattern is also similar to suckling, where a closed glottis prevents milk entering the airway while the diaphragm contracts, and observes that hiccups are extremely common in fetuses and newborns and become less frequent with age, which would make the reflex a developmental one that is learned from and then largely abandoned. A third, more prosaic account holds that it clears air swallowed during feeding. The home remedies that work at all appear to do so by raising carbon dioxide or stimulating the vagus and phrenic pathways, which is why breath holding, drinking cold water and swallowing granulated sugar have some plausible basis while most of the rest do not.

A sneeze clears the nasal airway through a coordinated brainstem-driven sequence, and its infectious importance comes from the size distribution of the droplet cloud rather than from peak speed, which is lower than commonly quoted. It can be triggered by bright light in a fifth to a third of people through an inherited crosstalk between nerve pathways. A hiccup contracts the diaphragm and then closes the vocal folds so fast that no useful air moves, and the leading explanations treat it as a retained ancient circuit or a fetal reflex.