Hiccups are a common bodily reflex experienced universally. These involuntary spasms of the diaphragm, the primary muscle involved in breathing, usually manifest as a brief, repetitive series of “hic” sounds. While sometimes annoying, hiccups are generally harmless and resolve on their own.
The Mechanics of a Hiccup
A hiccup originates from an involuntary contraction of the diaphragm, the dome-shaped muscle separating the chest and abdominal cavities. This sudden spasm causes an abrupt intake of air. Immediately following this rapid inhalation, the glottis, the opening between the vocal cords in the larynx, quickly closes. This abrupt closure blocks the incoming air, producing the characteristic “hic” sound. The entire process is part of a reflex arc involving the phrenic and vagus nerves, which transmit signals between the brain and the diaphragm and glottis.
The Evolutionary Link to Our Ancestors
One scientific theory suggests hiccups are a vestigial reflex from ancient aquatic or amphibious ancestors. This hypothesis proposes that the neural circuitry responsible for hiccups in humans, particularly involving the phrenic and vagus nerves, closely mirrors the brainstem circuits used by early vertebrates for gill-based respiration. For instance, tadpoles, during their transition from water to land, exhibit a breathing pattern that involves closing their glottis to prevent water from entering their developing lungs while simultaneously pumping water over their gills. This action bears a resemblance to the hiccup mechanism.
The convoluted path of the phrenic nerve in humans, traveling from the neck through the chest to the diaphragm, is a relic of this evolutionary history. In fish, this nerve’s origin is closer to the gills in the neck, which made sense for controlling gill movements. In mammals, the nerve’s lengthy route makes it susceptible to irritation, which can trigger the hiccup reflex. This reflex is a “leftover” motor pattern from our distant past, no longer serving a primary respiratory function but persisting in our biology.
Common Triggers of Hiccups
While the evolutionary theory explains the deep origins of the hiccup reflex, everyday factors can activate this ancient pathway in modern humans. Common triggers include eating too quickly, consuming carbonated beverages, and overeating, which can distend the stomach and irritate the diaphragm. Sudden excitement, stress, or even abrupt changes in temperature can also induce hiccups. These factors are believed to irritate the nerves involved in the hiccup reflex, prompting the diaphragm’s involuntary contractions and the subsequent glottis closure.
Hiccups in Other Species
Hiccups in other species further support their ancient evolutionary origins. Fetal hiccups are commonly observed in human babies and other mammals while still in the womb. These occur before the respiratory system is fully developed for air breathing, suggesting the reflex is not primarily linked to postnatal respiration. Many mammals, including dogs, cats, and horses, also experience hiccups. This widespread occurrence across species and developmental stages indicates a shared evolutionary heritage, suggesting hiccups are a deep-rooted physiological phenomenon rather than a unique human quirk.