Why do we hiccup?
In my third post in a series of weird human phenomenon’s we know little about, I want to explore the hiccups. It’s something we’ve all experienced, and it appears to occur at the most random of moments, but what’s the actual cause?
Unlike sneezing and coughing, there doesn’t appear to be any physiological significance of hiccupping, causing many to question why it has survived evolution. Why haven’t we, the amazing, physiological beings we are, weeded out this, frankly very annoying, occurrence?
First, let’s explore what a hiccup is, because I think if I was asked to explain it only a week ago, I’d have no clue. The sound we associate with a hiccup is caused by the forced and excessive contraction of the respiratory muscles, followed by the closure of the glottis 35 milliseconds later. The name hiccup actually came from the sound of the glottis closing, although I think it might be a little farfetched! Inhalation against the closed glottis is then forced, causing the pressure in our thorax to decrease sharply, the lower oesophageal sphincters to relax and exhalation is suppressed. The organs located in the thorax, including the oesophagus, heart and lungs also experience the drop-in pressure.
Normal breathing is disrupted by the hiccups. Air usually inhaled or exhaled cannot reach enter or exit due to the closed glottis. Image credit: Cruithne9 via Wikimedia Commons
So that’s what physically happens, but what causes it? If we look specifically at cases of pathological hiccuping caused by medical conditions or lesions, it is stimuli located in the stomach, oesophagus and diaphragm that trigger the mechanism. Afferent signals travel along the phrenic, vagus and sympathetic nerve branches and reach the medulla oblongata; the region of the brain responsible for muscle contraction. Signals from the integrating centre at the medulla travel along efferent pathways to the scalene, external intercostal, diaphragm, glottis and oesophageal muscles. Changes in the contraction of these muscles leads to an episode of hiccuping that may last for some time or only be transient.
Okay, so we know the pathway. We know the physical function of hiccuping. But what causes the initial innervation of hiccuping if it’s not caused by a medical condition or lesion. There are several theories that have been explored throughout history.
Early suggestions such as those made by Ferroni in 1899 consider the hiccup to be a strengthening exercise for the foetal respiratory muscles. This may hold some truth as hiccupping is observed in developing foetus’. Others suggest that hiccupping is important in infants when they are still in the phase of drinking milk as it allows for air to be removed from the stomach without milk entering the lungs.
Somewhat conversely, Straus et al. believe that hiccupping is a vestigial remnant of our transition from water to land beings. Lungs became developed in a series of early fish species that had to take advantage of the air above the water, whilst still retaining their gills. By moving the air over their gills and then forcing their glottis closed, water was prevented from entering the lungs. It’s believed hiccupping is a relic of the transition from gill to lung breathing as these fish species moved onto the land, similar to that experienced by tadpoles as they transform into frogs.
The transition from water to land is one theory that may explain hiccuping. Image credit: Bartu Lenka via Pikabay
These days, hiccuping often occurs after swallowing too much air, eating or drinking too quickly or as a response to a series of emotions, such as anxiety. These actions and feelings all cause an increase in the amount of oxygen in the stomach or the size of the stomach itself, stimulating the neural pathway of the hiccups.
So whilst the evolutionary importance of the hiccups is still debated in the scientific world, it seems that it did play a vital role in our existence today. And despite how far science has come, there still appears to be no distinct cure. So keep trying to swallow a glass of water whilst upside-down, being unexpectedly scared or holding your breath, however it’s unlikely that these methods will work every time.