What is the mechanism responsible for the periodicity of hiccups?

Question

Hiccups ([*singultus*]) are caused by involuntary spasm of the intercostal muscles and diaphragm, producing a sudden inspiration with associated glottic closure. It seems the neurobiology of the reflex has been relatively well worked out (nicely summarized in [this review]): - Afferent limb: the vagus, phrenic, and cervical sympathetic chain - Central mediators: the hypothalamus, medullary reticular formation, and brainstem respiratory centers - Efferent limb: primarily mediated by the phrenic nerve but also including the intercostals, and secondarily the recurrent laryngeal nerve to cause glottic closure To my knowledge, hiccups are always cyclical, with a period of between 1 and 30 seconds. According to the afore cited article, this interval is relatively constant it a given individual. The cycling stops after some time; [efferent vagal tone can suppress it][vt], or it stops spontaneously. My question is: - What is the mechanism responsible for this periodicity?*Something* must be triggering that afferent limb every 1 - 30s, but I can’t think of a normal physiologic process that triggers the firing of one or more nerves with such periodicity and then stops abruptly. [this review]: http://www.ncbi.nlm.nih.gov/pubmed/?term=22891647[vt]: http://www.ncbi.nlm.nih.gov/pubmed/?term=25055206[*singultus*]: http://www.memidex.com/singultus

Answer 1

Nerves have an Absolute Refactory Period, which vary from person to person. I mention this because hiccups usually have a cause, even if it is unknown to an individual. It is usually very difficult to find the cause, if not impossible. Causes can range from a hair in your ear, to an irritated throat, to a local electrolyte imbalance, which all cause the nerve to fire. Depending on the location (distance to the Phrenic nerve) of the trigger, and the time of an individual's Absolute Refractory Period, it will give you how often you get a hiccup. Each individual bout of hiccup will be regular, not every bout of hiccup.

Why this happens contentiously for a period of time and not just once is believed to be due to the hiccup being a reflex. It is theorized that since hiccups only occur in milk drinking mammals, that it is a reflex that would allow suckling babies to drink and breath at the same time. It would also allow them to "burp" them selves as well. If it is a reflex, which is cyclic, that would mean that there is a reentry point for the signal, leading to a repetitive reflex.

Why the Hiccup would stop would be the same principal as a TENS machine. As the hiccup is a cyclic nervous signal, the second signal must enter the nerve after Absolute Refractory Period to be propagated. If another signal happens to occur right before the hiccup signal, the hiccup signal would then occur during the Absolute Refractory Period, thus ending the propagation. This is why drinking cold water, getting scared, and holding ones breath (all actions that stimulate the vagus nerve) can stop hiccups.