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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, 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.

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Check out Van Der Pol (chaotic) oscillator. It can be used to model heart rhythms. If a stimulus is applied to the oscillator, it will return to the curved envelope discussed in the article.

However, if "pushed just right", the oscillator's current state will be pushed towards the very center of the loop, and it will stop oscillating. This is the mechanism by which a sudden blow to the chest can stop the heart.

Reading up on hiccups, their "almost regular" interval sounds similar to a chaotic oscillator. The abrupt stop may represent pushing the oscillator towards the middle of the limit cycle, from which it cannot recover, effectively stopping hiccups

enter image description here

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