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.