What are the mechanisms of action of auriculotherapy, auricular acupuncture or auricular neuromodulation?

The mechanism of auriculotherapy is based on the triad allodynia- sensitization - somatotopy

Auriculotherapy is based on three clinical observations that anyone can reproduce.

Allodynia

First, a bodily ("somatic") pain is associated with hypersensitivity in a defined area ("localized") of the auricular pavilion or concha. This hypersensitivity is called "allodynia" in medical terms. This localized area on the ear is referred to as a "point" in auriculotherapy and auricular acupuncture, or a "zone" in auricular neuromodulation. This allodynia is quite easy to reproduce. One simply needs to induce a peripheral nociceptive message (for example, by applying a clamp to the pulp of the thumb, but it works on other parts of the body as well!) and then search for a hypersensitive area on the auricle by applying pressure (this is ideally done with a calibrated pressure probe, and even better if you are familiar with the representation of the thumb on the auricle).

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Awareness raising

Second, stimulating the sensitized area generates a therapeutic effect. This observation is evident in the case of acute pain and is not specific to the ear. Healers ('rebouteux' in French) were already familiar with this practice and, under the pretext of 'putting a nerve back in place', would apply painful stimulation to a sensitized area. Today's osteopaths no longer 'put nerves back in place' but they use techniques known as "cutaneous" or "neurocutaneous," the most well-known being the "palpé-rouler" technique. On the ear, it's the same!

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Somatotopia

Third, the distribution of these hypersensitive areas follows a particular topographic organization, like a map, hence the term "auricular maps," many versions of which can be found on the internet. This topographic organization is called "somatotopy." It already exists in many brain structures: the oldest described is the somatotopic organization of the motor cortex on the ascending frontal area, known as the Penfield homunculus, named after its discoverer. Many other topographic organizations exist in other brain structures, such as the thalamus, the insula... and the ambiguous nucleus and the dorsal motor nucleus of the vagus, which are the two motor nuclei of the vagus nerve.
Everyone has in mind the image of the "inverted fetus," but this representation is misleading and leads to false reasoning. The reality is a bit more complex.

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Ear stimulation leads to a parasympathetic response

Stimulation of certain areas of the ear generates a parasympathetic response. Among these areas, the most studied has been the upper concha, which is 100% innervated by the vagus nerve (Peuker et al., 2002; Butt et al., 2019). More surprisingly, though observed only in animals, auricular stimulation also induces a sympathetic response (Strack et al., 1990).
Thanks to functional MRI, we know that stimulating different areas of the ear generates different responses (Yakunina et al., 2017). Activation or inhibition of certain brain areas (nuclei or areas of interest) is observed depending on the area stimulated on the auricular pavilion. Thus, stimulation of the auricular pavilion induces modulation of certain neural networks, hence the term "auricular neuromodulation," which differs depending on the area stimulated on the ear. Unfortunately, very few areas have been studied with functional MRI, while several hundred points have been described on the auricular pavilion. Additionally, the stimulation paradigm imposed by functional MRI does not exactly match common practice.
A notable point is that all information first passes through two relays: the nucleus of the solitary tract and the locus coeruleus. The nucleus of the solitary tract actively communicates with its motor counterparts, the ambiguous nucleus and the dorsal motor nucleus of the vagus, which we discussed in the previous paragraph.

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