How the Vagus Nerve Regulates Digestion
The vagus nerve’s right
branch plays a key role
in digestive signaling
by Jerry Bergman, PhD
Having taught anatomy for several decades, and attempting to keep up with the literature in this area due to my strong interest in the subject, one thing has become increasingly clear: every new discovery adds to the complexity of the vertebrate body.
I do not remember a single advancement that has shown any nerve or system to be simpler than previously believed. Instead, the organ or system is shown to be more complex. This is also true of the new insights into the vagus nerve reviewed below. The vagus nerve is proving to be even more complex than previously believed.
The Right Vagus Nerve. From Wikimedia Commons.
What the Vagus Nerve Does
The importance of the vagus nerve research was described by University of Texas professor Stephen Fontenot:
“After years of work, cognition and neuroscience doctoral student Hailey Welch is—for the first time—the lead author of a study published in an academic journal, a paper appearing in Cell Reports, which examined the role of the vagus nerve’s branches in digestive signaling.”[1]
The two vagus nerves—the right and left vagus nerves, collectively the tenth cranial nerves—are critically important carriers of sensory information from the viscera to the brain for analysis and anatomical response.[2]
The vagus nerve originates in the medulla oblongata of the brain stem and extends downward through the neck into the chest and abdomen. Along its course, it branches out to multiple organs, including the heart, lungs, liver, spleen, stomach, intestines, and kidneys. It carries sensory information from the cardiovascular, respiratory, and gastrointestinal (GI) systems to the central nervous system for processing.
Through these pathways, the vagus nerve helps to regulate autonomic functions including the heart rate, respiration, and digestion.[3]
The research by Welch et al. carried out at the University of Texas, charted the molecular and functional profiles of sensory neurons in the vagus nerve using three approaches: genetic sequencing, imaging, and functional assays.[4] The right vagus nerve includes sensory neurons responding to sensing signals from the digestive system.
The fact that almost twice as many of these neurons are on the right side as compared to the left, supports the conclusion that the right vagus nerve is more involved in sensing nutritional information that reinforces dietary behaviors. Specifically, Welch et al‘s findings explain why right-sided vagus nerve signaling activates the brain’s release of the “feel-good” hormone dopamine, rewarding the nutritional intake.
The Finding Created Problems for Evolution.
The Welch et al. article in Cell Reports never mentions evolution, but it clearly shows that the vagus nerve is critical for life in all vertebrates, including mammals, birds, reptiles, and amphibians. In some lower fish and mollusks, a functionally analogous structure composed of multiple nerves serves part of this role. None of the other references I consulted about the Welch et al. report mentioned evolution either.
The typical evolutionary explanation for the origin of the vagus nerve is that it evolved from its origins in the primitive brain stem to, after millions of years, fulfill its complex role in the modern human.[5] How this requirement was satisfied until the system evolved was not explained. Most references to the evolution of the vagus nerve focus on its development in the embryo, noting that it begins forming early in embryonic life and continues to develop after birth. All of the articles I found attempt to hypothesize its evolution based purely on assumptions, ignoring how it fulfilled its complex role before it evolved.[6]
The only article I was able to assess that went beyond this was on Darwin’s theory of the evolution of the vagus nerve:
Charles Darwin proposed that via the vagus nerve, the tenth cranial nerve, emotional facial expressions are evolved, adaptive, and serve a crucial communicative function. In line with this idea, the later-developed polyvagal theory assumes that the vagus nerve is the key phylogenetic substrate that regulates emotional and social behavior. The polyvagal theory assumes that optimal social interaction, which includes the recognition of emotion in faces, is modulated by the vagus nerve. So far, in humans, it has not yet been demonstrated that the vagus plays a causal role in emotion recognition.[7]
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From my literature review, I learned that no invertebrate possesses a vagus nerve, whereas all vertebrates do. I was unable to locate any study that empirically attempted to bridge this chasm. The vagus nerve is a defining feature of vertebrates, and no invertebrate has an anatomically and functionally homologous structure to the vagus nerve. Invertebrates generally have a simpler, or at least very differently designed, nervous systems. One example is nerve nets or ventral nerve cords with segmental ganglia. A few invertebrates, such as octopuses and squids, have very complex nervous systems, but these are organized very differently from those of vertebrates and are not homologous to the vagus nerve.
Conclusions
The Welch et al. Cell Reports article documented that the right vagus nerve is more complex than previously thought, widening the gap between vertebrate and invertebrate nervous system design. Although the study highlighted this increased complexity, it made no attempt to explain the finding through biological evolution. This is perhaps unsurprising given that, although evolutionists generally consider the vagus nerve to have evolved, I was unable to find any articles proposing a plausible mechanism for its evolution.
References
[1] Fontenot, Stephen, “Vagus nerve’s right branch plays a key role in digestive signaling,” Medical Express, 2025.
[2] Keer, Lisa, “The vagus nerve: A key player in your health and well-being,” https://www.massgeneral.org/news/article/vagus-nerve, 2024.
[3] Welch, Hailey F., et al., “Molecular and functional asymmetry in Cckar-expressing vagal sensory neurons,” Cell Reports 44(11):116507;
https://www.cell.com/cell-reports/pdf/S2211-1247(25)01278-1.pdf, 25 November 2025.
[4] Welch, et al., 2025.
[5] Zabara, Jacob, “Origin and Evolution of Vagal Nerve Stimulation: Implications for Understanding Brain Electrodynamics, Neuroendocrine Function, and Clinical Applications,” In: Bioelectromagnetic Medicine. CRC Press, Boca Raton, FL, 2004.
[6] Colzato, L., et al., “Darwin revisited: The vagus nerve is a causal element in controlling recognition of other’s emotions,” Cortex 92:95-102, July 2017.
[7] Colzato, et al., 2017.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,900 publications in 14 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,800 college libraries in 27 countries. So far over 80,000 copies of the 60 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.