Gut’s Second Brain: When Sensory Neurons Misfire, Digestive Disease Follows

Deep within the walls of the human intestine operates a neural network so sophisticated that scientists have long called it the second brain. The enteric nervous system contains hundreds of millions of neurons that can coordinate digestion largely without instructions from the brain or spinal cord. Now, a comprehensive review published in Frontiers in Cellular Neuroscience argues that a specific class of these neurons, known as intrinsic primary afferent neurons, or IPANs, may hold the key to understanding a range of stubborn gastrointestinal disorders, from irritable bowel syndrome to inflammatory bowel disease, chronic constipation, and visceral pain. The review, led by Linda Chia-Hui Yu and Wei-Ting Kuo of National Taiwan University, synthesizes evidence that when these sensory neurons miscode signals, the gut’s entire regulatory machinery can go awry.

IPANs function as the gut’s local sentinels. Morphologically, they correspond to what anatomists call Dogiel type II neurons, cells with smooth, multipolar bodies and long processes that wrap circumferentially around the intestinal wall. Electrophysiologically, they are characterized as AH-type neurons, distinguished by a prominent slow afterhyperpolarization, a prolonged electrical reset that limits repetitive firing and gives these cells their distinctive excitability profile. Unlike extrinsic sensory neurons housed in the dorsal root and nodose ganglia, which relay gut information to the central nervous system, IPANs detect mechanical deformation, chemical cues, nutrients, microbial metabolites, and immune-derived signals, then pass this information to interneurons, motor neurons, and secretomotor pathways entirely within the enteric network. This architecture allows the intestine to mount rapid, localized reflexes, peristalsis, secretion, and vasodilation, without waiting for central approval.

The review emphasizes that IPAN excitability is remarkably plastic. Inflammation, serotonergic signaling, cytokines, neurotrophins, and microbial products can all alter firing thresholds, modify the slow afterhyperpolarization, and reshape synaptic transmission. In experimental models, repetitive stimulation or exposure to inflammatory mediators produces lasting sensitization of these neurons, a form of neural memory that may persist long after the initial insult has resolved. The authors propose that such maladaptive plasticity could shift IPANs from adaptive sensory encoders into persistently hyperexcitable states, lowering reflex thresholds and amplifying enteric responses in ways that promote urgency, diarrhea, and visceral hypersensitivity. Conversely, suppressed sensory gain could impair the initiation of peristaltic and secretory reflexes, contributing to stasis, constipation, and defective mucosal clearance.

One of the most striking recent advances comes from molecular profiling. Single-cell and single-nucleus transcriptomic analyses have identified neuromedin U, or NMU, as a conserved marker of putative myenteric IPANs in both mice and humans, though its expression is strongly region dependent, marking roughly 50 to 75 percent of putative IPANs in the human small intestine but only 5 to 10 percent in the colon. In the mouse colon, researchers identified cadherins Cdh6 and Cdh8 as selective markers of Dogiel type II/AH sensory neurons, enabling cell-type-specific manipulation for the first time. Optogenetic activation of Cdh6-positive IPANs was shown to evoke retrograde colonic motor complexes, while pharmacological blockade of their hyperpolarization-activated current disrupted spontaneous generation of these motor patterns. This established that a molecularly defined IPAN population is both sufficient and necessary for a specific motor program under physiological conditions, a milestone in enteric neurobiology.

Perhaps the most conceptually transformative finding involves neuroimmunity. IPANs express a broad repertoire of cytokine, chemokine, and pattern-recognition receptors, including receptors for the type 2 cytokines IL-4 and IL-13, Toll-like receptors, and receptors for TNF, IL-1β, IL-6, and IL-17. Work by Wang and colleagues, using genetic and viral approaches combined with single-nucleus RNA sequencing, identified a population of IL-13 receptor-expressing IPANs as a functional node in gut neuroimmune signaling during helminth infection. In this circuit, group 2 innate lymphoid cells release IL-13, which activates IPANs through the IL-13 receptor, inducing the release of beta-calcitonin gene-related peptide. That neuropeptide then suppresses ILC2 activity, establishing a reciprocal feedback loop. When the researchers deleted the neuronal IL-13 receptor alpha 1 subunit, host defense against the parasite Heligmosomoides polygyrus was impaired, and combined administration of NMU and beta-CGRP restored parasite clearance, supporting a causal role for this neuronal pathway in protective immunity.

The microbiome adds another layer of complexity. Studies in germ-free rodents revealed that jejunal myenteric AH/Dogiel type II neurons exhibit impaired excitability, with elevated firing thresholds, reduced input resistance, and prolonged calcium-dependent slow afterhyperpolarization. Remarkably, conventionalizing adult germ-free animals with a normal microbiota normalized these electrophysiological properties. Subsequent work showed that pharmacological excitation of IPANs increased mesenteric nerve discharge by 21 percent in specific pathogen-free mice and 41 percent in conventionalized mice, but by only 2 percent in germ-free animals, suggesting that microbial input is required for normal signal transmission from IPANs to extrinsic afferent pathways. Pattern-recognition receptors such as TLR2 and TLR4 on enteric neurons and glia mediate some of these effects, and commensal-derived N-formyl peptides can directly activate IPANs through FPR1-dependent reactive oxygen species generation.

Serotonergic signaling provides some of the strongest pharmacological evidence for IPAN involvement in gut function, though the picture is far from simple. Enterochromaffin cell-derived serotonin activates 5-HT1P receptors on primary afferent neurons to initiate secretory reflexes, while 5-HT3 receptors on interneurons and secretomotor neurons participate in peristaltic signaling. Clinically, 5-HT4 receptor agonists such as prucalopride accelerate gastrointestinal transit, whereas 5-HT3 antagonists like alosetron delay transit and increase colonic compliance. Yet the review notes that mucosal removal can abolish cyclical serotonin release without preventing peristalsis, and Tph1 deletion does not necessarily impair transit, challenging a strictly mucosal serotonin-dependent reflex model and pointing instead to direct enteric mechanotransduction as a critical mechanism.

Despite this wealth of convergent evidence, the authors are candid about the field’s central limitation: no study has yet completed the canonical experimental sequence of selectively disrupting IPANs, inducing a gastrointestinal phenotype, and then reversing it through targeted rescue, particularly in human tissue. Pharmacological approaches often lack anatomical specificity because their targets are expressed by multiple cell types. Transcriptomic atlases predominantly define neuronal identity under healthy conditions rather than in disease. Germ-free models establish that microbial input is required for normal IPAN function but do not prove that dysbiosis directly corrupts afferent coding. Whether maladaptive IPAN sensory coding is a primary driver of disease, an amplifier of existing pathology, or merely a consequence of broader enteric dysfunction remains unresolved.

The therapeutic implications, however, are tantalizing. The antibiotic rifaximin, which modulates the gut microbiota, has been shown to reduce visceral hypersensitivity in both patients with diarrhea-predominant IBS and murine models, implicating microbial regulation of afferent signaling in human disease. If future studies can establish IPAN-specific causal links using conditional genetics, gnotobiotic reconstitution with defined microbial consortia, and validation in human enteric preparations, the review suggests that therapeutic recalibration of sensory gain and neuroimmune signaling could offer a rational strategy for restoring gut homeostasis in chronic inflammatory and functional gastrointestinal disorders. Such an approach would represent a fundamental shift from treating symptoms to correcting the underlying neural code that governs how the gut perceives and responds to its internal world.

For the millions of people worldwide living with IBS, IBD, chronic constipation, and unexplained visceral pain, the message from this synthesis is one of cautious optimism. The enteric nervous system is not merely a passive conduit for central commands but an autonomous integrative hub whose sensory neurons sit at the intersection of mechanical, chemical, microbial, and immunological signaling. Understanding how IPANs encode these signals in health, and how that encoding goes wrong in disease, may finally provide the mechanistic foundation needed to develop targeted therapies that restore, rather than merely suppress, the gut’s intrinsic intelligence.

Subject of Research: The role of maladaptive sensory coding by enteric nervous system neurons in gastrointestinal disorders

Article Title: Maladaptive sensory coding in the enteric nervous system playing a role in gastrointestinal disorders

Article References: Yu, L. C.-H., Huang, C.-Y., Lin, C.-Y., Tsai, Y.-S., Chang, Y.-C., Huang, C.-W., Chao, Z.-L., & Kuo, W.-T. (2026). Maladaptive sensory coding in the enteric nervous system playing a role in gastrointestinal disorders. Frontiers in Cellular Neuroscience, 20, Article 1903390. https://doi.org/10.3389/fncel.2026.1903390

Image Credits: AI Generated

DOI: 10.3389/fncel.2026.1903390

Keywords: enteric nervous system, intrinsic primary afferent neurons, irritable bowel syndrome, inflammatory bowel disease, visceral hypersensitivity, neuroimmune communication, gut microbiota, serotonin, IPANs, gastrointestinal motility, neural plasticity, Dogiel type II neurons