The enteric nervous system is 10 times stiffer than the brain

Alexis Peaucelle (IJPB); Nicolas R. Chevalier; Pierre Bourdoncle; Thomas Guilbert (IC UM3); Wang Xi

REVIEW 3 major objections 5 minor 17 references

The enteric nervous system is 10 times stiffer than the brain

T0 review · 3 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Mouse enteric nervous system is about ten times stiffer than brain tissue.

desk verdict First ENS stiffness map is worth having, but the '10x stiffer than brain' claim likely measures the muscle underneath, not the ganglia. read the letter →

arxiv 2506.08583 v1 pith:3BZRUGAP submitted 2025-06-10 physics.bio-ph q-bio.NC

Nicolas R. Chevalier , Alexis Peaucelle (IJPB) , Thomas Guilbert (IC UM3) , Pierre Bourdoncle , Wang Xi This is my paper

classification physics.bio-phq-bio.NC

keywords entericnervoussystemtissuebiomechanicsnano-indentationYoung'smoduluscollagenshellsecondharmonicgenerationmyentericplexusbrainstiffness

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper tries to establish that the enteric nervous system—the dense network of neurons and glia lining the gut wall—is about ten times stiffer than brain tissue, with a Young's modulus of roughly 3 kPa at shallow indentation and 6.6 kPa at deeper indentation, against a published brain range of 0.1–2 kPa. The stakes are biomechanical: the gut's nerve network is chronically squeezed, stretched, and sheared by muscle contractions and passing contents, unlike the skull-protected brain, so it must tolerate deformation without breaking. By raster-scanning a glass micro-indenter over peeled mouse duodenum, the authors measure ganglia, inter-ganglionic fibers, and surrounding smooth muscle as one mechanically homogeneous sheet, and use second harmonic generation to show that a collagen I shell wraps the nerves. If the claim is right, the 'second brain' is not a soft blob of nervous tissue but a mechanically reinforced structure matched to its moving environment.

What carries the argument

The machinery is microscope-scale indentation: a 51 µm glass sphere on a calibrated cantilever presses into the tissue while force and depth are recorded, and the Hertz contact model, corrected for finite sample thickness by a published formula, converts each force–depth curve into a local Young's modulus. Raster scanning at 5 µm steps and registering the maps against a neural-crest fluorescent marker lets the authors assign each measurement to ganglion, inter-ganglionic fiber, or muscle. Second harmonic generation then reveals the fibrillar collagen I shell around the ganglia and fibers, which is the proposed stiffening mechanism that explains the high modulus.

What would settle it

Run the identical indentation protocol (51 µm bead, 5–12 µm depth, and the same finite-thickness correction) on freshly prepared adult mouse cerebellum; if the measured modulus approaches 6 kPa instead of 0.3–0.45 kPa, the central ten-fold claim collapses.

Watch

Extended reading notes

Core claim

Using nano-indentation on adult mouse duodenum, with ganglia located by neural-crest fluorescence and moduli extracted from Hertz fits corrected for finite thickness, the authors report that enteric ganglia have a Young's modulus of about $3.0$ kPa at 3 µm indentation and $6.6$ kPa at 8 µm indentation, statistically indistinguishable from inter-ganglionic fibers and the surrounding longitudinal muscle ($6.5$ and $6.9$ kPa at depth). Glia-rich ganglion regions are not stiffer than neuron-rich regions. Published brain values from comparable indentation studies sit at 0.1–2 kPa, with the closest methodological baseline near 0.3–0.45 kPa. The paper concludes that mouse enteric ganglia are about an order of magnitude stiffer than central nervous system tissue, and attributes this to the fibrillar collagen I shell that remains tightly attached to the ganglia and fibers even after the overlying muscle is peeled away.

Load-bearing premise

The ten-fold difference assumes that brain stiffness values from the literature—measured with smaller beads, shallower indents, and sliced tissue—can be compared directly with the gut values reported here, so that the gap reflects biology rather than method.

Editorial extensions

If this is right

Enteric ganglia, inter-ganglionic fibers, and longitudinal muscle form a mechanically homogeneous sheet, so the ENS deforms with the gut wall rather than acting as a soft inclusion.
Glia-rich regions are no stiffer than neuron-rich regions, so at the length scale probed, glia are not acting as mechanical stiffeners inside enteric ganglia.
A collagen I shell remains anchored to the ganglia and inter-ganglionic fibers after muscle removal, making the shell a concrete structural candidate for the ENS's high stiffness.
An ENS an order of magnitude stiffer than brain tissue can better withstand the chronic tensile, compressive, and shear loads of gut motility, including the ~50% longitudinal strains reported for inter-ganglionic fibers during contractions.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Beyond the paper: the authors note in Methods that Hertz assumptions are only partially satisfied at the 8 µm indentation depth used for the 6.6 kPa value; that makes a same-probe brain measurement a necessary check on the headline comparison.
Beyond the paper: if the collagen shell is the main stiffener, then enzymatically removing collagen around fresh ganglia should measurably soften them; the paper proposes the shell's role but does not test it.
Beyond the paper: because ganglion and muscle stiffness match, enteric neuron mechanosensation may be tuned to the local muscle environment, so fibrotic or atrophic gut disease could shift how the ENS senses its surroundings.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

Read it for the first direct stiffness map of the mouse ENS, not for the headline. The internal data are solid and the methods are careful, but the '10 times stiffer than brain' claim is not supported by what was actually measured.

What's new: this is the first nanoindentation study of enteric ganglia, and it's done with proper attention to sample prep. They locate the ganglia with GCaMP, verify with IHC, use a 51 µm bead, and collect thousands of indentations across 16 ganglia from 6 mice. The internal comparison—ganglion vs. inter-ganglionic fibers vs. smooth muscle within the same scan—is well controlled and consistent. The finding that ENS regions have the same modulus as the surrounding muscle at both 3 µm and 8 µm indentation is a real, useful result. The SHG collagen shell around ganglia is also a nice observation.

The soft spot is the central claim. The brain comparison is cross-study, with different bead sizes, indentation depths, and slice preparations, and there is no internal brain measurement. That alone would warrant caution. But the bigger problem is the sample geometry. Myenteric ganglia are roughly 5 µm thick, average indentation was 8.2 µm, and even at 3 µm indentation the Hertz contact radius is about 8.7 µm—larger than the ganglion thickness. The stress field reaches the underlying 20 µm muscle. The finite-thickness correction treats the whole stack (muscle plus ganglion) as one homogeneous layer on glass; it cannot separate a thin soft film from a stiffer substrate. So the near-identical moduli for 'ganglion' and 'muscle' are exactly what you'd expect if both measurements are dominated by the muscle. The paper's own statement that 8 µm indentation measures the 'bulk elastic properties of the ganglia (~5 µm thickness)' is internally inconsistent. In other words, the intrinsic elastic modulus of enteric neurons and glia is never actually measured here. The '10x stiffer than brain' headline may be an artifact of comparing a composite gut-wall measurement to bare brain tissue.

What survives is an honest characterization of the gut wall: the ENS sits in a tissue that is mechanically homogeneous with the muscle and much stiffer than brain tissue. That is worth reporting, but it should be framed as a tissue-level result, not an ENS-specific one. A same-probe brain control or a careful bilayer model would fix the main issue.

Send it to peer review—it deserves real referees. They should push the authors to either measure brain with the same setup, model the layered sample, or rewrite the title and abstract to match the tissue-level claim.

Referee Report

3 major / 5 minor

Summary. The paper reports micro-indentation measurements on peeled mouse duodenum tunica muscularis, mapping Young's modulus and adhesion in enteric ganglia, inter-ganglionic fibers, and surrounding longitudinal muscle. Across 16 scans from 6 mice, the authors report ~3 kPa at 3 µm indentation depth and ~6.6 kPa at 8 µm depth, with no significant difference between the three tissue types, and they interpret this as showing that enteric ganglia are about an order of magnitude stiffer than brain tissue, based on comparison with published brain moduli. The paper also presents second harmonic generation imaging showing a collagen I shell around ganglia and inter-ganglionic fibers.

Significance. The internal comparison is carefully executed: the dataset is large (nearly 6000 indentations), the registration of indentation maps with immunohistochemistry is meticulous, and the within-scan comparison between ganglion and muscle is well controlled. The SHG evidence for a collagen shell is a valuable contribution. If the '10x stiffer than brain' claim were established, it would be an important result for gut biomechanics and neural tissue mechanics. However, the central claim currently relies on a cross-study comparison without an internal brain baseline, and the measured ganglion values are likely affected by the thin ganglion layer sitting on a thicker muscle substrate. The strengths of the paper lie in the internal mechanical mapping and the collagen imaging, not yet in the quantitative comparison to brain tissue.

major comments (3)

[Materials and Methods, Data analysis; Fig.1i,j] The finite-thickness correction (Long et al., formula in Data analysis) treats the tissue as a homogeneous elastic layer of thickness h = e + h_rel, with e ≈ 20 µm, so it cannot separate the ~5 µm ganglion layer from the underlying ~20 µm muscle. At the smallest analyzed indentation depth, δ = 3 µm, the Hertz contact radius is a = sqrt(Rδ) ≈ 8.7 µm, already exceeding the ganglion thickness, so the stress field reaches the muscle even in the low-depth analysis. The near-identical measured moduli for ganglion and muscle (3.0 vs 3.1 kPa at 3 µm; 6.6 vs 6.9 kPa at 8 µm) are exactly what would be expected if the ganglion measurement is a composite dominated by the muscle. To support the claim that the enteric ganglion itself is stiffer than brain, the authors need an experiment or model that isolates the ganglion layer, for example a smaller bead with contact radius much smaller than 5 µm, or an explicit bilayer model.
[Main text, paragraph after Fig.1] The statement that indentation depths of 8.2 ± 2.1 µm were chosen 'to measure the bulk elastic properties of the ganglia (~5 µm thickness)' is internally inconsistent: an indentation depth greater than the layer thickness cannot measure that layer's bulk modulus with a homogeneous Hertz model. The observed depth dependence of the apparent modulus (3.0 kPa at 3 µm versus 6.6 kPa at 8 µm) is consistent with increasing substrate influence, and the paper should discuss this as a substrate effect rather than as an intrinsic property of the ganglion.
[Table 1 and Conclusions] The central 'order of magnitude stiffer than brain' claim is based on comparing the measured ENS values with literature values obtained using different bead diameters (25–51 µm), indentation depths (1.5–8 µm), and sample preparations (sliced brain versus intact gut sheet), with no internal brain baseline measured using the same probe and protocol. This cross-study comparison is load-bearing for the title and abstract. The authors should either add a same-probe measurement of brain tissue or substantially soften the claim to reflect that the comparison is indirect and subject to uncontrolled methodological differences.

minor comments (5)

[Fig.1 caption] The caption reports significance asterisks (*p<0.05, ****p<0.0001), but the text states that there was no significant difference in elastic modulus between tissue types; please clarify which panels and which comparisons the asterisks refer to.
[Abstract and Significance Statement] The phrase 'an order of magnitude more resistant to deformation than brain tissue' is stronger than the data support because the comparison lacks an internal control; consider phrasing it as 'about an order of magnitude stiffer than published brain tissue values measured under different conditions.'
[Materials and Methods, Indentation] The paper acknowledges that Hertz assumptions were only partially met for the 8 µm indentations (δ/R = 0.32); please quantify or discuss the potential error introduced by using the Hertz model at this depth, since the headline modulus value of ~6.6 kPa is extracted from these curves.
[Data analysis] The statement that 'post-hoc recovery of the proper range of elastic modulus values for smooth muscle further justifies this approach' is somewhat circular, because the literature range for smooth muscle modulus is broad (3–15 kPa) and the measured value would fall within it for several different sample preparations.
[Data availability statement] Raw data are described as available upon request; depositing the full indentation dataset in a public repository would strengthen reproducibility and is increasingly expected for quantitative biomechanics papers.

Circularity Check

0 steps flagged · score 0.0 of 10

No circular reasoning: the ENS stiffness claim is an empirical measurement compared against independent external brain-tissue benchmarks.

full rationale

The central claim that enteric ganglia are about an order of magnitude stiffer than brain tissue is an empirical comparison, not a derivation from the claim itself. The ENS Young's moduli are extracted from measured force-indentation curves using the Hertz model, and the brain values are taken from published indentation studies by other groups (Table 1, e.g., Elkin, Christ, Menal). No parameter of the brain comparison is fitted from the ENS data, and no equation in the paper is defined in terms of the conclusion it supports. The finite-thickness correction is taken from an external analytical study (Long et al., ref. 8), not from the authors' prior work. The self-citations present (refs. 1, 16, 17) concern gut motility, neural-crest migration, and imaging methods; they do not supply the load-bearing stiffness comparison. The skeptical concern that the measured modulus may be influenced by the underlying muscle because indentations exceed the thin ganglion thickness is a measurement-validity or interpretation risk, not circularity: it questions whether the quantity measured is truly the intrinsic ENS stiffness, but the reported measurement remains independent of the conclusion. There is no fitted input renamed as a prediction, no uniqueness theorem imported from the authors, and no known result merely relabeled. The paper is self-contained against external benchmarks for its central quantitative claim, so the circularity score is 0.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

The central claim rests on standard contact mechanics and a published finite-thickness correction, plus several domain assumptions about sample prep and cross-study comparability. No free parameters are fitted to force the result.

assumptions (5)

standard math Hertz contact theory accurately models the indentation of the tissue by a spherical probe
Used to extract Young's modulus from force-displacement curves; assumptions only partially met at high indentation depth (Methods).
domain assumption The finite-thickness correction formula of Long et al. (2011) applies to this layered tissue on a rigid substrate
The sample is a ~20 µm muscle layer with ~5 µm ganglia on glass, and the correction is critical for absolute values.
domain assumption The peeled tunica muscularis preparation and adhesion to glass do not significantly alter the mechanical properties of the ENS
Protocol includes drying and rehydration steps; authors argue post-hoc that muscle modulus matches literature.
domain assumption Published brain tissue moduli from other studies are directly comparable to the ENS measurements
Central '10 times stiffer' claim depends on this cross-study comparison (Table 1).
domain assumption The tissue can be treated as a homogeneous elastic layer for the finite-thickness correction
The correction ignores the presence of the collagen shell and the composite nature of ganglia on muscle.

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Cite this review

Pith. "Pith review of The enteric nervous system is 10 times stiffer than the brain." pith.science (2026). https://pith.science/paper/3BZRUGAP

@misc{pith2026250608583,
  author       = {Pith},
  title        = {Pith review of: The enteric nervous system is 10 times stiffer than the brain},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/3BZRUGAP}},
  note         = {Machine review of arXiv:2506.08583}
}

abstract

Neural tissues of the central nervous system are among the softest and most fragile in the human body, protected from mechanical perturbation by the skull and the spine. In contrast, the enteric nervous system is embedded in a compliant, contractile tissue and subject to chronic, high-magnitude mechanical stress. Do neurons and glia of the enteric nervous system display specific mechanical properties to withstand these forces? Using nano-indentation combined with immunohistochemistry and second harmonic generation imaging of collagen, we discovered that enteric ganglia in adult mice are an order of magnitude more resistant to deformation than brain tissue. We found that glia-rich regions in ganglia have a similar stiffness to neuron-rich regions and to the surrounding smooth muscle, of ~3 kPa at 3 $\mu$m indentation depth and of ~7 kPa at 8 $\mu$m depth. Differences in the adhesion strength of the different tissue layers to the glass indenter were scarce. The collagen shell surrounding ganglia and inter-ganglionic fibers may play a key role in strengthening the enteric nervous system to resist the manifold mechanical challenges it faces.

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Reference graph

Works this paper leans on

17 extracted references · 17 canonical work pages

[1]

R., Agbesi, R

Chevalier, N. R., Agbesi, R. J. A., Ammouche, Y. & Dufour, S. How Smooth Muscle Contractions Shape the Developing Enteric Nervous System. Front. Cell Dev. Biol. 9, 1402 (2021)

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Lu, Y. B. et al. Viscoelastic properties of individual glial cells and neurons in the CNS. Proc. Natl. Acad. Sci. U. S. A. 103, 17759–17764 (2006)

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Christ, A. F. et al. Mechanical difference between white and gray matter in the rat cerebellum measured by scanning force microscopy. J. Biomech. 43, 2986–2992 (2010)

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[9]

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

[1] [1]

R., Agbesi, R

Chevalier, N. R., Agbesi, R. J. A., Ammouche, Y. & Dufour, S. How Smooth Muscle Contractions Shape the Developing Enteric Nervous System. Front. Cell Dev. Biol. 9, 1402 (2021)

work page 2021

[2] [2]

J., Dinning, P

Spencer, N. J., Dinning, P. G., Brookes, S. J. & Costa, M. Insights into the mechanisms underlying colonic motor patterns. J. Physiol. 594, 4099–4116 (2016)

work page 2016

[3] [3]

Lu, Y. B. et al. Viscoelastic properties of individual glial cells and neurons in the CNS. Proc. Natl. Acad. Sci. U. S. A. 103, 17759–17764 (2006)

work page 2006

[4] [4]

S., Azeloglu, E

Elkin, B. S., Azeloglu, E. U., Costa, K. D. & Morrison, B. Mechanical heterogeneity of the rat hippocampus measured by atomic force microscope indentation. J. Neurotrauma 24, 812–822 (2007)

work page 2007

[5] [5]

Christ, A. F. et al. Mechanical difference between white and gray matter in the rat cerebellum measured by scanning force microscopy. J. Biomech. 43, 2986–2992 (2010)

work page 2010

[6] [6]

& Nagayama, K

Matsumoto, T. & Nagayama, K. Tensile properties of vascular smooth muscle cells: Bridging vascular and cellular biomechanics. J. Biomech. 45, 745–755 (2012)

work page 2012

[7] [7]

Babu, P. K. V. & Radmacher, M. Mechanics of brain tissues studied by atomic force microscopy: A perspective. Front. Neurosci. 13, 1–9 (2019)

work page 2019

[8] [8]

S., Wu, M

Long, R., Hall, M. S., Wu, M. & Hui, C. Y. Effects of Gel Thickness on Microscopic Indentation Measurements of Gel Modulus. Biophys. J. 101, 643 (2011)

work page 2011