Mechanical characterization of squid giant axon membrane sheath and influence of the collagenous endoneurium on its properties

Annaclaudia Montanino,Eve Seuntjens,Svein Kleiven,Astrid Deryckere,Nele Famaey

doi:10.1038/s41598-019-45446-y

Annaclaudia Montanino, Eve Seuntjens + Show 3 more

Open Access

https://doi.org/10.1038/s41598-019-45446-y

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Journal: Scientific Reports	Publication Date: Jun 20, 2019
Citations: 4	License type: open-access

Affiliation: KTH Royal Institute of Technology, KU Leuven

Abstract

To understand traumas to the nervous system, the relation between mechanical load and functional impairment needs to be explained. Cellular-level computational models are being used to capture the mechanism behind mechanically-induced injuries and possibly predict these events. However, uncertainties in the material properties used in computational models undermine the validity of their predictions. For this reason, in this study the squid giant axon was used as a model to provide a description of the axonal mechanical behavior in a large strain and high strain rate regime ({boldsymbol{varepsilon }}={bf{10}}{boldsymbol{ % }},,mathop{{boldsymbol{varepsilon }}}limits^{cdot }={bf{1}}{{boldsymbol{s}}}^{-{bf{1}}}), which is relevant for injury investigations. More importantly, squid giant axon membrane sheaths were isolated and tested under dynamic uniaxial tension and relaxation. From the lumen outward, the membrane sheath presents: an axolemma, a layer of Schwann cells followed by the basement membrane and a prominent layer of loose connective tissue consisting of fibroblasts and collagen. Our results highlight the load-bearing role of this enwrapping structure and provide a constitutive description that could in turn be used in computational models. Furthermore, tests performed on collagen-depleted membrane sheaths reveal both the substantial contribution of the endoneurium to the total sheath’s response and an interesting increase in material nonlinearity when the collagen in this connective layer is digested. All in all, our results provide useful insights for modelling the axonal mechanical response and in turn will lead to a better understanding of the relationship between mechanical insult and electrophysiological outcome.

Highlights

Traumatic injury to the human central nervous system (CNS) and peripheral nervous system (PNS) arises as a result of the application of high dynamic loads to the head or the spinal cord and the limbs, respectively
With the aim of analyzing the mechanical-electrical response of a single axon, a seminal study was conducted on the squid giant axon[6,21]
To small axons in the PNS25, the squid giant axons (SGAs) is ensheated by a layer of non-myelinating Schwann cells and, always in analogy with peripheral nerve fibers, this axon is enwrapped in a collagenous layer: the endoneurium[26]

Summary

Introduction

Traumatic injury to the human central nervous system (CNS) and peripheral nervous system (PNS) arises as a result of the application of high dynamic loads to the head or the spinal cord and the limbs, respectively. In vitro models, such as organotypic and dissociated primary cultures, have been extensively used to examine the effect of mechanical perturbations on networks of unmyelinated axons (see[20] for an extensive review) Using these models, the compound response of myelinated or unmyelinated axons can be studied. The SGA is an unmyelinated axon, whose bioelectric properties were first discovered by Young et al.[22,23] This neurite’s diameter is considerably bigger than that of human or any other mammalian axons, it has proved a invaluable model to explain neurons’ electrophysiology[24]. Unrelated to axonal injury, the need of SGA’s sheath properties was made explicit in a study predicting axonal membrane displacement driven by the propagation of the action potential[33]

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