Strain partitioning between nerves and axons: Estimating axonal strain using sodium channel staining in intact peripheral nerves

[Display omitted] •Partitioning of axial strain from whole nerve to axonal fibres is not known.•Sodium channel extracellular subunits can be stained without tissue fixation/permeabilisation.•Sodium channel clustering at nodes of Ranvier can me imaged during nerve straining.•Axonal strains as a function of applied tissue strain can be estimated by digital image correlation. Peripheral nerves carry afferent and efferent signals between the central nervous system and the periphery of the body. When nerves are strained above physiological levels, conduction blocks occur, resulting in debilitating loss of motor and sensory function. Understanding the effects of strain on nerve function requires knowledge of the multi-scale mechanical behaviour of the tissue, and how this is transferred to the cellular environment. The aim of this work was to establish a technique to measure the partitioning of strain between tissue and axons in axially loaded peripheral nerves. This was achieved by staining extracellular domains of sodium channels clustered at nodes of Ranvier, without altering tissue mechanical properties by fixation or permeabilisation. Stained nerves were imaged by multi-photon microscopy during in situ tensile straining, and digital image correlation was used to measure axonal strain with increasing tissue strain. Strain was partitioned between tissue and axon scales by an average factor of 0.55. This technique allows non-invasive probing of cell-level strain within the physiological tissue environment. This technique can help understand the mechanisms behind the onset of conduction blocks in injured peripheral nerves, as well as to evaluate changes in multi-scale mechanical properties in diseased nerves.