Measurement force, speed and post-mortem time affect the ratio of CNS grey to white matter elasticity

For several decades, many attempts have been made to characterise the mechanical properties of grey and white matter, which constitute the two main compartments of the central nervous system (CNS), with various methods and contradictory results. In particular, the ratio of grey-to-white-matter elasticity is sometimes larger than 1 and sometimes smaller; the reason for this apparent discrepancy is currently unknown. Here, we exploited atomic force microscopy (AFM)-based indentation measurements to systematically investigate how the measurement force, measurement speed, post-mortem interval and temperature affect the measured elasticity of spinal cord tissue, and in particular the ratio of grey-to-white-matter elasticity (Kg/Kw). Within the explored parameter space, increasing measurement force and speed increased the measured elasticity of both grey and white matter. However, Kg/Kw declined from values as high as [~]5 at low forces and speeds to [~]1 for high forces and speeds. Kg/Kw also strongly depended on the anatomical plane in which the measurements were conducted and was considerably higher in transverse sections compared to longitudinal sections. Furthermore, the post-mortem interval impacted both the absolute measured tissue elasticity and Kg/Kw. Grey matter elasticity started decreasing [~]3 hours post-mortem until reaching a plateau after [~]6 hours. In contrast, white matter elasticity started declining from the beginning of the measurements until [~]6 hours post-mortem, when it also levelled off. As a result, Kg/Kw increased until [~]6 hours post-mortem before stabilising. Between 20{degrees}C and 38{degrees}C, both grey and white matter elasticity decreased at a similar rate, without affecting Kg/Kw. We have thus identified differences in the response of grey and white matter to varying strains and strain rates, and the post-mortem interval, and excluded temperature as a factor affecting Kg/Kw. These differential responses likely contribute to the contradictory results obtained with different methods working in different strain regimes. Statement of significanceWe here showed that the mechanical response of CNS grey and white matter to an applied force differentially depends on measurement parameters such as the speed and magnitude of the applied forces, the post-mortem interval, as well as the anatomical axis along which measurements are conducted. These results broaden our understanding of CNS mechanics and pave the way for better and more targeted experimental design of future experiments. Ultimately, they may help to reconcile seemingly contradictory results in the literature concerning the ratio of grey-to-white-matter elasticity.