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

Abstract

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 (K_g/K_w). Within the explored parameter space, increasing measurement force and speed increased the measured elasticity of both grey and white matter. However, K_g/K_w declined from values as high as ∼5 at low forces and speeds to ∼1 for high forces and speeds. K_g/K_w 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 K_g/K_w. 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, K_g/K_w increased until ∼6 hours post-mortem before stabilising. Between 20°C and 38°C, both grey and white matter elasticity decreased at a similar rate, without affecting K_g/K_w. 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 K_g/K_w. These differential responses likely contribute to the contradictory results obtained with different methods working in different strain regimes.