# Age-Dependent Changes in Thermo–Viscoelastic Properties of Human Brain by Non-Equilibrium Thermodynamics with Internal Variables

## TL;DR

This paper uses thermodynamics to show how the brain's viscoelastic properties change with age, linking these changes to neuron degeneration and reduced metabolic activity.

## Contribution

A novel thermodynamic approach is applied to analyze age-related changes in brain viscoelasticity and entropy production.

## Key findings

- The young brain is more rigid and viscous than the old brain due to neuron degeneration and myelin loss in aging.
- Older brains produce less entropy, indicating reduced metabolic activity and greater disorder in younger brains.
- Lower stiffness in the aging brain is linked to dendritic regression and neuronal death from oxidative stress.

## Abstract

The alterations to which neurons are subjected during their lifetime cause a general decline in cognitive activities as the years pass. The purpose of this research is to analyze the viscoelastic properties of the human brain using new mathematical procedures to allow for a thermodynamic characterization of this tissue. From this study, it is evident that the young brain is more rigid, less fluid, and more viscous than the old brain, and this is explained by remembering that the old one is characterized by neuron degeneration with partial myelin loss and a loss of “compactness”. Furthermore, in the brain, the oxidation of glucose for energy purposes is associated with the production of entropy, so the lower degree of entropy production monitored in the old brain compared to the young one inevitably indicates a lower metabolic activity of the neurons. In conclusion, this study of the rheological properties of the central nervous system through a thermodynamic approach has led to new characteristics of the young and old brain, allowing for new knowledge of the phenomena involved.

Over the years, neurons undergo functional changes initially linked to the maturation of the brain and then are progressively linked to normal aging. The curious relationship between brain decay, aging, and neuronal diseases has aroused the interest of numerous studies to better understand and contrast the evolution of these pathologies. The objective of this research is to apply the non-equilibrium thermodynamic theory with the internal variables of the study of the rheological properties of the brain, focusing on the study of viscoelastic properties. After a thermodynamic introduction of the principal rheological phenomena, this paper discusses the results by the application of our mathematical technique, which revealed a prevalence of anelastic properties in the old central nervous system compared to the young one. Furthermore, the entropy production trend tested identifies a greater disorder in the young brain in respect to the old one. The results obtained highlight that a lower stiffness in the old central nervous system may be interpreted with dendritic regression associated with neuronal death, both being potential consequences of an increased production of free radicals due to reduced antioxidant defenses and/or an altered mitochondrial dysfunction in aging.