Mechanical functioning of peripheral nerves: linkage with the “mushrooming” effect

Abstract

Biomechanical properties of nerve have been studied extensively. All neural matrix tissues have been suggested to be the main load-bearing component. Based on the ultrastructure it has been proposed that the architecture of the epineurium allows some degree of extensibility of the nerve. A role of the perineurium could be to withstand the positive endoneurial pressure. The hypothesis is that the mechanical behaviour of nerves is dependent on an interaction between the core swelling pressure and restraint by the outer sheath. Loss of this balance will alter that behaviour. To test this, rat sciatic nerves were subjected to mechanical loading at in vivo and ex vivo tension. Retraction of nerve segments was measured after excision and after incubation at 37°C or freezing. Swelling properties of the nerve were measured by immersion in water or PBS (phosphate buffer solution) with intact or opened epineurium. Results showed a significant decrease in strength and stiffness with an increase in strain of the nerve after excision, compared to in vivo. Retraction was on average 11%. Freezing or incubation at 37°C did not alter retraction. The swelling properties of the nerve demonstrated a significant difference between intact and opened epineurium and similar results for water and PBS, indicating that epineurium is a constraint and that the nerves are underhydrated. The proposed model for the intact nerve is a continuous connective tissue tube surrounding and constraining an inner swelling pressure of the neural core. Loss of integrity of the nerve has detrimental effects on its biomechanical properties.