Strain Distribution and Relaxation in the Stretched Axons of Sensory Neurons

Abstract

The mechanical environment of a neuron strongly influences its function. In response to an externally applied tensile load, a number of morphological responses have been demonstrated in the axons of cultured neurons. We have developed methods to understand cellular mechanisms governing these responses. Rat sensory neurons were seeded onto a flexible silicone substrate and were imaged during substrate stretch. This configuration resulted in uniform tensile loading along the length of the neuron. Stationary mitochondria, believed to be docked to the axonal cytoskeleton, were used as fiduciary markers for elements of the cytoskeleton. Their positions were determined before and after an applied substrate strain (percent change in length) of 10%, and used to calculate the resulting “instantaneous” strain of regions along the axon. There was dramatic heterogeneity in the measured strain along the length of the stretched axons. This variability was particularly evident in regions of the axon less than 35 microns long. Measured strain in regions longer than this was less variable and was closer to the expected 10% strain. These results suggest a length scale over which local structural elements may be altered to modulate the biomechanical response of the axon. Following the initial stretch, the substrate was held at 10% strain and the axons imaged for 20 minutes during “relaxation.” Compared to unstretched axons, mitochondrial pairs in stretched axons showed little coordinated movement with each other at all length scales. Additionally, mitochondria in stretched axons showed larger displacements during the initial phase of relaxation, but after 18 minutes, the displacements were much smaller than those seen in unstretched axons. Collectively, this work presents the axon as a dynamic and heterogeneous structure, which interacts mechanically with the extracellular environment in more complex ways than previously thought.