Mechanical and Electrical Responses of the Squid Giant Axon to Simple Elongation

There is a limited amount of information available on the mechanical and functional response of the nervous system to loading. While deformation of cerebral, spinal, or peripheral nerve tissue can have particularly severe consequences, most research in this area has concentrated on either demonstrating in-vivo functional changes and disclosing the effected anatomical pathways, or describing material deformations of the composite structure. Although such studies have successfully produced repeatable traumas, they have not addressed the mechanisms of these mechanically induced injuries. Therefore, a single cell model is required in order to gain further understanding of this complex phenomena. An isolated squid giant axon was subjected to controlled uniaxial loading and its mechanical and physiological responses were monitored with an instrument specifically designed for these experiments. These results determined that the mechanical properties of the isolated axon are similar to those of other soft tissues, and include features such as a nonlinear load-deflection curve and a hysteresis loop upon unloading. The mechanical response was modeled with the quasi-linear viscoelastic theory (Fung, 1972). The physiological response of the axon to quasi-static loading was a small reversible hyperpolarization; however, as the rate of loading was increased, the axon depolarized and the magnitude and the time needed to recover to the original resting potential increased in a nonlinear fashion. At elongations greater than twenty percent an irreversible injury occurs and the membrane potential does not completely recover to baseline.