Effects of local anesthetics on compound action potentials generated from the frog sciatic nerve

Abstract

The frog sciatic nerve provides a robust physiological preparation students may conveniently use to investigate the properties of action potentials. Electrical stimulation with standard physiology teaching equipment elicits compound action potentials that are easily recorded by upper level undergraduate students. The amplitude of compound action potentials increases with greater stimulation voltages, up until a maximum response is achieved. Plotting action potential size as a function of stimulating voltage produces a curve that illustrates the responsiveness of a nerve. In the current study, several local anesthetics (MS-222, procaine, lidocaine, benzocaine, and tetracaine) were used to reversibly suppress compound action potentials within a time frame consistent with the limitations of teaching labs. Highly responsive nerves generate steep response curves that reach asymptotes at relatively low stimulating voltages. Less active nerves require higher stimulating voltages and appear 'right shifted'. Anesthetized response curves may also appear 'flatter', exhibiting lower peak amplitude, when compared to fully active nerves. The magnitude of action potential suppression and time course of recovery depended upon the specific anesthetic applied. Nerves anesthetized with MS-222 were fastest to recover, reaching their original responsiveness within 20 minutes. Tetracaine had the most dramatic effects, with nerves typically requiring more than a day to fully recover physiological responses. Carefully dissected nerves maintained physiological responses for many days when stored in Ringer's solution at 4° C, making this preparation particularly useful for undergraduate lab experiences. Quantitative analyses may be performed on the data collected, providing students with opportunities to design and implement their own experiments.