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

The frog sciatic nerve provides a robust physiological preparation students may conveniently use to investigate the properties of compound 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 present 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 the fastest to recover, reaching their original responsiveness within 20 min. Tetracaine had the most dramatic effects, with nerves typically requiring more than a day to fully recover physiological responses. Carefully dissected nerves maintained their physiological responses for many days when stored in Ringer 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. The frog sciatic nerve preparation represents a "classical" physiology lab for demonstrating principles of action potentials. Local anesthetics provide an inexpensive tool to manipulate the physiological activity of nerves and other excitable tissues. Isolated nerves retain their physiological activity for up to several days when kept in Ringer solution at 4°C. Quantitative data analysis from this robust nerve preparation should present students with many opportunities for designing their own experiments with anesthetics.