The Chronaxie and Propagation Velocity of Canine Cervical Vagus Nerve Fibers In Vivo

A recent medical development is the stimulation of the left afferent vagus nerve to control seizures in man. It is well known that vagus nerves innervate the heart; however, when afferent fibers are stimulated so are efferent fibers. Basic to the understanding of afferent fiber stimulation is the fundamental excitability properties of the vagus nerve. In this study, we measured chronaxie, rheobase and propagation velocity in the intact dog cervical afferent vagus nerve. Five adult mongrel dogs of both sexes, weighing 22–29 kg, were sedated with thiopental sodium, intubated, and connected to an anesthesia machine delivering isoflurane and oxygen. The ventral side of the neck was dissected and two bipolar electrodes were then placed on the vagus nerve, as far from one another as possible, and data were collected in the form of the strength–duration curve using single rectangular current pulses. Two different fiber types, which we designated A and B, were identified electrophysiologically. The average values for chronaxie were 75.4 ± 24.5 s for A fibers and 82.3 ± 23.3 s for B fibers. Values for rheobase were 0.63 ± 0.18 mA for A fibers and 0.66 ± 0.22 mA for B fibers. Propagation velocities were 59.0 ± 9.6 m/s for A fibers and 43.4 ± 8.0 m/s for B fibers. With an effective electrode area of 5 mm², this yields current densities of 13.0 mA/cm² and 12.9 mA/cm² for A and B fibers respectively. We also identified fibers in the right vagus, which we called B_H, with propagation velocity of 5.75 ± 0.35 m/s, which appears to be responsible for cardiac slowing. Fibers with a high propagation velocity appeared to have a short chronaxie and fibers with a low propagation velocity appeared to have a long chronaxie; however, no definitive relationship between propagation velocity and chronaxie was found.