THE EFFECT OF CARBON DIOXIDE ON NERVE

Abstract

The influence of CO2 on the action and resting potentials of isolated frog nerves was studied with the aid of a moving coil galvanometer, as previously described. The rate of passing the gas through the nerve chamber affected observations. Threshold changes to induction shocks and condenser discharges were studied on muscle nerve preparations or, using action potentials as the index of response, on isolated nerves. Exposing a stretch of nerve to CO2 caused a marked increase of total action potential, up to 5 times the initial values. This was true for CO2 concentrations (in O) from 10 to 100%. The high responses were maintained during hrs. of exposure except in the case of 100% CO2 in which the responses fell to zero in 2-3 hrs. This fall was probably due to asphyxia. The rise was relatively much greater for stimuli at 90 per sec. than at 300, and is interpreted as due to prolongation of action potentials. There was usually but little change in the responses below the exposed region; so that few fibers could be blocked. The refractory period was increased. On re-admitting O, there was a further rise in action potentials to a peak value, 5-15% above that reached during exposure to CO2, followed by a fall to initial or somewhat lower values. The first change in response on admitting CO2 appeared in 45 sec.; the maximum was reached in 20 minutes. These delays were probably not due to time required for diffusion into the nerve but measured the progress of a slow change initiated by CO2. On removing CO2, the first effect appeared in 2-3 min., the peak was reached in 3-15 min., an hr. or more being required for the subsequent fall. Similar but less marked changes were registered at electrodes leading from a stretch of nerve nearer the stimulating electrode, not exposed to CO2. A nerve in CO2 did not show the usual progressive increase of response (equilibration) with repetition of activity. The resting potential was very sensitive to the rate of bubbling of gas through the chamber. When this factor was controlled, there still appeared, when CO2 was admitted, a sharp but brief rise in potential followed by a greater and maintained fall. On replacing CO2 by O there was a further brief fall and then a gradual rise to a new level, usually lower than at the start. The threshold of the exposed region of the nerve, but no other, was markedly increased, sometimes after a preliminary fall, by CO2. It could be tripled for induction shocks and increased over 10 times for long condenser discharges. The chronaxie was decreased and the voltage-duration curve altered, in that currents of shorter duration became relatively more effective than longer ones. The changes were reversed on removing the CO2. The rise after admitting the gas was complete in 2-4 min., the return to normal after its removal not for an hr. or more. These observations are interpreted as indicating a slowing of critical chemical reactions in nerve under the influence of CO2.