Peripheral nerve at extreme low temperatures 1: effects of temperature on the action potential.

Hypothermia is an important means of neuroprotection. Understanding the effects of temperature on a physiologic measurement such as the nerve action potential (NAP) is important in monitoring its effects. The effects of hypothermia on the NAP amplitude, conduction velocity, and response to paired pulse stimulation were quantified in a rat sciatic nerve preparation from 37 to 10 degrees C. The time course of temperature related changes and the effect of repeated cycles of cooling and rewarming are explored using the following measures of the NAP: peak-to-peak amplitude, conduction velocity, duration, area under the curve and response to paired pulse stimuli. The NAP amplitude initially increases as temperature is reduced to 27 degrees C and then drops to roughly 50% of its baseline value by 16 degrees C while the area under the curve increases gradually until it begins to decline at 16 degrees C. Permanent loss of the NAP appears only after cooling below 10 degrees C for extended periods. Although the dependence of amplitude on temperature is approximately sigmoidal, the conduction velocity declines linearly at a rate of 2.8m/s/ degrees C. The response to paired pulse stimulation is strongly dependent on both temperature and the interstimulus interval with the responses at shorter interstimulus intervals being more temperature sensitive. With repetitive cycles of cooling and rewarming, the NAP amplitude declines by roughly 4% with every cycle without changes in the temperature at which the NAP amplitude reaches 50% of baseline. Only minor differences in conduction velocity are seen during cooling and rewarming.