Effects of thermal acclimation on nervous conduction and muscle contraction in the frog Rana temporaria

The effects of season and acclimation temperature on the latency of the leg withdrawal reflex and three of its components have been studied: conductio n velocity in the sciatic nerve, spinal conduction time, and contraction li me of gastrocnemius muscle. The latency of the leg withdrawal reflex was ma rkedly shortened by cold acclimation: the reaction times were at 6 degrees C 1.54 s in 4 degrees C acclimated and 3.97 s in 24 degrees C acclimated wi nter frogs. Also, the temperature dependence of the reflex latency was redu ced by cold acclimation. Thus, frogs acclimated to cold responded to extern al stimuli in cold more rapidly than warm-acclimated ones. This cold adapta tion of the reflex could not be explained by changes in its studied compone nts. These made up only one-tenth of the reflex response time, and either d id not show significant cold acclimation (muscle contraction and spinal con duction times in summer) or showed inverse acclimation, especially when mea sured at high temperatures (i.e. conduction velocities were reduced by accl imation to cold). Thus, the cold acclimation of the reflex response probabl y resides in the sensory component of the response. The inverse temperature adaptation response of conduction velocities may reflect a reduced ion per meability across cellular membranes in cold which decreases metabolic energ y expenditure during inactive periods. (C) 1999 Elsevier Science Inc. All r ights reserved.