Effect of Cooling on Neuromuscular Transmission in the Rat

¹ From the National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Maryland

When the rat was being cooled from normal body temperature to 10°C, the resting membrane potentials recorded from the endplate region of the gracilis anticus muscle of the rat showed no significant change, with a mean value of –79.5 ± 4.6 mv (S.D.). The resting membrane potentials at body temperatures below 10°C were lower; the mean value at 10°–4°C was –71 ± 8.5 mv, and at 4°–0°C, –32 ± 10.2 mv. Below 0°C resting membrane potential was not recorded. The responses to nerve stimulation did not change at body temperatures between 36° and 15°C. Below 15°C the duration of the spike responses became longer and at times the responses showed no spike potentials; at 5°–4°C no spikes but endplate potentials were recorded. The miniature endplate potentials could be recorded at body temperature as low as 4°C but not below. The performance of neuromuscular transmission was most efficient at body temperature above 15°C, and was impeded at 15°C and blocked at 5°C. At 4°C the nerve ceased to conduct although contraction of the muscle could be produced by direct stimulation at body temperatures as low as 1°C. During the process of cooling some of the muscle fibers would fibrillate. In frost bite lesions, produced by cooling the extremity to –2°C for 2 hours, muscle resting potentials and miniature endplate potentials were absent and there was no response to direct or indirect stimulation of the muscle.

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