Effects of hypothermia on auditory brain-stem and somatosensory evoked responses. A model of a synaptic and axonal lesion.

Auditory nerve brain-stem (ABR) and somatosensory evoked responses (SER) were recorded in cats as body temperature was uniformly lowered from 37 to 27 degrees C. Analysis of the results showed that the alterations in the evoked responses were due to disturbances induced both in axonal propagation and synaptic transmission by the hypothermia. By studying the first wave of the SER, which is solely an axonal event, and by assuming reasonable values for the total synaptic delay and axonal propagation times along the ABR pathway, it was concluded that this lesion model induced an effect on synaptic transmission 1.3-1.7 times greater than that on axonal propagation. There was a strong inverse correlation between wave latency and body temperature, with slightly steeper slopes for the longer latency waves. Wave amplitudes were not correlated with temperature. Furthermore, the wave latencies and amplitudes were generally not dependent on stimulus rate.