Corticospinal and spinal excitability during progressive skin and core cooling

Cold stress in survival situations can impair fine and gross motor control. This reduces muscle control and performance which can lead to life threatening consequences. Effect of cooling on muscle performance is mostly on the muscle tissue and local. However, less is known about its effect on the central nervous system. Therefore, the purpose of this study was to characterize corticospinal and spinal excitability that occurs during whole-body cooling, resulting in a reduction in both skin (Tsk) and core (Tco) temperature and shivering. Eight subjects (four male, four female) wore a liquid perfused suit and cooled by 2°C water pumped into the suit for 90 minutes and rewarmed with 41°C water for 30 minutes. Meanwhile, stimulation blocks consisting of 10 transcranial magnetic stimulations (eliciting MEPs), 8 trans-mastoid (eliciting CMEPs) and 2 brachial plexus (eliciting Mmax) electrical stimulations, were delivered at baseline, and 30, 60, and 90 minutes of cooling and once after 30 minutes of rewarming. 90 min cooling reduced Tsk to 18.2 ± 1.1°C (P <0.001) while Tco did not change (P = 0.92). Shivering EMG of all eight subjects was observed and recorded. At 100 min, met heat production was significantly higher than baseline (P = 0.013). Shivering stopped after 15 min of the start of the rewarming all subjects stopped shivering. Regardless of cooling or rewarming, Mmax, MEP, and MEP/Mmax did not change from baseline, but CMEP and CMEP/Mmax increased at the end of rewarming by 64% (P = 0.001) and (58%) (P = 0.02) respectively. These results suggest that a reduced Tco increases spinal excitability. However, corticospinal excitability remained unaltered.