Author(s): WALDRON, M., PEEL, J., PAGE, J., SCOTT, G., HEFFERNAN, S., TALLENT, J., JEFFRIES, O., Institution: SWANSEA UNIVERSITY , Country: UNITED KINGDOM, Abstract-ID: 748

INTRODUCTION:
During hyperthermia, integration of afferent input from central and peripheral thermoreceptors initiates efferent sympathetic signals to activate sweat glands in pulsatile bursts. In this feedback system, core temperature (Tcore) is the primary controlled variable, with skin temperature (Tskin) providing ‘auxiliary’ dynamic feedback [1]. However, the connectivity of efferent signalling between Tcore, Tskin and sweating, as well as the synchrony of sweating between skin sites, across individuals of varying thermoregulatory control has not been investigated. Therefore, we evaluated i) connectivity between Tcore, Tskin and local eccrine sweating of the arm and chest during passive heating and ii) the temporal synchrony of arm and chest sweating.
METHODS:
Thirteen participants completed 90 min passive heating, twice, across one-week (38±0°C, relative humidity 56±2%). Local sweat rates were continuously measured on the ventral forearm and chest, alongside Tcore (rectal) and mean Tskin (chest, arm and leg). Changes in Tcore across the protocol and thermosensitivity (Tcore change at sweat onset) were measured to determine thermoregulatory control of the sample. Transfer entropy (TE) was used to quantify information flow between Tcore, Tskin and all sweating sites, with higher values demonstrating greater connectivity. Relationships between thermoregulatory control measures and TE were evaluated using repeated measures correlations. Cross-correlation was used to assess sweating synchrony across the two sites. Paired t-tests assessed differences between TE values. Data are means±SD.
RESULTS:
The TE for Tcore-arm (0.007±0.005 bits) and Tcore-chest (0.009±0.010) was lower (P<0.01 and P <0.01) than Tskin-arm (0.019±0.014 bits) and Tskin-chest (0.022±0.015 bits), respectively, denoting higher information flow from the skin to sweating sites. There were no differences between arm and chest Tskin TE values (P = 0.31). Both Tskin-arm TE and Tskin-chest TE were inversely related to the Tcore increases (r= -0.56 and -0.57, respectively) and thermosensitivity values (r= -0.37 & -0.46, respectively), while Tcore TE values were unrelated (P>0.05). Cross-correlation analysis revealed no time-lag between chest and arm site peak relationships, which were strongly correlated (r=0.98±0.03).
CONCLUSION:
The notion of Tskin as a dynamic auxiliary feedback mechanism [1] is supported, with reliance upon informational flow between Tskin and sweating of the arm and chest to maintain control of Tcore, which has relatively less TE to sweating sites. Individuals with smaller increases in Tcore and faster sweating onset possessed greater connectivity between Tskin and sweating sites. The temporal correspondence confirms the assumed synchrony between sweating sites, indicating that the pattern of this informational flow is likely to be similar across anatomical locations. These findings may inform future passive assessments of thermoeffector feedback loops.

References
1. Romanovsky, 2014, Acta Physiol.